docs/mm: make GFP flags descriptions usable as kernel-doc

This patch adds DOC: headings for GFP flag descriptions and adjusts the
formatting to fit sphinx expectations of paragraphs.

Link: http://lkml.kernel.org/r/1532626360-16650-7-git-send-email-rppt@linux.vnet.ibm.com
Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 154 insertions, 137 deletions

diff --git a/include/linux/gfp.h b/include/linux/gfp.h
index a6afcec53795..24bcc5eec6b4 100644
--- a/include/linux/gfp.h
+++ b/include/linux/gfp.h
@@ -59,29 +59,32 @@ struct vm_area_struct;
 #define __GFP_MOVABLE	((__force gfp_t)___GFP_MOVABLE)  /* ZONE_MOVABLE allowed */
 #define GFP_ZONEMASK	(__GFP_DMA|__GFP_HIGHMEM|__GFP_DMA32|__GFP_MOVABLE)
 
-/*
+/**
+ * DOC: Page mobility and placement hints
+ *
  * Page mobility and placement hints
+ * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  *
  * These flags provide hints about how mobile the page is. Pages with similar
  * mobility are placed within the same pageblocks to minimise problems due
  * to external fragmentation.
  *
- * __GFP_MOVABLE (also a zone modifier) indicates that the page can be
- *   moved by page migration during memory compaction or can be reclaimed.
+ * %__GFP_MOVABLE (also a zone modifier) indicates that the page can be
+ * moved by page migration during memory compaction or can be reclaimed.
  *
- * __GFP_RECLAIMABLE is used for slab allocations that specify
- *   SLAB_RECLAIM_ACCOUNT and whose pages can be freed via shrinkers.
+ * %__GFP_RECLAIMABLE is used for slab allocations that specify
+ * SLAB_RECLAIM_ACCOUNT and whose pages can be freed via shrinkers.
  *
- * __GFP_WRITE indicates the caller intends to dirty the page. Where possible,
- *   these pages will be spread between local zones to avoid all the dirty
- *   pages being in one zone (fair zone allocation policy).
+ * %__GFP_WRITE indicates the caller intends to dirty the page. Where possible,
+ * these pages will be spread between local zones to avoid all the dirty
+ * pages being in one zone (fair zone allocation policy).
  *
- * __GFP_HARDWALL enforces the cpuset memory allocation policy.
+ * %__GFP_HARDWALL enforces the cpuset memory allocation policy.
  *
- * __GFP_THISNODE forces the allocation to be satisified from the requested
- *   node with no fallbacks or placement policy enforcements.
+ * %__GFP_THISNODE forces the allocation to be satisified from the requested
+ * node with no fallbacks or placement policy enforcements.
  *
- * __GFP_ACCOUNT causes the allocation to be accounted to kmemcg.
+ * %__GFP_ACCOUNT causes the allocation to be accounted to kmemcg.
  */
 #define __GFP_RECLAIMABLE ((__force gfp_t)___GFP_RECLAIMABLE)
 #define __GFP_WRITE	((__force gfp_t)___GFP_WRITE)
@@ -89,54 +92,60 @@ struct vm_area_struct;
 #define __GFP_THISNODE	((__force gfp_t)___GFP_THISNODE)
 #define __GFP_ACCOUNT	((__force gfp_t)___GFP_ACCOUNT)
 
-/*
+/**
+ * DOC: Watermark modifiers
+ *
  * Watermark modifiers -- controls access to emergency reserves
+ * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  *
- * __GFP_HIGH indicates that the caller is high-priority and that granting
- *   the request is necessary before the system can make forward progress.
- *   For example, creating an IO context to clean pages.
+ * %__GFP_HIGH indicates that the caller is high-priority and that granting
+ * the request is necessary before the system can make forward progress.
+ * For example, creating an IO context to clean pages.
