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

["NeilBrown" <neilb@suse.de>]

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@suse.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@suse.de>
> > Signed-off-by: Mel Gorman <mgorman@techsingularity.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@sgi.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.
>