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

#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

 * %__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

 * %__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,

 * %__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

 * %__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)

#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

 * %__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

 * %__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

 * used in conjunction with %__GFP_HIGH

 *

 * __GFP_MEMALLOC allows access to all memory. This should only be used when

 * %__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

 * %__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.

 * %__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

 * %__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

 * The following three modifiers might be used to override some of these

 * implicit rules

 *

 * __GFP_NORETRY: The VM implementation will try only very lightweight

 * %__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

 * %__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.

 * 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

 * free some non-essential memory, doing so could benefit the system

 * as a whole.

 *

 * __GFP_NOFAIL: The VM implementation _must_ retry infinitely: the caller

 * %__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.

#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)

#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_FOO flags as necessary.

 *

 * GFP_ATOMIC users can not sleep and need the allocation to succeed. A lower

 * %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 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

 * %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

 * %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

 * %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.

 * %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

 * %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.

 * %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

 * 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

 * 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

 * %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,

 * %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

 * %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.

 * 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

 * %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.