gfp.h File Reference

List of Get Free Pages (GFP) Flags. More...

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Macros
#define	___GFP_DMA   0x001U
	DMA.
#define	___GFP_HIGHMEM   0x002U
	HIGHMEM.
#define	___GFP_DMA32   0x004U
	DMA32.
#define	___GFP_RECLAIMABLE   0x010U
	RECLAIMABLE.
#define	___GFP_HIGH   0x020U
	HIGH.
#define	___GFP_IO   0x040U
	IO.
#define	___GFP_FS   0x080U
	FS.
#define	___GFP_ZERO   0x100U
	ZERO.
#define	___GFP_ATOMIC   0x200U
	ATOMIC.
#define	___GFP_DIRECT_RECLAIM   0x400U
	DIRECT_RECLAIM.
#define	___GFP_KSWAPD_RECLAIM   0x800U
	KSWAPD_RECLAIM.
#define	__GFP_DMA   ___GFP_DMA
	DMA.
#define	__GFP_HIGHMEM   ___GFP_HIGHMEM
	HIGHMEM.
#define	__GFP_DMA32   ___GFP_DMA32
#define	GFP_ZONEMASK   (__GFP_DMA | __GFP_HIGHMEM | __GFP_DMA32)
	All of the above.
#define	__GFP_ATOMIC   ___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.
#define	__GFP_HIGH   ___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.
#define	__GFP_IO   ___GFP_IO
	Can start physical IO.
#define	__GFP_FS   ___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.
#define	__GFP_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.
#define	__GFP_KSWAPD_RECLAIM   ___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.
#define	__GFP_RECLAIM   (___GFP_DIRECT_RECLAIM | ___GFP_KSWAPD_RECLAIM)
	Is shorthand to allow/forbid both direct and kswapd reclaim.
#define	GFP_ATOMIC   (__GFP_HIGH | __GFP_ATOMIC | __GFP_KSWAPD_RECLAIM)
	Users can not sleep and need the allocation to succeed. A lower watermark is applied to allow access to "atomic reserves".
#define	GFP_KERNEL   (__GFP_RECLAIM | __GFP_IO | __GFP_FS)
	is typical for kernel-internal allocations. The caller requires ZONE_NORMAL or a lower zone for direct access but can direct reclaim.
#define	GFP_NOWAIT   (__GFP_KSWAPD_RECLAIM)
	is for kernel allocations that should not stall for direct reclaim, start physical IO or use any filesystem callback.
#define	GFP_NOIO   (__GFP_RECLAIM)
	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.
#define	GFP_NOFS   (__GFP_RECLAIM | __GFP_IO)
	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.
#define	GFP_USER   (__GFP_RECLAIM | __GFP_IO | __GFP_FS)
	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.
#define	GFP_DMA   (__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.
#define	GFP_HIGHUSER   (GFP_USER | __GFP_HIGHMEM)
	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.

Typedefs
typedef unsigned int	gfp_t
	Type used for GFP_FLAGS.

Detailed Description

List of Get Free Pages (GFP) Flags.

Copyright

(c) 2014-2024 This file is distributed under the MIT License. See LICENSE.md for details.