Re: [PATCH 06/10] mm, page_alloc: Rename __GFP_WAIT to __GFP_RECLAIM

[Michal Hocko <mhocko@kernel.org>]

On Tue 29-09-15 14:37:21, Mel Gorman wrote:
[...]
> mm: page_alloc: Hide some GFP internals and document the bits and flag combinations
> 
> Andrew started the following
> 
> 	We have quite a history of remote parts of the kernel using
> 	weird/wrong/inexplicable combinations of __GFP_ flags.	I tend
> 	to think that this is because we didn't adequately explain the
> 	interface.
> 
> 	And I don't think that gfp.h really improved much in this area as
> 	a result of this patchset.  Could you go through it some time and
> 	decide if we've adequately documented all this stuff?
> 
> This patches first moves some GFP flag combinations that are part of the MM
> internals to mm/internal.h. The rest of the patch documents the __GFP_FOO
> bits under various headings and then documents the flag combinations. It
> will not help callers that are brain damaged but the clarity might motivate
> some fixes and avoid future mistakes.
> 
> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>

Yes this looks like a clear improvement.
Acked-by: Michal Hocko <mhocko@suse.com>

> ---
>  include/linux/gfp.h | 252 +++++++++++++++++++++++++++++++++++-----------------
>  mm/internal.h       |  19 ++++
>  mm/shmem.c          |   2 +
>  mm/vmalloc.c        |   2 +
>  4 files changed, 193 insertions(+), 82 deletions(-)
> 
> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> index 369227202ac2..67654f08a28b 100644
> --- a/include/linux/gfp.h
> +++ b/include/linux/gfp.h
> @@ -39,9 +39,7 @@ struct vm_area_struct;
>  /* If the above are modified, __GFP_BITS_SHIFT may need updating */
>  
>  /*
> - * GFP bitmasks..
> - *
> - * Zone modifiers (see linux/mmzone.h - low three bits)
> + * Physical address zone modifiers (see linux/mmzone.h - low four bits)
>   *
>   * Do not put any conditional on these. If necessary modify the definitions
>   * without the underscores and use them consistently. The definitions here may
> @@ -50,121 +48,209 @@ struct vm_area_struct;
>  #define __GFP_DMA	((__force gfp_t)___GFP_DMA)
>  #define __GFP_HIGHMEM	((__force gfp_t)___GFP_HIGHMEM)
>  #define __GFP_DMA32	((__force gfp_t)___GFP_DMA32)
> -#define __GFP_MOVABLE	((__force gfp_t)___GFP_MOVABLE)  /* Page is movable */
> +#define __GFP_MOVABLE	((__force gfp_t)___GFP_MOVABLE)  /* ZONE_MOVABLE allowed */
>  #define GFP_ZONEMASK	(__GFP_DMA|__GFP_HIGHMEM|__GFP_DMA32|__GFP_MOVABLE)
> +
>  /*
> - * Action modifiers - doesn't change the zoning
> + * Page mobility and placement hints
>   *
> - * __GFP_REPEAT: Try hard to allocate the memory, but the allocation attempt
> - * _might_ fail.  This depends upon the particular VM implementation.
> + * 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_NOFAIL: The VM implementation _must_ retry infinitely: the caller
> - * cannot handle allocation failures. 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.
> + * __GFP_MOVABLE (also a zone modifier) indicates that the page can be
> + *   moved by page migration during memory compaction or can be reclaimed.
>   *
> - * __GFP_NORETRY: The VM implementation must not retry indefinitely and will
> - * return NULL when direct reclaim and memory compaction have failed to allow
> - * the allocation to succeed.  The OOM killer is not called with the current
> - * implementation.
> + * __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_MOVABLE: Flag that this page will be movable by the page migration
> - * mechanism or reclaimed
> + * __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.
>   */
> -#define __GFP_ATOMIC	((__force gfp_t)___GFP_ATOMIC)  /* Caller cannot wait or reschedule */
> -#define __GFP_HIGH	((__force gfp_t)___GFP_HIGH)	/* Should access emergency pools? */
> -#define __GFP_IO	((__force gfp_t)___GFP_IO)	/* Can start physical IO? */
> -#define __GFP_FS	((__force gfp_t)___GFP_FS)	/* Can call down to low-level FS? */
> -#define __GFP_COLD	((__force gfp_t)___GFP_COLD)	/* Cache-cold page required */
> -#define __GFP_NOWARN	((__force gfp_t)___GFP_NOWARN)	/* Suppress page allocation failure warning */
> -#define __GFP_REPEAT	((__force gfp_t)___GFP_REPEAT)	/* See above */
> -#define __GFP_NOFAIL	((__force gfp_t)___GFP_NOFAIL)	/* See above */
> -#define __GFP_NORETRY	((__force gfp_t)___GFP_NORETRY) /* See above */
> -#define __GFP_MEMALLOC	((__force gfp_t)___GFP_MEMALLOC)/* Allow access to emergency reserves */
> -#define __GFP_COMP	((__force gfp_t)___GFP_COMP)	/* Add compound page metadata */
> -#define __GFP_ZERO	((__force gfp_t)___GFP_ZERO)	/* Return zeroed page on success */
> -#define __GFP_NOMEMALLOC ((__force gfp_t)___GFP_NOMEMALLOC) /* Don't use emergency reserves.
