diff options
Diffstat (limited to 'Documentation/block/biodoc.txt')
| -rw-r--r-- | Documentation/block/biodoc.txt | 1213 |
1 files changed, 1213 insertions, 0 deletions
diff --git a/Documentation/block/biodoc.txt b/Documentation/block/biodoc.txt new file mode 100644 index 000000000000..6dd274d7e1cf --- /dev/null +++ b/Documentation/block/biodoc.txt @@ -0,0 +1,1213 @@ + Notes on the Generic Block Layer Rewrite in Linux 2.5 + ===================================================== + +Notes Written on Jan 15, 2002: + Jens Axboe <axboe@suse.de> + Suparna Bhattacharya <suparna@in.ibm.com> + +Last Updated May 2, 2002 +September 2003: Updated I/O Scheduler portions + Nick Piggin <piggin@cyberone.com.au> + +Introduction: + +These are some notes describing some aspects of the 2.5 block layer in the +context of the bio rewrite. The idea is to bring out some of the key +changes and a glimpse of the rationale behind those changes. + +Please mail corrections & suggestions to suparna@in.ibm.com. + +Credits: +--------- + +2.5 bio rewrite: + Jens Axboe <axboe@suse.de> + +Many aspects of the generic block layer redesign were driven by and evolved +over discussions, prior patches and the collective experience of several +people. See sections 8 and 9 for a list of some related references. + +The following people helped with review comments and inputs for this +document: + Christoph Hellwig <hch@infradead.org> + Arjan van de Ven <arjanv@redhat.com> + Randy Dunlap <rddunlap@osdl.org> + Andre Hedrick <andre@linux-ide.org> + +The following people helped with fixes/contributions to the bio patches +while it was still work-in-progress: + David S. Miller <davem@redhat.com> + + +Description of Contents: +------------------------ + +1. Scope for tuning of logic to various needs + 1.1 Tuning based on device or low level driver capabilities + - Per-queue parameters + - Highmem I/O support + - I/O scheduler modularization + 1.2 Tuning based on high level requirements/capabilities + 1.2.1 I/O Barriers + 1.2.2 Request Priority/Latency + 1.3 Direct access/bypass to lower layers for diagnostics and special + device operations + 1.3.1 Pre-built commands +2. New flexible and generic but minimalist i/o structure or descriptor + (instead of using buffer heads at the i/o layer) + 2.1 Requirements/Goals addressed + 2.2 The bio struct in detail (multi-page io unit) + 2.3 Changes in the request structure +3. Using bios + 3.1 Setup/teardown (allocation, splitting) + 3.2 Generic bio helper routines + 3.2.1 Traversing segments and completion units in a request + 3.2.2 Setting up DMA scatterlists + 3.2.3 I/O completion + 3.2.4 Implications for drivers that do not interpret bios (don't handle + multiple segments) + 3.2.5 Request command tagging + 3.3 I/O submission +4. The I/O scheduler +5. Scalability related changes + 5.1 Granular locking: Removal of io_request_lock + 5.2 Prepare for transition to 64 bit sector_t +6. Other Changes/Implications + 6.1 Partition re-mapping handled by the generic block layer +7. A few tips on migration of older drivers +8. A list of prior/related/impacted patches/ideas +9. Other References/Discussion Threads + +--------------------------------------------------------------------------- + +Bio Notes +-------- + +Let us discuss the changes in the context of how some overall goals for the +block layer are addressed. + +1. Scope for tuning the generic logic to satisfy various requirements + +The block layer design supports adaptable abstractions to handle common +processing with the ability to tune the logic to an appropriate extent +depending on the nature of the device and the requirements of the caller. +One of the objectives of the rewrite was to increase the degree of tunability +and to enable higher level code to utilize underlying device/driver +capabilities to the maximum extent for better i/o performance. This is +important especially in the light of ever improving hardware capabilities +and application/middleware software designed to take advantage of these +capabilities. + +1.1 Tuning based on low level device / driver capabilities + +Sophisticated devices with large built-in caches, intelligent i/o scheduling +optimizations, high memory DMA support, etc may find some of the +generic processing an overhead, while for less capable devices the +generic functionality is essential for performance or correctness reasons. +Knowledge of some of the capabilities or parameters of the device should be +used at the generic block layer to take the right decisions on +behalf of the driver. + +How is this achieved ? + +Tuning at a per-queue level: + +i. Per-queue limits/values exported to the generic layer by the driver + +Various parameters that the generic i/o scheduler logic uses are set at +a per-queue level (e.g maximum request size, maximum number of segments in +a scatter-gather list, hardsect size) + +Some parameters that were earlier available as global arrays indexed by +major/minor are now directly associated with the queue. Some of these may +move into the block device structure in the future. Some characteristics +have been incorporated into a queue flags field rather than separate fields +in themselves. There are blk_queue_xxx functions to set the parameters, +rather than update the fields directly + +Some new queue property settings: + + blk_queue_bounce_limit(q, u64 dma_address) + Enable I/O to highmem pages, dma_address being the + limit. No highmem default. + + blk_queue_max_sectors(q, max_sectors) + Maximum size request you can handle in units of 512 byte + sectors. 255 default. + + blk_queue_max_phys_segments(q, max_segments) + Maximum physical segments you can handle in a request. 128 + default (driver limit). (See 3.2.2) + + blk_queue_max_hw_segments(q, max_segments) + Maximum dma segments the hardware can handle in a request. 128 + default (host adapter limit, after dma remapping). + (See 3.2.2) + + blk_queue_max_segment_size(q, max_seg_size) + Maximum size of a clustered segment, 64kB default. + + blk_queue_hardsect_size(q, hardsect_size) + Lowest possible sector size that the hardware can operate + on, 512 bytes default. + +New queue flags: + + QUEUE_FLAG_CLUSTER (see 3.2.2) + QUEUE_FLAG_QUEUED (see 3.2.4) + + +ii. High-mem i/o capabilities are now considered the default + +The generic bounce buffer logic, present in 2.4, where the block layer would +by default copyin/out i/o requests on high-memory buffers to low-memory buffers +assuming that the driver wouldn't be able to handle it directly, has been +changed in 2.5. The bounce logic is now applied only for memory ranges +for which the device cannot handle i/o. A driver can specify this by +setting the queue bounce limit for the request queue for the device +(blk_queue_bounce_limit()). This avoids the inefficiencies of the copyin/out +where a device is capable of handling high memory i/o. + +In order to enable high-memory i/o where the device is capable of supporting +it, the pci dma mapping routines and associated data structures have now been +modified to accomplish a direct page -> bus translation, without requiring +a virtual address mapping (unlike