Re: [PATCH 6/6] mm: discard __GFP_ATOMIC

From: Mike Rapoport
Date: Thu Jan 05 2023 - 08:54:22 EST

Hi Mel,

On Tue, Nov 29, 2022 at 03:17:01PM +0000, Mel Gorman wrote:
> From: NeilBrown <neilb@xxxxxxx>
>
> __GFP_ATOMIC serves little purpose. Its 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
> - allows __GFP_HIGH allocations to ignore watermark boosting as well
> as GFP_ATOMIC requests.
> - 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.
>
> [mgorman: Minor adjustments to rework on top of a series]
> Link: https://lkml.kernel.org/r/163712397076.13692.4727608274002939094@xxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: NeilBrown <neilb@xxxxxxx>
> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> ---
> Documentation/mm/balance.rst | 2 +-

Documentation/core-api/memory-allocation.rst needs an update as well, and
there are other mentions of GFP_ATOMIC in Documentation/

> drivers/iommu/tegra-smmu.c | 4 ++--
> include/linux/gfp_types.h | 12 ++++--------
> include/trace/events/mmflags.h | 1 -
> lib/test_printf.c | 8 ++++----
> mm/internal.h | 2 +-
> mm/page_alloc.c | 13 +++----------
> tools/perf/builtin-kmem.c | 1 -
> 8 files changed, 15 insertions(+), 28 deletions(-)
>
> diff --git a/Documentation/mm/balance.rst b/Documentation/mm/balance.rst
> index 6a1fadf3e173..e38e9d83c1c7 100644
> --- a/Documentation/mm/balance.rst
> +++ b/Documentation/mm/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 5b1af40221ec..af8d0e685260 100644
> --- a/drivers/iommu/tegra-smmu.c
> +++ b/drivers/iommu/tegra-smmu.c
> @@ -671,12 +671,12 @@ 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 (gfpflags_allow_blocking(gfp))
> spin_unlock_irqrestore(&as->lock, *flags);
>
> page = alloc_page(gfp | __GFP_DMA | __GFP_ZERO);
>
> - if (!(gfp & __GFP_ATOMIC))
> + if (gfpflags_allow_blocking(gfp))
> spin_lock_irqsave(&as->lock, *flags);
>
> /*
> diff --git a/include/linux/gfp_types.h b/include/linux/gfp_types.h
> index d88c46ca82e1..5088637fe5c2 100644
> --- a/include/linux/gfp_types.h
> +++ b/include/linux/gfp_types.h
> @@ -31,7 +31,7 @@ typedef unsigned int __bitwise gfp_t;
> #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 @@ typedef unsigned int __bitwise gfp_t;
> *
> * %__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 @@ typedef unsigned int __bitwise gfp_t;
> * %__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)
> @@ -329,7 +325,7 @@ typedef unsigned int __bitwise gfp_t;
> * 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 e87cb2b80ed3..11524cda4a95 100644
> --- a/include/trace/events/mmflags.h
> +++ b/include/trace/events/mmflags.h
> @@ -31,7 +31,6 @@
> gfpflag_string(__GFP_HIGHMEM), \
> gfpflag_string(GFP_DMA32), \
> gfpflag_string(__GFP_HIGH), \
> - gfpflag_string(__GFP_ATOMIC), \
> gfpflag_string(__GFP_IO), \
> gfpflag_string(__GFP_FS), \
> gfpflag_string(__GFP_NOWARN), \
> diff --git a/lib/test_printf.c b/lib/test_printf.c
> index 4bd15a593fbd..fe13de1bed5f 100644
> --- a/lib/test_printf.c
> +++ b/lib/test_printf.c
> @@ -686,17 +686,17 @@ 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 98b1e526559d..48926b290cd5 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -24,7 +24,7 @@ struct folio_batch;
> #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_NOLOCKDEP)
> + __GFP_NOLOCKDEP)
>
> /* The GFP flags allowed during early boot */
> #define GFP_BOOT_MASK (__GFP_BITS_MASK & ~(__GFP_RECLAIM|__GFP_IO|__GFP_FS))
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 6bee987ec9a3..ad6c4705a79d 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -4081,13 +4081,14 @@ static inline bool zone_watermark_fast(struct zone *z, unsigned int order,
> if (__zone_watermark_ok(z, order, mark, highest_zoneidx, alloc_flags,
> 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_MIN_RESERVE) && z->watermark_boost
> && ((alloc_flags & ALLOC_WMARK_MASK) == WMARK_MIN))) {
> mark = z->_watermark[WMARK_MIN];
> return __zone_watermark_ok(z, order, mark, highest_zoneidx,
> @@ -5052,14 +5053,6 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> unsigned int zonelist_iter_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;
> -
> restart:
> compaction_retries = 0;
> no_progress_loops = 0;
> diff --git a/tools/perf/builtin-kmem.c b/tools/perf/builtin-kmem.c
> index ebfab2ca1702..4a06d83f2ac5 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" },
> --
> 2.35.3
>
>

--
Sincerely yours,
Mike.