> -							 * This takes precedence over the
> -							 * __GFP_MEMALLOC flag if both are
> -							 * set
> -							 */
> -#define __GFP_HARDWALL   ((__force gfp_t)___GFP_HARDWALL) /* Enforce hardwall cpuset memory allocs */
> -#define __GFP_THISNODE	((__force gfp_t)___GFP_THISNODE)/* No fallback, no policies */
> -#define __GFP_RECLAIMABLE ((__force gfp_t)___GFP_RECLAIMABLE) /* Page is reclaimable */
> -#define __GFP_NOACCOUNT	((__force gfp_t)___GFP_NOACCOUNT) /* Don't account to kmemcg */
> -#define __GFP_NOTRACK	((__force gfp_t)___GFP_NOTRACK)  /* Don't track with kmemcheck */
> -
> -#define __GFP_OTHER_NODE ((__force gfp_t)___GFP_OTHER_NODE) /* On behalf of other node */
> -#define __GFP_WRITE	((__force gfp_t)___GFP_WRITE)	/* Allocator intends to dirty page */
> +#define __GFP_RECLAIMABLE ((__force gfp_t)___GFP_RECLAIMABLE)
> +#define __GFP_WRITE	((__force gfp_t)___GFP_WRITE)
> +#define __GFP_HARDWALL   ((__force gfp_t)___GFP_HARDWALL)
> +#define __GFP_THISNODE	((__force gfp_t)___GFP_THISNODE)
>  
>  /*
> - * A caller that is willing to wait may enter direct reclaim and will
> - * wake kswapd to reclaim pages in the background until the high
> - * watermark is met. A caller may wish to clear __GFP_DIRECT_RECLAIM to
> - * avoid unnecessary delays when a fallback option is available but
> - * still allow kswapd to reclaim in the background. The kswapd flag
> - * can be cleared when the reclaiming of pages would cause unnecessary
> - * disruption.
> + * 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_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_NOMEMALLOC is used to explicitly forbid access to emergency reserves.
> + *   This takes precedence over the __GFP_MEMALLOC flag if both are set.
> + *
> + * __GFP_NOACCOUNT ignores the accounting for kmemcg limit enforcement.
>   */
> -#define __GFP_RECLAIM ((__force gfp_t)(___GFP_DIRECT_RECLAIM|___GFP_KSWAPD_RECLAIM))
> +#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)
> +#define __GFP_NOACCOUNT	((__force gfp_t)___GFP_NOACCOUNT)
> +
> +/*
> + * Reclaim modifiers
> + *
> + * __GFP_IO can start physical IO.
> + *
> + * __GFP_FS can call down to the low-level FS. 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_KSWAPD_RECLAIM indicates that the caller wants 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_REPEAT: Try hard to allocate the memory, but the allocation attempt
> + *   _might_ fail.  This depends upon the particular VM implementation.
> + *
> + * __GFP_NOFAIL: The VM implementation _must_ retry infinitely: the caller
> + *   cannot handle allocation failures. 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.
> + *
> + * __GFP_NORETRY: The VM implementation must not retry indefinitely and will
> + *   return NULL when direct reclaim and memory compaction have failed to allow
> + *   the allocation to succeed.  The OOM killer is not called with the current
> + *   implementation.
> + */
> +#define __GFP_IO	((__force gfp_t)___GFP_IO)
> +#define __GFP_FS	((__force gfp_t)___GFP_FS)
>  #define __GFP_DIRECT_RECLAIM	((__force gfp_t)___GFP_DIRECT_RECLAIM) /* Caller can reclaim */
>  #define __GFP_KSWAPD_RECLAIM	((__force gfp_t)___GFP_KSWAPD_RECLAIM) /* kswapd can wake */
> +#define __GFP_RECLAIM ((__force gfp_t)(___GFP_DIRECT_RECLAIM|___GFP_KSWAPD_RECLAIM))
> +#define __GFP_REPEAT	((__force gfp_t)___GFP_REPEAT)
> +#define __GFP_NOFAIL	((__force gfp_t)___GFP_NOFAIL)
> +#define __GFP_NORETRY	((__force gfp_t)___GFP_NORETRY)
>  
>  /*
> - * This may seem redundant, but it's a way of annotating false positives vs.