the earlier scheme of virtual address +-> bus translation). So this works uniformly for high-memory pages (which +do not have a correponding kernel virtual address space mapping) and +low-memory pages. + +Note: Please refer to DMA-mapping.txt for a discussion on PCI high mem DMA +aspects and mapping of scatter gather lists, and support for 64 bit PCI. + +Special handling is required only for cases where i/o needs to happen on +pages at physical memory addresses beyond what the device can support. In these +cases, a bounce bio representing a buffer from the supported memory range +is used for performing the i/o with copyin/copyout as needed depending on +the type of the operation. For example, in case of a read operation, the +data read has to be copied to the original buffer on i/o completion, so a +callback routine is set up to do this, while for write, the data is copied +from the original buffer to the bounce buffer prior to issuing the +operation. Since an original buffer may be in a high memory area that's not +mapped in kernel virtual addr, a kmap operation may be required for +performing the copy, and special care may be needed in the completion path +as it may not be in irq context. Special care is also required (by way of +GFP flags) when allocating bounce buffers, to avoid certain highmem +deadlock possibilities. + +It is also possible that a bounce buffer may be allocated from high-memory +area that's not mapped in kernel virtual addr, but within the range that the +device can use directly; so the bounce page may need to be kmapped during +copy operations. [Note: This does not hold in the current implementation, +though] + +There are some situations when pages from high memory may need to +be kmapped, even if bounce buffers are not necessary. For example a device +may need to abort DMA operations and revert to PIO for the transfer, in +which case a virtual mapping of the page is required. For SCSI it is also +done in some scenarios where the low level driver cannot be trusted to +handle a single sg entry correctly. The driver is expected to perform the +kmaps as needed on such occasions using the __bio_kmap_atomic and bio_kmap_irq +routines as appropriate. A driver could also use the blk_queue_bounce() +routine on its own to bounce highmem i/o to low memory for specific requests +if so desired. + +iii. The i/o scheduler algorithm itself can be replaced/set as appropriate + +As in 2.4, it is possible to plugin a brand new i/o scheduler for a particular +queue or pick from (copy) existing generic schedulers and replace/override +certain portions of it. The 2.5 rewrite provides improved modularization +of the i/o scheduler. There are more pluggable callbacks, e.g for init, +add request, extract request, which makes it possible to abstract specific +i/o scheduling algorithm aspects and details outside of the generic loop. +It also makes it possible to completely hide the implementation details of +the i/o scheduler from block drivers. + +I/O scheduler wrappers are to be used instead of accessing the queue directly. +See section 4. The I/O scheduler for details. + +1.2 Tuning Based on High level code capabilities + +i. Application capabilities for raw i/o + +This comes from some of the high-performance database/middleware +requirements where an application prefers to make its own i/o scheduling +decisions based on an understanding of the access patterns and i/o +characteristics + +ii. High performance filesystems or other higher level kernel code's +capabilities + +Kernel components like filesystems could also take their own i/o scheduling +decisions for optimizing performance. Journalling filesystems may need +some control over i/o ordering. + +What kind of support exists at the generic block layer for this ? + +The flags and rw fields in the bio structure can be used for some tuning +from above e.g indicating that an i/o is just a readahead request, or for +marking barrier requests (discussed next), or priority settings (currently +unused). As far as user applications are concerned they would need an +additional mechanism either via open flags or ioctls, or some other upper +level mechanism to communicate such settings to block. + +1.2.1 I/O Barriers + +There is a way to enforce strict ordering for i/os through barriers. +All requests before a barrier point must be serviced before the barrier +request and any other requests arriving after the barrier will not be +serviced until after the barrier has completed. This is useful for higher +level control on write ordering, e.g flushing a log of committed updates +to disk before the corresponding updates themselves. + +A flag in the bio structure, BIO_BARRIER is used to identify a barrier i/o. +The generic i/o scheduler would make sure that it places the barrier request and +all other requests coming after it after all the previous requests in the +queue. Barriers may be implemented in different ways depending on the +driver. A SCSI driver for example could make use of ordered tags to +preserve the necessary ordering with a lower impact on throughput. For IDE +this might be two sync cache flush: a pre and post flush when encountering +a barrier write. + +There is a provision for queues to indicate what kind of barriers they +can provide. This is as of yet unmerged, details will be added here once it +is in the kernel. + +1.2.2 Request Priority/Latency + +Todo/Under discussion: +Arjan's proposed request priority scheme allows higher levels some broad + control (high/med/low) over the priority of an i/o request vs other pending + requests in the queue. For example it allows reads for bringing in an + executable page on demand to be given a higher priority over pending write + requests which haven't aged too much on the queue. Potentially this priority + could even be exposed to applications in some manner, providing higher level + tunability. Time based aging avoids starvation of lower priority + requests. Some bits in the bi_rw flags field in the bio structure are + intended to be used for this priority information. + + +1.3 Direct Access to Low level Device/Driver Capabilities (Bypass mode) + (e.g Diagnostics, Systems Management) + +There are situations where high-level code needs to have direct access to +the low level device capabilities or requires the ability to issue commands +to the device bypassing some of the intermediate i/o layers. +These could, for example, be special control commands issued through ioctl +interfaces, or could be raw read/write commands that stress the drive's +capabilities for certain kinds of fitness tests. Having direct interfaces at +multiple levels without having to pass through upper layers makes +it possible to perform bottom up validation of the i/o path, layer by +layer, starting from the media. + +The normal i/o submission interfaces, e.g submit_bio, could be bypassed +for specially crafted requests which such ioctl or diagnostics +interfaces would typically use, and the elevator add_request routine +can instead be used to directly insert such requests in the queue or preferably +the blk_do_rq routine can be used