  *
- * __GFP_ATOMIC indicates that the caller cannot reclaim or sleep and is
- *   high priority. Users are typically interrupt handlers. This may be
- *   used in conjunction with __GFP_HIGH
+ * %__GFP_ATOMIC indicates that the caller cannot reclaim or sleep and is
+ * high priority. Users are typically interrupt handlers. This may be
+ * used in conjunction with %__GFP_HIGH
  *
- * __GFP_MEMALLOC allows access to all memory. This should only be used when
- *   the caller guarantees the allocation will allow more memory to be freed
- *   very shortly e.g. process exiting or swapping. Users either should
- *   be the MM or co-ordinating closely with the VM (e.g. swap over NFS).
+ * %__GFP_MEMALLOC allows access to all memory. This should only be used when
+ * the caller guarantees the allocation will allow more memory to be freed
+ * very shortly e.g. process exiting or swapping. Users either should
+ * be the MM or co-ordinating closely with the VM (e.g. swap over NFS).
  *
- * __GFP_NOMEMALLOC is used to explicitly forbid access to emergency reserves.
- *   This takes precedence over the __GFP_MEMALLOC flag if both are set.
+ * %__GFP_NOMEMALLOC is used to explicitly forbid access to emergency reserves.
+ * This takes precedence over the %__GFP_MEMALLOC flag if both are set.
  */
 #define __GFP_ATOMIC	((__force gfp_t)___GFP_ATOMIC)
 #define __GFP_HIGH	((__force gfp_t)___GFP_HIGH)
 #define __GFP_MEMALLOC	((__force gfp_t)___GFP_MEMALLOC)
 #define __GFP_NOMEMALLOC ((__force gfp_t)___GFP_NOMEMALLOC)
 
-/*
+/**
+ * DOC: Reclaim modifiers
+ *
  * Reclaim modifiers
+ * ~~~~~~~~~~~~~~~~~
  *
- * __GFP_IO can start physical IO.
+ * %__GFP_IO can start physical IO.
  *
- * __GFP_FS can call down to the low-level FS. Clearing the flag avoids the
- *   allocator recursing into the filesystem which might already be holding
- *   locks.
+ * %__GFP_FS can call down to the low-level FS. Clearing the flag avoids the
+ * allocator recursing into the filesystem which might already be holding
+ * locks.
  *
- * __GFP_DIRECT_RECLAIM indicates that the caller may enter direct reclaim.
- *   This flag can be cleared to avoid unnecessary delays when a fallback
- *   option is available.
+ * %__GFP_DIRECT_RECLAIM indicates that the caller may enter direct reclaim.
+ * This flag can be cleared to avoid unnecessary delays when a fallback
+ * option is available.
  *
- * __GFP_KSWAPD_RECLAIM indicates that the caller wants to wake kswapd when
- *   the low watermark is reached and have it reclaim pages until the high
- *   watermark is reached. A caller may wish to clear this flag when fallback
- *   options are available and the reclaim is likely to disrupt the system. The
- *   canonical example is THP allocation where a fallback is cheap but
- *   reclaim/compaction may cause indirect stalls.
+ * %__GFP_KSWAPD_RECLAIM indicates that the caller wants to wake kswapd when
+ * the low watermark is reached and have it reclaim pages until the high
+ * watermark is reached. A caller may wish to clear this flag when fallback
+ * options are available and the reclaim is likely to disrupt the system. The
+ * canonical example is THP allocation where a fallback is cheap but
+ * reclaim/compaction may cause indirect stalls.
  *
- * __GFP_RECLAIM is shorthand to allow/forbid both direct and kswapd reclaim.
+ * %__GFP_RECLAIM is shorthand to allow/forbid both direct and kswapd reclaim.
  *
  * The default allocator behavior depends on the request size. We have a concept
- * of so called costly allocations (with order > PAGE_ALLOC_COSTLY_ORDER).
+ * of so called costly allocations (with order > %PAGE_ALLOC_COSTLY_ORDER).
