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Message-id: <167295562534.13974.9600718707761592893@noble.neil.brown.name>
Date: Fri, 06 Jan 2023 08:53:45 +1100
From: "NeilBrown" <neilb@...e.de>
To: "Mike Rapoport" <rppt@...nel.org>
Cc: "Mel Gorman" <mgorman@...hsingularity.net>,
"Linux-MM" <linux-mm@...ck.org>,
"Andrew Morton" <akpm@...ux-foundation.org>,
"Michal Hocko" <mhocko@...e.com>,
"Thierry Reding" <thierry.reding@...il.com>,
"Matthew Wilcox" <willy@...radead.org>,
"Vlastimil Babka" <vbabka@...e.cz>,
"LKML" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 6/6] mm: discard __GFP_ATOMIC
On Fri, 06 Jan 2023, Mike Rapoport wrote:
> Hi Mel,
>
> On Tue, Nov 29, 2022 at 03:17:01PM +0000, Mel Gorman wrote:
> > From: NeilBrown <neilb@...e.de>
> >
> > __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@noble.neil.brown.name
> > Signed-off-by: NeilBrown <neilb@...e.de>
> > Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>
> > ---
> > 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/
Note that this patch removes __GFP_ATOMIC, but does not change the
behaviour of GFP_ATOMIC. So I don't believe there is any other
documentation that need changing.
Thanks,
NeilBrown
>
> > 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@....com>
> >
> > -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.
>
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