to place the request on the queue and +wait for completion. Alternatively, sometimes the caller might just +invoke a lower level driver specific interface with the request as a +parameter. + +If the request is a means for passing on special information associated with +the command, then such information is associated with the request->special +field (rather than misuse the request->buffer field which is meant for the +request data buffer's virtual mapping). + +For passing request data, the caller must build up a bio descriptor +representing the concerned memory buffer if the underlying driver interprets +bio segments or uses the block layer end*request* functions for i/o +completion. Alternatively one could directly use the request->buffer field to +specify the virtual address of the buffer, if the driver expects buffer +addresses passed in this way and ignores bio entries for the request type +involved. In the latter case, the driver would modify and manage the +request->buffer, request->sector and request->nr_sectors or +request->current_nr_sectors fields itself rather than using the block layer +end_request or end_that_request_first completion interfaces. +(See 2.3 or Documentation/block/request.txt for a brief explanation of +the request structure fields) + +[TBD: end_that_request_last should be usable even in this case; +Perhaps an end_that_direct_request_first routine could be implemented to make +handling direct requests easier for such drivers; Also for drivers that +expect bios, a helper function could be provided for setting up a bio +corresponding to a data buffer] + +<JENS: I dont understand the above, why is end_that_request_first() not +usable? Or _last for that matter. I must be missing something> +<SUP: What I meant here was that if the request doesn't have a bio, then + end_that_request_first doesn't modify nr_sectors or current_nr_sectors, + and hence can't be used for advancing request state settings on the + completion of partial transfers. The driver has to modify these fields + directly by hand. + This is because end_that_request_first only iterates over the bio list, + and always returns 0 if there are none associated with the request. + _last works OK in this case, and is not a problem, as I mentioned earlier +> + +1.3.1 Pre-built Commands + +A request can be created with a pre-built custom command to be sent directly +to the device. The cmd block in the request structure has room for filling +in the command bytes. (i.e rq->cmd is now 16 bytes in size, and meant for +command pre-building, and the type of the request is now indicated +through rq->flags instead of via rq->cmd) + +The request structure flags can be set up to indicate the type of request +in such cases (REQ_PC: direct packet command passed to driver, REQ_BLOCK_PC: +packet command issued via blk_do_rq, REQ_SPECIAL: special request). + +It can help to pre-build device commands for requests in advance. +Drivers can now specify a request prepare function (q->prep_rq_fn) that the +block layer would invoke to pre-build device commands for a given request, +or perform other preparatory processing for the request. This is routine is +called by elv_next_request(), i.e. typically just before servicing a request. +(The prepare function would not be called for requests that have REQ_DONTPREP +enabled) + +Aside: + Pre-building could possibly even be done early, i.e before placing the + request on the queue, rather than construct the command on the fly in the + driver while servicing the request queue when it may affect latencies in + interrupt context or responsiveness in general. One way to add early + pre-building would be to do it whenever we fail to merge on a request. + Now REQ_NOMERGE is set in the request flags to skip this one in the future, + which means that it will not change before we feed it to the device. So + the pre-builder hook can be invoked there. + + +2. Flexible and generic but minimalist i/o structure/descriptor. + +2.1 Reason for a new structure and requirements addressed + +Prior to 2.5, buffer heads were used as the unit of i/o at the generic block +layer, and the low level request structure was associated with a chain of +buffer heads for a contiguous i/o request. This led to certain inefficiencies +when it came to large i/o requests and readv/writev style operations, as it +forced such requests to be broken up into small chunks before being passed +on to the generic block layer, only to be merged by the i/o scheduler +when the underlying device was capable of handling the i/o in one shot. +Also, using the buffer head as an i/o structure for i/os that didn't originate +from the buffer cache unecessarily added to the weight of the descriptors +which were generated for each such chunk. + +The following were some of the goals and expectations considered in the +redesign of the block i/o data structure in 2.5. + +i. Should be appropriate as a descriptor for both raw and buffered i/o - + avoid cache related fields which are irrelevant in the direct/page i/o path, + or filesystem block size alignment restrictions which may not be relevant + for raw i/o. +ii. Ability to represent high-memory buffers (which do not have a virtual + address mapping in kernel address space). +iii.Ability to represent large i/os w/o unecessarily breaking them up (i.e + greater than PAGE_SIZE chunks in one shot) +iv. At the same time, ability to retain independent identity of i/os from + different sources or i/o units requiring individual completion (e.g. for + latency reasons) +v. Ability to represent an i/o involving multiple physical memory segments + (including non-page aligned page fragments, as specified via readv/writev) + without unecessarily breaking it up, if the underlying device is capable of + handling it. +vi. Preferably should be based on a memory descriptor structure that can be + passed around different types of subsystems or layers, maybe even + networking, without duplication or extra copies of data/descriptor fields + themselves in the process +vii.Ability to handle the possibility of splits/merges as the structure passes + through layered drivers (lvm, md, evms), with minimal overhead. + +The solution was to define a new structure (bio) for the block layer, +instead of using the buffer head structure (bh) directly, the idea being +avoidance of some associated baggage and limitations. The bio structure +is uniformly used for all i/o at the block layer ; it forms a part of the +bh structure for buffered i/o, and in the case of raw/direct i/o kiobufs are +mapped to bio structures. + +2.2 The bio struct + +The bio structure uses a vector representation pointing to an array of tuples +of <page, offset, len> to describe the i/o buffer, and has various other +fields describing i/o parameters and state that needs to be maintained for +performing the i/o. + +Notice that this representation means that a bio has no virtual address +mapping at all (unlike buffer heads). + +struct bio_vec { + struct page *bv_page; + unsigned short bv_len; + unsigned short bv_offset; +}; + +/* + * main unit of I/O for the block layer and lower