  * !costly allocations are too essential to fail so they are implicitly
  * non-failing by default (with some exceptions like OOM victims might fail so
  * the caller still has to check for failures) while costly requests try to be
@@ -144,40 +153,40 @@ struct vm_area_struct;
  * The following three modifiers might be used to override some of these
  * implicit rules
  *
- * __GFP_NORETRY: The VM implementation will try only very lightweight
- *   memory direct reclaim to get some memory under memory pressure (thus
- *   it can sleep). It will avoid disruptive actions like OOM killer. The
- *   caller must handle the failure which is quite likely to happen under
- *   heavy memory pressure. The flag is suitable when failure can easily be
- *   handled at small cost, such as reduced throughput
- *
- * __GFP_RETRY_MAYFAIL: The VM implementation will retry memory reclaim
- *   procedures that have previously failed if there is some indication
- *   that progress has been made else where.  It can wait for other
- *   tasks to attempt high level approaches to freeing memory such as
- *   compaction (which removes fragmentation) and page-out.
- *   There is still a definite limit to the number of retries, but it is
- *   a larger limit than with __GFP_NORETRY.
- *   Allocations with this flag may fail, but only when there is
- *   genuinely little unused memory. While these allocations do not
- *   directly trigger the OOM killer, their failure indicates that
- *   the system is likely to need to use the OOM killer soon.  The
- *   caller must handle failure, but can reasonably do so by failing
- *   a higher-level request, or completing it only in a much less
- *   efficient manner.
- *   If the allocation does fail, and the caller is in a position to
- *   free some non-essential memory, doing so could benefit the system
- *   as a whole.
- *
- * __GFP_NOFAIL: The VM implementation _must_ retry infinitely: the caller
- *   cannot handle allocation failures. The allocation could block
- *   indefinitely but will never return with failure. Testing for
- *   failure is pointless.
- *   New users should be evaluated carefully (and the flag should be
- *   used only when there is no reasonable failure policy) but it is
- *   definitely preferable to use the flag rather than opencode endless
- *   loop around allocator.
- *   Using this flag for costly allocations is _highly_ discouraged.
+ * %__GFP_NORETRY: The VM implementation will try only very lightweight
+ * memory direct reclaim to get some memory under memory pressure (thus
+ * it can sleep). It will avoid disruptive actions like OOM killer. The
+ * caller must handle the failure which is quite likely to happen under
+ * heavy memory pressure. The flag is suitable when failure can easily be
+ * handled at small cost, such as reduced throughput
+ *
+ * %__GFP_RETRY_MAYFAIL: The VM implementation will retry memory reclaim
+ * procedures that have previously failed if there is some indication
+ * that progress has been made else where.  It can wait for other
+ * tasks to attempt high level approaches to freeing memory such as
+ * compaction (which removes fragmentation) and page-out.
+ * There is still a definite limit to the number of retries, but it is
+ * a larger limit than with %__GFP_NORETRY.
+ * Allocations with this flag may fail, but only when there is
+ * genuinely little unused memory. While these allocations do not
+ * directly trigger the OOM killer, their failure indicates that
+ * the system is likely to need to use the OOM killer soon.  The
+ * caller must handle failure, but can reasonably do so by failing
+ * a higher-level request, or completing it only in a much less
+ * efficient manner.
+ * If the allocation does fail, and the caller is in a position to
+ * free some non-essential memory, doing so could benefit the system
+ * as a whole.
+ *
+ * %__GFP_NOFAIL: The VM implementation _must_ retry infinitely: the caller
+ * cannot handle allocation failures. The allocation could block
+ * indefinitely but will never return with failure. Testing for
+ * failure is pointless.
+ * New users should be evaluated carefully (and the flag should be
+ * used only when there is no reasonable failure policy) but it is
+ * definitely preferable to use the flag rather than opencode endless
+ * loop around allocator.
+ * Using this flag for costly allocations is _highly_ discouraged.