> - * allocations that simply cannot be supported (e.g. page tables).
> + * Action modifiers
> + * 
> + * __GFP_COLD indicates that the caller does not expect to be used in the near
> + *   future. Where possible, a cache-cold page will be returned.
> + *
> + * __GFP_NOWARN suppresses allocation failure reports.
> + *
> + * __GFP_COMP address compound page metadata.
> + *
> + * __GFP_ZERO returns a zeroed page on success.
> + *
> + * __GFP_NOTRACK avoids tracking with kmemcheck.
> + *
> + * __GFP_NOTRACK_FALSE_POSITIVE is an alias of __GFP_NOTRACK. It's a means of
> + *   distinguishing in the source between false positives and allocations that
> + *   cannot be supported (e.g. page tables).
> + *
> + * __GFP_OTHER_NODE is for allocations that are on a remote node but that
> + *   should not be accounted for as a remote allocation in vmstat. A
> + *   typical user would be khugepaged collapsing a huge page on a remote
> + *   node.
>   */
> +#define __GFP_COLD	((__force gfp_t)___GFP_COLD)
> +#define __GFP_NOWARN	((__force gfp_t)___GFP_NOWARN)
> +#define __GFP_COMP	((__force gfp_t)___GFP_COMP)
> +#define __GFP_ZERO	((__force gfp_t)___GFP_ZERO)
> +#define __GFP_NOTRACK	((__force gfp_t)___GFP_NOTRACK)
>  #define __GFP_NOTRACK_FALSE_POSITIVE (__GFP_NOTRACK)
> +#define __GFP_OTHER_NODE ((__force gfp_t)___GFP_OTHER_NODE)
>  
> -#define __GFP_BITS_SHIFT 26	/* Room for N __GFP_FOO bits */
> +/* Room for N __GFP_FOO bits */
> +#define __GFP_BITS_SHIFT 26
>  #define __GFP_BITS_MASK ((__force gfp_t)((1 << __GFP_BITS_SHIFT) - 1))
>  
>  /*
> - * GFP_ATOMIC callers can not sleep, need the allocation to succeed.
> - * A lower watermark is applied to allow access to "atomic reserves"
> + * 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_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.
> + *
> + * GFP_NOFS will use direct reclaim but will not use any filesystem interfaces.
> + *
> + * 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_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_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_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 is used for THP allocations. They are compound allocations
> + *   that will fail quickly if memory is not available and will not wake
> + *   kswapd on failure.
>   */
>  #define GFP_ATOMIC	(__GFP_HIGH|__GFP_ATOMIC|__GFP_KSWAPD_RECLAIM)
> +#define GFP_KERNEL	(__GFP_RECLAIM | __GFP_IO | __GFP_FS)
>  #define GFP_NOWAIT	(__GFP_KSWAPD_RECLAIM)
>  #define GFP_NOIO	(__GFP_RECLAIM)
>  #define GFP_NOFS	(__GFP_RECLAIM | __GFP_IO)
> -#define GFP_KERNEL	(__GFP_RECLAIM | __GFP_IO | __GFP_FS)
>  #define GFP_TEMPORARY	(__GFP_RECLAIM | __GFP_IO | __GFP_FS | \
>  			 __GFP_RECLAIMABLE)
>  #define GFP_USER	(__GFP_RECLAIM | __GFP_IO | __GFP_FS | __GFP_HARDWALL)
> +#define GFP_DMA		__GFP_DMA
> +#define GFP_DMA32	__GFP_DMA32
>  #define GFP_HIGHUSER	(GFP_USER | __GFP_HIGHMEM)
>  #define GFP_HIGHUSER_MOVABLE	(GFP_HIGHUSER | __GFP_MOVABLE)
>  #define GFP_TRANSHUGE	((GFP_HIGHUSER_MOVABLE | __GFP_COMP | \
>  			 __GFP_NOMEMALLOC | __GFP_NORETRY | __GFP_NOWARN) & \
>  			 ~__GFP_KSWAPD_RECLAIM)
>  
> -/* This mask makes up all the page movable related flags */
> +/* Convert GFP flags to their corresponding migrate type */
>  #define GFP_MOVABLE_MASK (__GFP_RECLAIMABLE|__GFP_MOVABLE)
>  #define GFP_MOVABLE_SHIFT 3