layers (ie drivers) + */ +struct bio { + sector_t bi_sector; + struct bio *bi_next; /* request queue link */ + struct block_device *bi_bdev; /* target device */ + unsigned long bi_flags; /* status, command, etc */ + unsigned long bi_rw; /* low bits: r/w, high: priority */ + + unsigned int bi_vcnt; /* how may bio_vec's */ + unsigned int bi_idx; /* current index into bio_vec array */ + + unsigned int bi_size; /* total size in bytes */ + unsigned short bi_phys_segments; /* segments after physaddr coalesce*/ + unsigned short bi_hw_segments; /* segments after DMA remapping */ + unsigned int bi_max; /* max bio_vecs we can hold + used as index into pool */ + struct bio_vec *bi_io_vec; /* the actual vec list */ + bio_end_io_t *bi_end_io; /* bi_end_io (bio) */ + atomic_t bi_cnt; /* pin count: free when it hits zero */ + void *bi_private; + bio_destructor_t *bi_destructor; /* bi_destructor (bio) */ +}; + +With this multipage bio design: + +- Large i/os can be sent down in one go using a bio_vec list consisting + of an array of <page, offset, len> fragments (similar to the way fragments + are represented in the zero-copy network code) +- Splitting of an i/o request across multiple devices (as in the case of + lvm or raid) is achieved by cloning the bio (where the clone points to + the same bi_io_vec array, but with the index and size accordingly modified) +- A linked list of bios is used as before for unrelated merges (*) - this + avoids reallocs and makes independent completions easier to handle. +- Code that traverses the req list needs to make a distinction between + segments of a request (bio_for_each_segment) and the distinct completion + units/bios (rq_for_each_bio). +- Drivers which can't process a large bio in one shot can use the bi_idx + field to keep track of the next bio_vec entry to process. + (e.g a 1MB bio_vec needs to be handled in max 128kB chunks for IDE) + [TBD: Should preferably also have a bi_voffset and bi_vlen to avoid modifying + bi_offset an len fields] + +(*) unrelated merges -- a request ends up containing two or more bios that + didn't originate from the same place. + +bi_end_io() i/o callback gets called on i/o completion of the entire bio. + +At a lower level, drivers build a scatter gather list from the merged bios. +The scatter gather list is in the form of an array of <page, offset, len> +entries with their corresponding dma address mappings filled in at the +appropriate time. As an optimization, contiguous physical pages can be +covered by a single entry where <page> refers to the first page and <len> +covers the range of pages (upto 16 contiguous pages could be covered this +way). There is a helper routine (blk_rq_map_sg) which drivers can use to build +the sg list. + +Note: Right now the only user of bios with more than one page is ll_rw_kio, +which in turn means that only raw I/O uses it (direct i/o may not work +right now). The intent however is to enable clustering of pages etc to +become possible. The pagebuf abstraction layer from SGI also uses multi-page +bios, but that is currently not included in the stock development kernels. +The same is true of Andrew Morton's work-in-progress multipage bio writeout +and readahead patches. + +2.3 Changes in the Request Structure + +The request structure is the structure that gets passed down to low level +drivers. The block layer make_request function builds up a request structure, +places it on the queue and invokes the drivers request_fn. The driver makes +use of block layer helper routine elv_next_request to pull the next request +off the queue. Control or diagnostic functions might bypass block and directly +invoke underlying driver entry points passing in a specially constructed +request structure. + +Only some relevant fields (mainly those which changed or may be referred +to in some of the discussion here) are listed below, not necessarily in +the order in which they occur in the structure (see include/linux/blkdev.h) +Refer to Documentation/block/request.txt for details about all the request +structure fields and a quick reference about the layers which are +supposed to use or modify those fields. + +struct request { + struct list_head queuelist; /* Not meant to be directly accessed by + the driver. + Used by q->elv_next_request_fn + rq->queue is gone + */ + . + . + unsigned char cmd[16]; /* prebuilt command data block */ + unsigned long flags; /* also includes earlier rq->cmd settings */ + . + . + sector_t sector; /* this field is now of type sector_t instead of int + preparation for 64 bit sectors */ + . + . + + /* Number of scatter-gather DMA addr+len pairs after + * physical address coalescing is performed. + */ + unsigned short nr_phys_segments; + + /* Number of scatter-gather addr+len pairs after + * physical and DMA remapping hardware coalescing is performed. + * This is the number of scatter-gather entries the driver + * will actually have to deal with after DMA mapping is done. + */ + unsigned short nr_hw_segments; + + /* Various sector counts */ + unsigned long nr_sectors; /* no. of sectors left: driver modifiable */ + unsigned long hard_nr_sectors; /* block internal copy of above */ + unsigned int current_nr_sectors; /* no. of sectors left in the + current segment:driver modifiable */ + unsigned long hard_cur_sectors; /* block internal copy of the above */ + . + . + int tag; /* command tag associated with request */ + void *special; /* same as before */ + char *buffer; /* valid only for low memory buffers upto + current_nr_sectors */ + . + . + struct bio *bio, *biotail; /* bio list instead of bh */ + struct request_list *rl; +} + +See the rq_flag_bits definitions for an explanation of the various flags +available. Some bits are used by the block layer or i/o scheduler. + +The behaviour of the various sector counts are almost the same as before, +except that since we have multi-segment bios, current_nr_sectors refers +to the numbers of sectors in the current segment being processed which could +be one of the many segments in the current bio (i.e i/o completion unit). +The nr_sectors value refers to the total number of sectors in the whole +request that remain to be transferred (no change). The purpose of the +hard_xxx values is for block to remember these counts every time it hands +over the request to the driver. These values are updated by block on +end_that_request_first, i.e. every time the driver completes a part of the +transfer and invokes block end*request helpers to mark this. The +driver should not modify these values. The block layer sets up the +nr_sectors and current_nr_sectors fields (based on the corresponding +hard_xxx values and the number of bytes transferred) and updates it on +every transfer that invokes end_that_request_first. It does the same for the +buffer, bio, bio->bi_idx fields too. + +The buffer field is just a virtual address mapping of the current segment +of the i/o buffer in cases where the buffer resides in low-memory. For high +memory i/o, this field is not valid and must not be used by drivers. + +Code that sets up its own request structures and passes them down to +a driver needs to