Re: [PATCH] MM: discard __GFP_ATOMIC

From: Michal Hocko
Date: Thu Nov 18 2021 - 04:22:46 EST

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[Cc Mel]

On Wed 17-11-21 15:39:30, Neil Brown wrote:
>
> __GFP_ATOMIC serves little purpose.
> It's main effect is to set ALLOC_HARDER which adds a few little boosts to
> increase the chance of an allocation succeeding, one of which is to
> lower the water-mark at which it will succeed.
>
> It is *always* paired with __GFP_HIGH which sets ALLOC_HIGH which also
> adjusts this watermark. It is probable that other users of __GFP_HIGH
> should benefit from the other little bonuses that __GFP_ATOMIC gets.
>
> __GFP_ATOMIC also gives a warning if used with __GFP_DIRECT_RECLAIM.
> There is little point to this. We already get a might_sleep() warning
> if __GFP_DIRECT_RECLAIM is set.
>
> __GFP_ATOMIC allows the "watermark_boost" to be side-stepped. It is
> probable that testing ALLOC_HARDER is a better fit here.
>
> __GFP_ATOMIC is used by tegra-smmu.c to check if the allocation might
> sleep. This should test __GFP_DIRECT_RECLAIM instead.
>
> This patch:
> - removes __GFP_ATOMIC
> - causes __GFP_HIGH to set ALLOC_HARDER unless __GFP_NOMEMALLOC is set
> (as well as ALLOC_HIGH).
> - makes other adjustments as suggested by the above.
>
> The net result is not change to GFP_ATOMIC allocations. Other
> allocations that use __GFP_HIGH will benefit from a few different extra
> privileges. This affects:
> xen, dm, md, ntfs3
> the vermillion frame buffer
> hibernation
> ksm
> swap
> all of which likely produce more benefit than cost if these selected
> allocation are more likely to succeed quickly.
>
> Signed-off-by: NeilBrown <neilb@xxxxxxx>
> ---
>
> This patch came out of my attempts to review and document some of the
> __GFP flags. I couldn't find that __GFP_ATOMIC meant anything really
> useful, so I thought that discarding it would be best.
>
> Obviously GFP_ATOMIC is useful - just not __GFP_ATOMIC (and the fact
> that both names exist could be seen as a warning sign).
>
> Thanks,
> NeilBrown
>
>
> Documentation/vm/balance.rst | 2 +-
> drivers/iommu/tegra-smmu.c | 4 ++--
> include/linux/gfp.h | 12 ++++--------
> include/trace/events/mmflags.h | 1 -
> lib/test_printf.c | 8 ++++----
> mm/internal.h | 3 +--
> mm/memcontrol.c | 2 +-
> mm/page_alloc.c | 16 ++++------------
> tools/perf/builtin-kmem.c | 1 -
> tools/testing/radix-tree/linux/gfp.h | 3 +--
> 10 files changed, 18 insertions(+), 34 deletions(-)
>
> diff --git a/Documentation/vm/balance.rst b/Documentation/vm/balance.rst
> index 6a1fadf3e173..e38e9d83c1c7 100644
> --- a/Documentation/vm/balance.rst
> +++ b/Documentation/vm/balance.rst
> @@ -6,7 +6,7 @@ Memory Balancing
>
> Started Jan 2000 by Kanoj Sarcar <kanoj@xxxxxxx>
>
> -Memory balancing is needed for !__GFP_ATOMIC and !__GFP_KSWAPD_RECLAIM as
> +Memory balancing is needed for !__GFP_HIGH and !__GFP_KSWAPD_RECLAIM as
> well as for non __GFP_IO allocations.
>
> The first reason why a caller may avoid reclaim is that the caller can not
> diff --git a/drivers/iommu/tegra-smmu.c b/drivers/iommu/tegra-smmu.c
> index e900e3c46903..c5fa8b8673b6 100644
> --- a/drivers/iommu/tegra-smmu.c
> +++ b/drivers/iommu/tegra-smmu.c
> @@ -676,12 +676,12 @@ static struct page *as_get_pde_page(struct tegra_smmu_as *as,
> * allocate page in a sleeping context if GFP flags permit. Hence
> * spinlock needs to be unlocked and re-locked after allocation.
> */
> - if (!(gfp & __GFP_ATOMIC))