> -
> -/* Control page allocator reclaim behavior */
> -#define GFP_RECLAIM_MASK (__GFP_RECLAIM|__GFP_HIGH|__GFP_IO|__GFP_FS|\
> -			__GFP_NOWARN|__GFP_REPEAT|__GFP_NOFAIL|\
> -			__GFP_NORETRY|__GFP_MEMALLOC|__GFP_NOMEMALLOC)
> -
> -/* Control slab gfp mask during early boot */
> -#define GFP_BOOT_MASK (__GFP_BITS_MASK & ~(__GFP_RECLAIM|__GFP_IO|__GFP_FS))
> -
> -/* Control allocation constraints */
> -#define GFP_CONSTRAINT_MASK (__GFP_HARDWALL|__GFP_THISNODE)
> -
> -/* Do not use these with a slab allocator */
> -#define GFP_SLAB_BUG_MASK (__GFP_DMA32|__GFP_HIGHMEM|~__GFP_BITS_MASK)
> -
> -/* Flag - indicates that the buffer will be suitable for DMA.  Ignored on some
> -   platforms, used as appropriate on others */
> -
> -#define GFP_DMA		__GFP_DMA
> -
> -/* 4GB DMA on some platforms */
> -#define GFP_DMA32	__GFP_DMA32
> -
> -/* Convert GFP flags to their corresponding migrate type */
>  static inline int gfpflags_to_migratetype(const gfp_t gfp_flags)
>  {
>  	VM_WARN_ON((gfp_flags & GFP_MOVABLE_MASK) == GFP_MOVABLE_MASK);
> @@ -177,6 +263,8 @@ static inline int gfpflags_to_migratetype(const gfp_t gfp_flags)
>  	/* Group based on mobility */
>  	return (gfp_flags & GFP_MOVABLE_MASK) >> GFP_MOVABLE_SHIFT;
>  }
> +#undef GFP_MOVABLE_MASK
> +#undef GFP_MOVABLE_SHIFT
>  
>  static inline bool gfpflags_allow_blocking(const gfp_t gfp_flags)
>  {
> diff --git a/mm/internal.h b/mm/internal.h
> index 83fb0bfffc13..f99f0ff6935d 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -14,6 +14,25 @@
>  #include <linux/fs.h>
>  #include <linux/mm.h>
>  
> +/*
> + * The set of flags that only affect watermark checking and reclaim
> + * behaviour. This is used by the MM to obey the caller constraints
> + * about IO, FS and watermark checking while ignoring placement
> + * hints such as HIGHMEM usage.
> + */
> +#define GFP_RECLAIM_MASK (__GFP_RECLAIM|__GFP_HIGH|__GFP_IO|__GFP_FS|\
> +			__GFP_NOWARN|__GFP_REPEAT|__GFP_NOFAIL|\
> +			__GFP_NORETRY|__GFP_MEMALLOC|__GFP_NOMEMALLOC)
> +
> +/* The GFP flags allowed during early boot */
> +#define GFP_BOOT_MASK (__GFP_BITS_MASK & ~(__GFP_RECLAIM|__GFP_IO|__GFP_FS))
> +
> +/* Control allocation cpuset and node placement constraints */
> +#define GFP_CONSTRAINT_MASK (__GFP_HARDWALL|__GFP_THISNODE)
> +
> +/* Do not use these with a slab allocator */
> +#define GFP_SLAB_BUG_MASK (__GFP_DMA32|__GFP_HIGHMEM|~__GFP_BITS_MASK)
> +
>  void free_pgtables(struct mmu_gather *tlb, struct vm_area_struct *start_vma,
>  		unsigned long floor, unsigned long ceiling);
>  
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 48ce82926d93..469b639018b0 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -73,6 +73,8 @@ static struct vfsmount *shm_mnt;
>  #include <asm/uaccess.h>
>  #include <asm/pgtable.h>
>  
> +#include "internal.h"
> +
>  #define BLOCKS_PER_PAGE  (PAGE_CACHE_SIZE/512)
>  #define VM_ACCT(size)    (PAGE_CACHE_ALIGN(size) >> PAGE_SHIFT)
>  
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 9ad4dcb0631c..af6d519aa21b 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -35,6 +35,8 @@
>  #include <asm/tlbflush.h>
>  #include <asm/shmparam.h>
>  
> +#include "internal.h"
> +
>  struct vfree_deferred {
>  	struct llist_head list;
>  	struct work_struct wq;

-- 
Michal Hocko
SUSE Labs

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