be careful about interoperation with the block layer helper +functions which the driver uses. (Section 1.3) + +3. Using bios + +3.1 Setup/Teardown + +There are routines for managing the allocation, and reference counting, and +freeing of bios (bio_alloc, bio_get, bio_put). + +This makes use of Ingo Molnar's mempool implementation, which enables +subsystems like bio to maintain their own reserve memory pools for guaranteed +deadlock-free allocations during extreme VM load. For example, the VM +subsystem makes use of the block layer to writeout dirty pages in order to be +able to free up memory space, a case which needs careful handling. The +allocation logic draws from the preallocated emergency reserve in situations +where it cannot allocate through normal means. If the pool is empty and it +can wait, then it would trigger action that would help free up memory or +replenish the pool (without deadlocking) and wait for availability in the pool. +If it is in IRQ context, and hence not in a position to do this, allocation +could fail if the pool is empty. In general mempool always first tries to +perform allocation without having to wait, even if it means digging into the +pool as long it is not less that 50% full. + +On a free, memory is released to the pool or directly freed depending on +the current availability in the pool. The mempool interface lets the +subsystem specify the routines to be used for normal alloc and free. In the +case of bio, these routines make use of the standard slab allocator. + +The caller of bio_alloc is expected to taken certain steps to avoid +deadlocks, e.g. avoid trying to allocate more memory from the pool while +already holding memory obtained from the pool. +[TBD: This is a potential issue, though a rare possibility + in the bounce bio allocation that happens in the current code, since + it ends up allocating a second bio from the same pool while + holding the original bio ] + +Memory allocated from the pool should be released back within a limited +amount of time (in the case of bio, that would be after the i/o is completed). +This ensures that if part of the pool has been used up, some work (in this +case i/o) must already be in progress and memory would be available when it +is over. If allocating from multiple pools in the same code path, the order +or hierarchy of allocation needs to be consistent, just the way one deals +with multiple locks. + +The bio_alloc routine also needs to allocate the bio_vec_list (bvec_alloc()) +for a non-clone bio. There are the 6 pools setup for different size biovecs, +so bio_alloc(gfp_mask, nr_iovecs) will allocate a vec_list of the +given size from these slabs. + +The bi_destructor() routine takes into account the possibility of the bio +having originated from a different source (see later discussions on +n/w to block transfers and kvec_cb) + +The bio_get() routine may be used to hold an extra reference on a bio prior +to i/o submission, if the bio fields are likely to be accessed after the +i/o is issued (since the bio may otherwise get freed in case i/o completion +happens in the meantime). + +The bio_clone() routine may be used to duplicate a bio, where the clone +shares the bio_vec_list with the original bio (i.e. both point to the +same bio_vec_list). This would typically be used for splitting i/o requests +in lvm or md. + +3.2 Generic bio helper Routines + +3.2.1 Traversing segments and completion units in a request + +The macros bio_for_each_segment() and rq_for_each_bio() should be used for +traversing the bios in the request list (drivers should avoid directly +trying to do it themselves). Using these helpers should also make it easier +to cope with block changes in the future. + + rq_for_each_bio(bio, rq) + bio_for_each_segment(bio_vec, bio, i) + /* bio_vec is now current segment */ + +I/O completion callbacks are per-bio rather than per-segment, so drivers +that traverse bio chains on completion need to keep that in mind. Drivers +which don't make a distinction between segments and completion units would +need to be reorganized to support multi-segment bios. + +3.2.2 Setting up DMA scatterlists + +The blk_rq_map_sg() helper routine would be used for setting up scatter +gather lists from a request, so a driver need not do it on its own. + + nr_segments = blk_rq_map_sg(q, rq, scatterlist); + +The helper routine provides a level of abstraction which makes it easier +to modify the internals of request to scatterlist conversion down the line +without breaking drivers. The blk_rq_map_sg routine takes care of several +things like collapsing physically contiguous segments (if QUEUE_FLAG_CLUSTER +is set) and correct segment accounting to avoid exceeding the limits which +the i/o hardware can handle, based on various queue properties. + +- Prevents a clustered segment from crossing a 4GB mem boundary +- Avoids building segments that would exceed the number of physical + memory segments that the driver can handle (phys_segments) and the + number that the underlying hardware can handle at once, accounting for + DMA remapping (hw_segments) (i.e. IOMMU aware limits). + +Routines which the low level driver can use to set up the segment limits: + +blk_queue_max_hw_segments() : Sets an upper limit of the maximum number of +hw data segments in a request (i.e. the maximum number of address/length +pairs the host adapter can actually hand to the device at once) + +blk_queue_max_phys_segments() : Sets an upper limit on the maximum number +of physical data segments in a request (i.e. the largest sized scatter list +a driver could handle) + +3.2.3 I/O completion + +The existing generic block layer helper routines end_request, +end_that_request_first and end_that_request_last can be used for i/o +completion (and setting things up so the rest of the i/o or the next +request can be kicked of) as before. With the introduction of multi-page +bio support, end_that_request_first requires an additional argument indicating +the number of sectors completed. + +3.2.4 Implications for drivers that do not interpret bios (don't handle + multiple segments) + +Drivers that do not interpret bios e.g those which do not handle multiple +segments and do not support i/o into high memory addresses (require bounce +buffers) and expect only virtually mapped buffers, can access the rq->buffer +field. As before the driver should use current_nr_sectors to determine the +size of remaining data in the current segment (that is the maximum it can +transfer in one go unless it interprets segments), and rely on the block layer +end_request, or end_that_request_first/last to take care of all accounting +and transparent mapping of the next bio segment when a segment boundary +is crossed on completion of a transfer. (The end*request* functions should +be used if only if the request has come down from block/bio path, not for +direct access requests which only specify rq->buffer without a valid rq->bio) + +3.2.5 Generic request command tagging + +3.2.5.1 Tag helpers + +Block now offers some simple generic functionality to help support command +queueing (typically known as tagged command queueing), ie manage more than +one outstanding command on a queue at any given time. + + blk_queue_init_tags(request_queue_t *q, int depth) + + Initialize internal command tagging structures for a maximum + depth of 'depth'. + + blk_queue_free_tags((request_queue_t *q) + + Teardown tag info associated with the queue. This will be done + automatically by block if blk_queue_cleanup() is called on a queue + that is using tagging. + +The above are initialization and exit management, the main helpers during +normal operations are: + + blk_queue_start_tag(request_queue_t *q, struct request *rq) + + Start tagged operation for this request. A free tag number between + 0 and 'depth' is assigned to the request (rq->tag holds this number), + and 'rq' is added to the internal tag management. If the maximum depth + for this queue is already achieved (or if the tag wasn't started for + some other reason), 1 is returned. Otherwise 0 is returned. + + blk_queue_end_tag(request_queue_t *q, struct request *rq) + + End tagged operation on this request. 