> + if (gfp & __GFP_DIRECT_RECLAIM)
> spin_unlock_irqrestore(&as->lock, *flags);
>
> page = alloc_page(gfp | __GFP_DMA | __GFP_ZERO);
>
> - if (!(gfp & __GFP_ATOMIC))
> + if (gfp & __GFP_DIRECT_RECLAIM)
> spin_lock_irqsave(&as->lock, *flags);
>
> /*
> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> index b976c4177299..676c813bd93f 100644
> --- a/include/linux/gfp.h
> +++ b/include/linux/gfp.h
> @@ -39,7 +39,7 @@ struct vm_area_struct;
> #define ___GFP_IO 0x40u
> #define ___GFP_FS 0x80u
> #define ___GFP_ZERO 0x100u
> -#define ___GFP_ATOMIC 0x200u
> +/* 0x200u unused */
> #define ___GFP_DIRECT_RECLAIM 0x400u
> #define ___GFP_KSWAPD_RECLAIM 0x800u
> #define ___GFP_WRITE 0x1000u
> @@ -116,11 +116,8 @@ struct vm_area_struct;
> *
> * %__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
> + * For example creating an IO context to clean pages and requests
> + * from atomic context.
> *
> * %__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
> @@ -135,7 +132,6 @@ struct vm_area_struct;
> * %__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)
> @@ -320,7 +316,7 @@ struct vm_area_struct;
> * 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_ATOMIC (__GFP_HIGH|__GFP_KSWAPD_RECLAIM)
> #define GFP_KERNEL (__GFP_RECLAIM | __GFP_IO | __GFP_FS)
> #define GFP_KERNEL_ACCOUNT (GFP_KERNEL | __GFP_ACCOUNT)
> #define GFP_NOWAIT (__GFP_KSWAPD_RECLAIM)
> diff --git a/include/trace/events/mmflags.h b/include/trace/events/mmflags.h
> index 116ed4d5d0f8..30f492256b8c 100644
> --- a/include/trace/events/mmflags.h
> +++ b/include/trace/events/mmflags.h
> @@ -29,7 +29,6 @@
> {(unsigned long)__GFP_HIGHMEM, "__GFP_HIGHMEM"}, \
> {(unsigned long)GFP_DMA32, "GFP_DMA32"}, \
> {(unsigned long)__GFP_HIGH, "__GFP_HIGH"}, \
> - {(unsigned long)__GFP_ATOMIC, "__GFP_ATOMIC"}, \
> {(unsigned long)__GFP_IO, "__GFP_IO"}, \
> {(unsigned long)__GFP_FS, "__GFP_FS"}, \
> {(unsigned long)__GFP_NOWARN, "__GFP_NOWARN"}, \
> diff --git a/lib/test_printf.c b/lib/test_printf.c
> index 07309c45f327..8010de49b6c5 100644
> --- a/lib/test_printf.c
> +++ b/lib/test_printf.c
> @@ -673,17 +673,17 @@ flags(void)
> gfp = GFP_ATOMIC|__GFP_DMA;
> test("GFP_ATOMIC|GFP_DMA", "%pGg", &gfp);
>
> - gfp = __GFP_ATOMIC;
> - test("__GFP_ATOMIC", "%pGg", &gfp);
> + gfp = __GFP_HIGH;
> + test("__GFP_HIGH", "%pGg", &gfp);
>
> /* Any flags not translated by the table should remain numeric */
> gfp = ~__GFP_BITS_MASK;
> snprintf(cmp_buffer, BUF_SIZE, "%#lx", (unsigned long) gfp);
> test(cmp_buffer, "%pGg", &gfp);
>
> - snprintf(cmp_buffer, BUF_SIZE, "__GFP_ATOMIC|%#lx",
> + snprintf(cmp_buffer, BUF_SIZE, "__GFP_HIGH|%#lx",
> (unsigned long) gfp);
> - gfp |= __GFP_ATOMIC;
> + gfp |= __GFP_HIGH;
> test(cmp_buffer, "%pGg", &gfp);
>
> kfree(cmp_buffer);
> diff --git a/mm/internal.h b/mm/internal.h
> index 3b79a5c9427a..4564067065f9 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -20,8 +20,7 @@
> */
> #define GFP_RECLAIM_MASK (__GFP_RECLAIM|__GFP_HIGH|__GFP_IO|__GFP_FS|\
> __GFP_NOWARN|__GFP_RETRY_MAYFAIL|__GFP_NOFAIL|\
> - __GFP_NORETRY|__GFP_MEMALLOC|__GFP_NOMEMALLOC|\
> - __GFP_ATOMIC)
> + __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))
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 781605e92015..1ff55347e4b9 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2621,7 +2621,7 @@ static int try_charge_memcg(struct mem_cgroup *memcg, gfp_t gfp_mask,
> * put the burden of reclaim on regular allocation requests
> * and let these go through as privileged allocations.