  */
 #define __GFP_IO	((__force gfp_t)___GFP_IO)
 #define __GFP_FS	((__force gfp_t)___GFP_FS)
@@ -188,14 +197,17 @@ struct vm_area_struct;
 #define __GFP_NOFAIL	((__force gfp_t)___GFP_NOFAIL)
 #define __GFP_NORETRY	((__force gfp_t)___GFP_NORETRY)
 
-/*
+/**
+ * DOC: Action modifiers
+ *
  * Action modifiers
+ * ~~~~~~~~~~~~~~~~
  *
- * __GFP_NOWARN suppresses allocation failure reports.
+ * %__GFP_NOWARN suppresses allocation failure reports.
  *
- * __GFP_COMP address compound page metadata.
+ * %__GFP_COMP address compound page metadata.
  *
- * __GFP_ZERO returns a zeroed page on success.
+ * %__GFP_ZERO returns a zeroed page on success.
  */
 #define __GFP_NOWARN	((__force gfp_t)___GFP_NOWARN)
 #define __GFP_COMP	((__force gfp_t)___GFP_COMP)
@@ -208,66 +220,71 @@ struct vm_area_struct;
 #define __GFP_BITS_SHIFT (23 + IS_ENABLED(CONFIG_LOCKDEP))
 #define __GFP_BITS_MASK ((__force gfp_t)((1 << __GFP_BITS_SHIFT) - 1))
 
-/*
+/**
+ * DOC: Useful GFP flag combinations
+ *
+ * Useful GFP flag combinations
+ * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ *
  * Useful GFP flag combinations that are commonly used. It is recommended
  * that subsystems start with one of these combinations and then set/clear
- * __GFP_FOO flags as necessary.
- *
- * GFP_ATOMIC users can not sleep and need the allocation to succeed. A lower
- *   watermark is applied to allow access to "atomic reserves"
- *
- * GFP_KERNEL is typical for kernel-internal allocations. The caller requires
- *   ZONE_NORMAL or a lower zone for direct access but can direct reclaim.
- *
- * GFP_KERNEL_ACCOUNT is the same as GFP_KERNEL, except the allocation is
- *   accounted to kmemcg.
- *
- * GFP_NOWAIT is for kernel allocations that should not stall for direct
- *   reclaim, start physical IO or use any filesystem callback.
- *
- * GFP_NOIO will use direct reclaim to discard clean pages or slab pages
- *   that do not require the starting of any physical IO.
- *   Please try to avoid using this flag directly and instead use
- *   memalloc_noio_{save,restore} to mark the whole scope which cannot
- *   perform any IO with a short explanation why. All allocation requests
- *   will inherit GFP_NOIO implicitly.
- *
- * GFP_NOFS will use direct reclaim but will not use any filesystem interfaces.
- *   Please try to avoid using this flag directly and instead use
- *   memalloc_nofs_{save,restore} to mark the whole scope which cannot/shouldn't
- *   recurse into the FS layer with a short explanation why. All allocation
- *   requests will inherit GFP_NOFS implicitly.
- *
- * GFP_USER is for userspace allocations that also need to be directly
- *   accessibly by the kernel or hardware. It is typically used by hardware
- *   for buffers that are mapped to userspace (e.g. graphics) that hardware
- *   still must DMA to. cpuset limits are enforced for these allocations.
- *
- * GFP_DMA exists for historical reasons and should be avoided where possible.
- *   The flags indicates that the caller requires that the lowest zone be
- *   used (ZONE_DMA or 16M on x86-64). Ideally, this would be removed but
- *   it would require careful auditing as some users really require it and
- *   others use the flag to avoid lowmem reserves in ZONE_DMA and treat the
- *   lowest zone as a type of emergency reserve.
- *
- * GFP_DMA32 is similar to GFP_DMA except that the caller requires a 32-bit
- *   address.
- *
- * GFP_HIGHUSER is for userspace allocations that may be mapped to userspace,
- *   do not need to be directly accessible by the kernel but that cannot
- *   move once in use. An example may be a hardware allocation that maps
- *   data directly into userspace but has no addressing limitations.
- *
- * GFP_HIGHUSER_MOVABLE is for userspace allocations that the kernel does not
- *   need direct access to but can use kmap() when access is required. They
- *   are expected to be movable via page reclaim or page migration. Typically,
- *   pages on the LRU would also be allocated with GFP_HIGHUSER_MOVABLE.