'rq' is removed from the internal + book keeping structures. + +To minimize struct request and queue overhead, the tag helpers utilize some +of the same request members that are used for normal request queue management. +This means that a request cannot both be an active tag and be on the queue +list at the same time. blk_queue_start_tag() will remove the request, but +the driver must remember to call blk_queue_end_tag() before signalling +completion of the request to the block layer. This means ending tag +operations before calling end_that_request_last()! For an example of a user +of these helpers, see the IDE tagged command queueing support. + +Certain hardware conditions may dictate a need to invalidate the block tag +queue. For instance, on IDE any tagged request error needs to clear both +the hardware and software block queue and enable the driver to sanely restart +all the outstanding requests. There's a third helper to do that: + + blk_queue_invalidate_tags(request_queue_t *q) + + Clear the internal block tag queue and readd all the pending requests + to the request queue. The driver will receive them again on the + next request_fn run, just like it did the first time it encountered + them. + +3.2.5.2 Tag info + +Some block functions exist to query current tag status or to go from a +tag number to the associated request. These are, in no particular order: + + blk_queue_tagged(q) + + Returns 1 if the queue 'q' is using tagging, 0 if not. + + blk_queue_tag_request(q, tag) + + Returns a pointer to the request associated with tag 'tag'. + + blk_queue_tag_depth(q) + + Return current queue depth. + + blk_queue_tag_queue(q) + + Returns 1 if the queue can accept a new queued command, 0 if we are + at the maximum depth already. + + blk_queue_rq_tagged(rq) + + Returns 1 if the request 'rq' is tagged. + +3.2.5.2 Internal structure + +Internally, block manages tags in the blk_queue_tag structure: + + struct blk_queue_tag { + struct request **tag_index; /* array or pointers to rq */ + unsigned long *tag_map; /* bitmap of free tags */ + struct list_head busy_list; /* fifo list of busy tags */ + int busy; /* queue depth */ + int max_depth; /* max queue depth */ + }; + +Most of the above is simple and straight forward, however busy_list may need +a bit of explaining. Normally we don't care too much about request ordering, +but in the event of any barrier requests in the tag queue we need to ensure +that requests are restarted in the order they were queue. This may happen +if the driver needs to use blk_queue_invalidate_tags(). + +Tagging also defines a new request flag, REQ_QUEUED. This is set whenever +a request is currently tagged. You should not use this flag directly, +blk_rq_tagged(rq) is the portable way to do so. + +3.3 I/O Submission + +The routine submit_bio() is used to submit a single io. Higher level i/o +routines make use of this: + +(a) Buffered i/o: +The routine submit_bh() invokes submit_bio() on a bio corresponding to the +bh, allocating the bio if required. ll_rw_block() uses submit_bh() as before. + +(b) Kiobuf i/o (for raw/direct i/o): +The ll_rw_kio() routine breaks up the kiobuf into page sized chunks and +maps the array to one or more multi-page bios, issuing submit_bio() to +perform the i/o on each of these. + +The embedded bh array in the kiobuf structure has been removed and no +preallocation of bios is done for kiobufs. [The intent is to remove the +blocks array as well, but it's currently in there to kludge around direct i/o.] +Thus kiobuf allocation has switched back to using kmalloc rather than vmalloc. + +Todo/Observation: + + A single kiobuf structure is assumed to correspond to a contiguous range + of data, so brw_kiovec() invokes ll_rw_kio for each kiobuf in a kiovec. + So right now it wouldn't work for direct i/o on non-contiguous blocks. + This is to be resolved. The eventual direction is to replace kiobuf + by kvec's. + + Badari Pulavarty has a patch to implement direct i/o correctly using + bio and kvec. + + +(c) Page i/o: +Todo/Under discussion: + + Andrew Morton's multi-page bio patches attempt to issue multi-page + writeouts (and reads) from the page cache, by directly building up + large bios for submission completely bypassing the usage of buffer + heads. This work is still in progress. + + Christoph Hellwig had some code that uses bios for page-io (rather than + bh). This isn't included in bio as yet. Christoph was also working on a + design for representing virtual/real extents as an entity and modifying + some of the address space ops interfaces to utilize this abstraction rather + than buffer_heads. (This is somewhat along the lines of the SGI XFS pagebuf + abstraction, but intended to be as lightweight as possible). + +(d) Direct access i/o: +Direct access requests that do not contain bios would be submitted differently +as discussed earlier in section 1.3. + +Aside: + + Kvec i/o: + + Ben LaHaise's aio code uses a slighly different structure instead + of kiobufs, called a kvec_cb. This contains an array of <page, offset, len> + tuples (very much like the networking code), together with a callback function + and data pointer. This is embedded into a brw_cb structure when passed + to brw_kvec_async(). + + Now it should be possible to directly map these kvecs to a bio. Just as while + cloning, in this case rather than PRE_BUILT bio_vecs, we set the bi_io_vec + array pointer to point to the veclet array in kvecs. + + TBD: In order for this to work, some changes are needed in the way multi-page + bios are handled today. The values of the tuples in such a vector passed in + from higher level code should not be modified by the block layer in the course + of its request processing, since that would make it hard for the higher layer + to continue to use the vector descriptor (kvec) after i/o completes. Instead, + all such transient state should either be maintained in the request structure, + and passed on in some way to the endio completion routine. + + +4. The I/O scheduler +I/O schedulers are now per queue. They should be runtime switchable