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Commit 263fade5 authored by Mike Rapoport's avatar Mike Rapoport Committed by Linus Torvalds
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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.comSigned-off-by: default avatarMike Rapoport <rppt@linux.vnet.ibm.com>
Reviewed-by: default avatarAndrew 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: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 41f35b39
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Showing with 154 additions and 137 deletions
include/linux/gfp.h
View file @ 263fade5
......@@ -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)
......
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