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Diffstat (limited to 'Documentation/vm/locking')
| -rw-r--r-- | Documentation/vm/locking | 131 |
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diff --git a/Documentation/vm/locking b/Documentation/vm/locking new file mode 100644 index 000000000000..c3ef09ae3bb1 --- /dev/null +++ b/Documentation/vm/locking @@ -0,0 +1,131 @@ +Started Oct 1999 by Kanoj Sarcar <kanojsarcar@yahoo.com> + +The intent of this file is to have an uptodate, running commentary +from different people about how locking and synchronization is done +in the Linux vm code. + +page_table_lock & mmap_sem +-------------------------------------- + +Page stealers pick processes out of the process pool and scan for +the best process to steal pages from. To guarantee the existence +of the victim mm, a mm_count inc and a mmdrop are done in swap_out(). +Page stealers hold kernel_lock to protect against a bunch of races. +The vma list of the victim mm is also scanned by the stealer, +and the page_table_lock is used to preserve list sanity against the +process adding/deleting to the list. This also guarantees existence +of the vma. Vma existence is not guaranteed once try_to_swap_out() +drops the page_table_lock. To guarantee the existence of the underlying +file structure, a get_file is done before the swapout() method is +invoked. The page passed into swapout() is guaranteed not to be reused +for a different purpose because the page reference count due to being +present in the user's pte is not released till after swapout() returns. + +Any code that modifies the vmlist, or the vm_start/vm_end/ +vm_flags:VM_LOCKED/vm_next of any vma *in the list* must prevent +kswapd from looking at the chain. + +The rules are: +1. To scan the vmlist (look but don't touch) you must hold the + mmap_sem with read bias, i.e. down_read(&mm->mmap_sem) +2. To modify the vmlist you need to hold the mmap_sem with + read&write bias, i.e. down_write(&mm->mmap_sem) *AND* + you need to take the page_table_lock. +3. The swapper takes _just_ the page_table_lock, this is done + because the mmap_sem can be an extremely long lived lock + and the swapper just cannot sleep on that. +4. The exception to this rule is expand_stack, which just + takes the read lock and the page_table_lock, this is ok + because it doesn't really modify fields anybody relies on. +5. You must be able to guarantee that while holding page_table_lock + or page_table_lock of mm A, you will not try to get either lock + for mm B. + +The caveats are: +1. find_vma() makes use of, and updates, the mmap_cache pointer hint. +The update of mmap_cache is racy (page stealer can race with other code +that invokes find_vma with mmap_sem held), but that is okay, since it +is a hint. This can be fixed, if desired, by having find_vma grab the +page_table_lock. + + +Code that add/delete elements from the vmlist chain are +1. callers of insert_vm_struct +2. callers of merge_segments +3. callers of avl_remove + +Code that changes vm_start/vm_end/vm_flags:VM_LOCKED of vma's on +the list: +1. expand_stack +2. mprotect +3. mlock +4. mremap + +It is advisable that changes to vm_start/vm_end be protected, although +in some cases it is not really needed. Eg, vm_start is modified by +expand_stack(), it is hard to come up with a destructive scenario without +having the vmlist protection in this case. + +The page_table_lock nests with the inode i_mmap_lock and the kmem cache +c_spinlock spinlocks. This is okay, since the kmem code asks for pages after +dropping c_spinlock. The page_table_lock also nests with pagecache_lock and +pagemap_lru_lock spinlocks, and no code asks for memory with these locks +held. + +The page_table_lock is grabbed while holding the kernel_lock spinning monitor. + +The page_table_lock is a spin lock. + +Note: PTL can also be used to guarantee that no new clones using the +mm start up ... this is a loose form of stability on mm_users. For +example, it is used in copy_mm to protect against a racing tlb_gather_mmu +single address space optimization, so that the zap_page_range (from +vmtruncate) does not lose sending ipi's to cloned threads that might +be spawned underneath it and go to user mode to drag in pte's into tlbs. + +swap_list_lock/swap_device_lock +------------------------------- +The swap devices are chained in priority order from the "swap_list" header. +The "swap_list" is used for the round-robin swaphandle allocation strategy. +The #free swaphandles is maintained in "nr_swap_pages". These two together +are protected by the swap_list_lock. + +The swap_device_lock, which is per swap device, protects the reference +counts on the corresponding swaphandles, maintained in the "swap_map" +array, and the "highest_bit" and "lowest_bit" fields. + +Both of these are spinlocks, and are never acquired from intr level. The +locking hierarchy is swap_list_lock -> swap_device_lock. + +To prevent races between swap space deletion or async readahead swapins +deciding whether a swap handle is being used, ie worthy of being read in +from disk, and an unmap -> swap_free making the handle unused, the swap +delete and readahead code grabs a temp reference on the swaphandle to +prevent warning messages from swap_duplicate <- read_swap_cache_async. + +Swap cache locking +------------------ +Pages are added into the swap cache with kernel_lock held, to make sure +that multiple pages are not being added (and hence lost) by associating +all of them with the same swaphandle. + +Pages are guaranteed not to be removed from the scache if the page is +"shared": ie, other processes hold reference on the page or the associated +swap handle. The only code that does not follow this rule is shrink_mmap, +which deletes pages from the swap cache if no process has a reference on +the page (multiple processes might have references on the corresponding +swap handle though). lookup_swap_cache() races with shrink_mmap, when +establishing a reference on a scache page, so, it must check whether the +page it located is still in the swapcache, or shrink_mmap deleted it. +(This race is due to the fact that shrink_mmap looks at the page ref +count with pagecache_lock, but then drops pagecache_lock before deleting +the page from the scache). + +do_wp_page and do_swap_page have MP races in them while trying to figure +out whether a page is "shared", by looking at the page_count + swap_count. +To preserve the sum of the counts, the page lock _must_ be acquired before +calling is_page_shared (else processes might switch their swap_count refs +to the page count refs, after the page count ref has been snapshotted). + +Swap device deletion code currently breaks all the scache assumptions, +since it grabs neither mmap_sem nor page_table_lock. |