and modular +but aren't yet. Jens has most bits to do this, but the sysfs implementation is +missing. + +A block layer call to the i/o scheduler follows the convention elv_xxx(). This +calls elevator_xxx_fn in the elevator switch (drivers/block/elevator.c). Oh, +xxx and xxx might not match exactly, but use your imagination. If an elevator +doesn't implement a function, the switch does nothing or some minimal house +keeping work. + +4.1. I/O scheduler API + +The functions an elevator may implement are: (* are mandatory) +elevator_merge_fn called to query requests for merge with a bio + +elevator_merge_req_fn " " " with another request + +elevator_merged_fn called when a request in the scheduler has been + involved in a merge. It is used in the deadline + scheduler for example, to reposition the request + if its sorting order has changed. + +*elevator_next_req_fn returns the next scheduled request, or NULL + if there are none (or none are ready). + +*elevator_add_req_fn called to add a new request into the scheduler + +elevator_queue_empty_fn returns true if the merge queue is empty. + Drivers shouldn't use this, but rather check + if elv_next_request is NULL (without losing the + request if one exists!) + +elevator_remove_req_fn This is called when a driver claims ownership of + the target request - it now belongs to the + driver. It must not be modified or merged. + Drivers must not lose the request! A subsequent + call of elevator_next_req_fn must return the + _next_ request. + +elevator_requeue_req_fn called to add a request to the scheduler. This + is used when the request has alrnadebeen + returned by elv_next_request, but hasn't + completed. If this is not implemented then + elevator_add_req_fn is called instead. + +elevator_former_req_fn +elevator_latter_req_fn These return the request before or after the + one specified in disk sort order. Used by the + block layer to find merge possibilities. + +elevator_completed_req_fn called when a request is completed. This might + come about due to being merged with another or + when the device completes the request. + +elevator_may_queue_fn returns true if the scheduler wants to allow the + current context to queue a new request even if + it is over the queue limit. This must be used + very carefully!! + +elevator_set_req_fn +elevator_put_req_fn Must be used to allocate and free any elevator + specific storate for a request. + +elevator_init_fn +elevator_exit_fn Allocate and free any elevator specific storage + for a queue. + +4.2 I/O scheduler implementation +The generic i/o scheduler algorithm attempts to sort/merge/batch requests for +optimal disk scan and request servicing performance (based on generic +principles and device capabilities), optimized for: +i. improved throughput +ii. improved latency +iii. better utilization of h/w & CPU time + +Characteristics: + +i. Binary tree +AS and deadline i/o schedulers use red black binary trees for disk position +sorting and searching, and a fifo linked list for time-based searching. This +gives good scalability and good availablility of information. Requests are +almost always dispatched in disk sort order, so a cache is kept of the next +request in sort order to prevent binary tree lookups. + +This arrangement is not a generic block layer characteristic however, so +elevators may implement queues as they please. + +ii. Last merge hint +The last merge hint is part of the generic queue layer. I/O schedulers must do +some management on it. For the most part, the most important thing is to make +sure q->last_merge is cleared (set to NULL) when the request on it is no longer +a candidate for merging (for example if it has been sent to the driver). + +The last merge performed is cached as a hint for the subsequent request. If +sequential data is being submitted, the hint is used to perform merges without +any scanning. This is not sufficient when there are multiple processes doing +I/O though, so a "merge hash" is used by some schedulers. + +iii. Merge hash +AS and deadline use a hash table indexed by the last sector of a request. This +enables merging code to quickly look up "back merge" candidates, even when +multiple I/O streams are being performed at once on one disk. + +"Front merges", a new request being merged at the front of an existing request, +are far less common than "back merges" due to the nature of most I/O patterns. +Front merges are handled by the binary trees in AS and deadline schedulers. + +iv. Handling barrier cases +A request with flags REQ_HARDBARRIER or REQ_SOFTBARRIER must not be ordered +around. That is, they must be processed after all older requests, and before +any newer ones. This includes merges! + +In AS and deadline schedulers, barriers have the effect of flushing the reorder +queue. The performance cost of this will vary from nothing to a lot depending +on i/o patterns and device characteristics. Obviously they won't improve +performance, so their use should be kept to a minimum. + +v. Handling insertion position directives +A request may be inserted with a position directive. The directives are one of +ELEVATOR_INSERT_BACK, ELEVATOR_INSERT_FRONT, ELEVATOR_INSERT_SORT. + +ELEVATOR_INSERT_SORT is a general directive for non-barrier requests. +ELEVATOR_INSERT_BACK is used to insert a barrier to the back of the queue. +ELEVATOR_INSERT_FRONT is used to insert a barrier to the front of the queue, and +overrides the ordering requested by any previous barriers. In practice this is +harmless and required, because it is used for SCSI requeueing. This does not +require flushing the reorder queue, so does not impose a performance penalty. + +vi. Plugging the queue to batch requests in anticipation of opportunities for + merge/sort optimizations + +This is just the same as in 2.4 so far, though per-device unplugging +support is anticipated for 2.5. Also with a priority-based i/o scheduler, +such decisions could be based on request priorities. + +Plugging is an approach that the current i/o scheduling algorithm resorts to so +that it collects up enough requests in the queue to be able to take +advantage of the sorting/merging logic in the elevator. If the +queue is empty when a request comes in, then it plugs the request queue +(sort of like plugging the bottom of a vessel to get fluid to build up) +till it fills up with a few more requests, before starting to service +the requests. This provides an opportunity to merge/sort the requests before +passing them down to the device. There are various conditions when the queue is +unplugged (to open up the flow again), either through a scheduled task or +could be on demand. For example wait_on_buffer sets the unplugging going +(by running tq_disk) so the read gets satisfied soon. So in the read case, +the queue gets explicitly unplugged as part of waiting for completion, +in fact all queues get unplugged as a side-effect. + +Aside: + This is kind of controversial territory, as it's not clear if plugging is + always the right thing to do. Devices typically have their own queues, + and allowing a big queue to build up in software, while letting the