- *
- * GFP_TRANSHUGE and GFP_TRANSHUGE_LIGHT are used for THP allocations. They are
- *   compound allocations that will generally fail quickly if memory is not
- *   available and will not wake kswapd/kcompactd on failure. The _LIGHT
- *   version does not attempt reclaim/compaction at all and is by default used
- *   in page fault path, while the non-light is used by khugepaged.
+ * %__GFP_FOO flags as necessary.
+ *
+ * %GFP_ATOMIC users can not sleep and need the allocation to succeed. A lower
+ * watermark is applied to allow access to "atomic reserves"
+ *
+ * %GFP_KERNEL is typical for kernel-internal allocations. The caller requires
+ * %ZONE_NORMAL or a lower zone for direct access but can direct reclaim.
+ *
+ * %GFP_KERNEL_ACCOUNT is the same as GFP_KERNEL, except the allocation is
+ * accounted to kmemcg.
+ *
+ * %GFP_NOWAIT is for kernel allocations that should not stall for direct
+ * reclaim, start physical IO or use any filesystem callback.
+ *
+ * %GFP_NOIO will use direct reclaim to discard clean pages or slab pages
+ * that do not require the starting of any physical IO.
+ * Please try to avoid using this flag directly and instead use
+ * memalloc_noio_{save,restore} to mark the whole scope which cannot
+ * perform any IO with a short explanation why. All allocation requests
+ * will inherit GFP_NOIO implicitly.
+ *
+ * %GFP_NOFS will use direct reclaim but will not use any filesystem interfaces.
+ * Please try to avoid using this flag directly and instead use
+ * memalloc_nofs_{save,restore} to mark the whole scope which cannot/shouldn't
+ * recurse into the FS layer with a short explanation why. All allocation
+ * requests will inherit GFP_NOFS implicitly.
+ *
+ * %GFP_USER is for userspace allocations that also need to be directly
+ * accessibly by the kernel or hardware. It is typically used by hardware
+ * for buffers that are mapped to userspace (e.g. graphics) that hardware
+ * still must DMA to. cpuset limits are enforced for these allocations.
+ *
+ * %GFP_DMA exists for historical reasons and should be avoided where possible.
+ * The flags indicates that the caller requires that the lowest zone be
+ * used (%ZONE_DMA or 16M on x86-64). Ideally, this would be removed but
+ * it would require careful auditing as some users really require it and
+ * others use the flag to avoid lowmem reserves in %ZONE_DMA and treat the
+ * lowest zone as a type of emergency reserve.
+ *
+ * %GFP_DMA32 is similar to %GFP_DMA except that the caller requires a 32-bit
+ * address.
+ *
+ * %GFP_HIGHUSER is for userspace allocations that may be mapped to userspace,
+ * do not need to be directly accessible by the kernel but that cannot
+ * move once in use. An example may be a hardware allocation that maps
+ * data directly into userspace but has no addressing limitations.
+ *
+ * %GFP_HIGHUSER_MOVABLE is for userspace allocations that the kernel does not
+ * need direct access to but can use kmap() when access is required. They
+ * are expected to be movable via page reclaim or page migration. Typically,
+ * pages on the LRU would also be allocated with %GFP_HIGHUSER_MOVABLE.
+ *
+ * %GFP_TRANSHUGE and %GFP_TRANSHUGE_LIGHT are used for THP allocations. They
+ * are compound allocations that will generally fail quickly if memory is not
+ * available and will not wake kswapd/kcompactd on failure. The _LIGHT
+ * version does not attempt reclaim/compaction at all and is by default used
+ * in page fault path, while the non-light is used by khugepaged.
  */
 #define GFP_ATOMIC	(__GFP_HIGH|__GFP_ATOMIC|__GFP_KSWAPD_RECLAIM)
 #define GFP_KERNEL	(__GFP_RECLAIM | __GFP_IO | __GFP_FS)