> */
> - if (gfp_mask & __GFP_ATOMIC)
> + if (gfp_mask & __GFP_HIGH)
> goto force;
>
> /*
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index c5952749ad40..4fff90ca145b 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3932,12 +3932,12 @@ static inline bool zone_watermark_fast(struct zone *z, unsigned int order,
> free_pages))
> return true;
> /*
> - * Ignore watermark boosting for GFP_ATOMIC order-0 allocations
> + * Ignore watermark boosting for GFP_HIGH order-0 allocations
> * when checking the min watermark. The min watermark is the
> * point where boosting is ignored so that kswapd is woken up
> * when below the low watermark.
> */
> - if (unlikely(!order && (gfp_mask & __GFP_ATOMIC) && z->watermark_boost
> + if (unlikely(!order && (alloc_flags & ALLOC_HARDER) && z->watermark_boost
> && ((alloc_flags & ALLOC_WMARK_MASK) == WMARK_MIN))) {
> mark = z->_watermark[WMARK_MIN];
> return __zone_watermark_ok(z, order, mark, highest_zoneidx,
> @@ -4661,12 +4661,12 @@ gfp_to_alloc_flags(gfp_t gfp_mask)
> * The caller may dip into page reserves a bit more if the caller
> * cannot run direct reclaim, or if the caller has realtime scheduling
> * policy or is asking for __GFP_HIGH memory. GFP_ATOMIC requests will
> - * set both ALLOC_HARDER (__GFP_ATOMIC) and ALLOC_HIGH (__GFP_HIGH).
> + * set both ALLOC_HARDER (unless __GFP_NOMEMALLOC) and ALLOC_HIGH.
> */
> alloc_flags |= (__force int)
> (gfp_mask & (__GFP_HIGH | __GFP_KSWAPD_RECLAIM));
>
> - if (gfp_mask & __GFP_ATOMIC) {
> + if (gfp_mask & __GFP_HIGH) {
> /*
> * Not worth trying to allocate harder for __GFP_NOMEMALLOC even
> * if it can't schedule.
> @@ -4859,14 +4859,6 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> unsigned int cpuset_mems_cookie;
> int reserve_flags;
>
> - /*
> - * We also sanity check to catch abuse of atomic reserves being used by
> - * callers that are not in atomic context.
> - */
> - if (WARN_ON_ONCE((gfp_mask & (__GFP_ATOMIC|__GFP_DIRECT_RECLAIM)) ==
> - (__GFP_ATOMIC|__GFP_DIRECT_RECLAIM)))
> - gfp_mask &= ~__GFP_ATOMIC;
> -
> retry_cpuset:
> compaction_retries = 0;
> no_progress_loops = 0;
> diff --git a/tools/perf/builtin-kmem.c b/tools/perf/builtin-kmem.c
> index da03a341c63c..17e6eeb6e196 100644
> --- a/tools/perf/builtin-kmem.c
> +++ b/tools/perf/builtin-kmem.c
> @@ -640,7 +640,6 @@ static const struct {
> { "__GFP_HIGHMEM", "HM" },
> { "GFP_DMA32", "D32" },
> { "__GFP_HIGH", "H" },
> - { "__GFP_ATOMIC", "_A" },
> { "__GFP_IO", "I" },
> { "__GFP_FS", "F" },
> { "__GFP_NOWARN", "NWR" },
> diff --git a/tools/testing/radix-tree/linux/gfp.h b/tools/testing/radix-tree/linux/gfp.h
> index 32159c08a52e..0a0741104dfe 100644
> --- a/tools/testing/radix-tree/linux/gfp.h
> +++ b/tools/testing/radix-tree/linux/gfp.h
> @@ -12,7 +12,6 @@
> #define __GFP_FS 0x80u
> #define __GFP_NOWARN 0x200u
> #define __GFP_ZERO 0x8000u
> -#define __GFP_ATOMIC 0x80000u
> #define __GFP_ACCOUNT 0x100000u
> #define __GFP_DIRECT_RECLAIM 0x400000u
> #define __GFP_KSWAPD_RECLAIM 0x2000000u
> @@ -20,7 +19,7 @@
> #define __GFP_RECLAIM (__GFP_DIRECT_RECLAIM|__GFP_KSWAPD_RECLAIM)
>
> #define GFP_ZONEMASK 0x0fu
> -#define GFP_ATOMIC (__GFP_HIGH|__GFP_ATOMIC|__GFP_KSWAPD_RECLAIM)
> +#define GFP_ATOMIC (__GFP_HIGH|__GFP_KSWAPD_RECLAIM)
> #define GFP_KERNEL (__GFP_RECLAIM | __GFP_IO | __GFP_FS)
> #define GFP_NOWAIT (__GFP_KSWAPD_RECLAIM)
>
> --
> 2.33.1

--
Michal Hocko
SUSE Labs

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