device be + idle for a while may not always make sense. The trick is to handle the fine + balance between when to plug and when to open up. Also now that we have + multi-page bios being queued in one shot, we may not need to wait to merge + a big request from the broken up pieces coming by. + + Per-queue granularity unplugging (still a Todo) may help reduce some of the + concerns with just a single tq_disk flush approach. Something like + blk_kick_queue() to unplug a specific queue (right away ?) + or optionally, all queues, is in the plan. + +4.3 I/O contexts +I/O contexts provide a dynamically allocated per process data area. They may +be used in I/O schedulers, and in the block layer (could be used for IO statis, +priorities for example). See *io_context in drivers/block/ll_rw_blk.c, and +as-iosched.c for an example of usage in an i/o scheduler. + + +5. Scalability related changes + +5.1 Granular Locking: io_request_lock replaced by a per-queue lock + +The global io_request_lock has been removed as of 2.5, to avoid +the scalability bottleneck it was causing, and has been replaced by more +granular locking. The request queue structure has a pointer to the +lock to be used for that queue. As a result, locking can now be +per-queue, with a provision for sharing a lock across queues if +necessary (e.g the scsi layer sets the queue lock pointers to the +corresponding adapter lock, which results in a per host locking +granularity). The locking semantics are the same, i.e. locking is +still imposed by the block layer, grabbing the lock before +request_fn execution which it means that lots of older drivers +should still be SMP safe. Drivers are free to drop the queue +lock themselves, if required. Drivers that explicitly used the +io_request_lock for serialization need to be modified accordingly. +Usually it's as easy as adding a global lock: + + static spinlock_t my_driver_lock = SPIN_LOCK_UNLOCKED; + +and passing the address to that lock to blk_init_queue(). + +5.2 64 bit sector numbers (sector_t prepares for 64 bit support) + +The sector number used in the bio structure has been changed to sector_t, +which could be defined as 64 bit in preparation for 64 bit sector support. + +6. Other Changes/Implications + +6.1 Partition re-mapping handled by the generic block layer + +In 2.5 some of the gendisk/partition related code has been reorganized. +Now the generic block layer performs partition-remapping early and thus +provides drivers with a sector number relative to whole device, rather than +having to take partition number into account in order to arrive at the true +sector number. The routine blk_partition_remap() is invoked by +generic_make_request even before invoking the queue specific make_request_fn, +so the i/o scheduler also gets to operate on whole disk sector numbers. This +should typically not require changes to block drivers, it just never gets +to invoke its own partition sector offset calculations since all bios +sent are offset from the beginning of the device. + + +7. A Few Tips on Migration of older drivers + +Old-style drivers that just use CURRENT and ignores clustered requests, +may not need much change. The generic layer will automatically handle +clustered requests, multi-page bios, etc for the driver. + +For a low performance driver or hardware that is PIO driven or just doesn't +support scatter-gather changes should be minimal too. + +The following are some points to keep in mind when converting old drivers +to bio. + +Drivers should use elv_next_request to pick up requests and are no longer +supposed to handle looping directly over the request list. +(struct request->queue has been removed) + +Now end_that_request_first takes an additional number_of_sectors argument. +It used to handle always just the first buffer_head in a request, now +it will loop and handle as many sectors (on a bio-segment granularity) +as specified. + +Now bh->b_end_io is replaced by bio->bi_end_io, but most of the time the +right thing to use is bio_endio(bio, uptodate) instead. + +If the driver is dropping the io_request_lock from its request_fn strategy, +then it just needs to replace that with q->queue_lock instead. + +As described in Sec 1.1, drivers can set max sector size, max segment size +etc per queue now. Drivers that used to define their own merge functions i +to handle things like this can now just use the blk_queue_* functions at +blk_init_queue time. + +Drivers no longer have to map a {partition, sector offset} into the +correct absolute location anymore, this is done by the block layer, so +where a driver received a request ala this before: + + rq->rq_dev = mk_kdev(3, 5); /* /dev/hda5 */ + rq->sector = 0; /* first sector on hda5 */ + + it will now see + + rq->rq_dev = mk_kdev(3, 0); /* /dev/hda */ + rq->sector = 123128; /* offset from start of disk */ + +As mentioned, there is no virtual mapping of a bio. For DMA, this is +not a problem as the driver probably never will need a virtual mapping. +Instead it needs a bus mapping (pci_map_page for a single segment or +use blk_rq_map_sg for scatter gather) to be able to ship it to the driver. For +PIO drivers (or drivers that need to revert to PIO transfer once in a +while (IDE for example)), where the CPU is doing the actual data +transfer a virtual mapping is needed. If the driver supports highmem I/O, +(Sec 1.1, (ii) ) it needs to use __bio_kmap_atomic and bio_kmap_irq to +temporarily map a bio into the virtual address space. + + +8. Prior/Related/Impacted patches + +8.1. Earlier kiobuf patches (sct/axboe/chait/hch/mkp) +- orig kiobuf & raw i/o patches (now in 2.4 tree) +- direct kiobuf based i/o to devices (no intermediate bh's) +- page i/o using kiobuf +- kiobuf splitting for lvm (mkp) +- elevator support for kiobuf request merging (axboe) +8.2. Zero-copy networking (Dave Miller) +8.3. SGI XFS - pagebuf patches - use of kiobufs +8.4. Multi-page pioent patch for bio (Christoph Hellwig) +8.5. Direct i/o implementation (Andrea Arcangeli) since 2.4.10-pre11 +8.6. Async i/o implementation patch (Ben LaHaise) +8.7. EVMS layering design (IBM EVMS team) +8.8. Larger page cache size patch (Ben LaHaise) and + Large page size (Daniel Phillips) + => larger contiguous physical memory buffers +8.9. VM reservations patch (Ben LaHaise) +8.10. Write clustering patches ? (Marcelo/Quintela/Riel ?) +8.11. Block device in page cache patch (Andrea Archangeli) - now in 2.4.10+ +8.12. Multiple block-size transfers for faster raw i/o (Shailabh Nagar, + Badari) +8.13 Priority based i/o scheduler - prepatches (Arjan van de Ven) +8.14 IDE Taskfile i/o patch (Andre Hedrick) +8.15 Multi-page writeout and readahead patches (Andrew Morton) +8.16 Direct i/o patches for 2.5 using kvec and bio (Badari Pulavarthy) + +9. Other References: + +9.1 The Splice I/O Model - Larry McVoy (and subsequent discussions on lkml, +and Linus' comments - Jan 2001) +9.2 Discussions about kiobuf and bh design on lkml between sct, linus, alan +et al - Feb-March 2001 (many of the initial thoughts that led to bio were +brought up in this discusion thread) +9.3 Discussions on mempool on lkml - Dec 2001. + |