mm: discard __GFP_ATOMIC

__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
Link: https://lkml.kernel.org/r/20230113111217.14134-7-mgorman@techsingularity.net
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 3 insertions, 10 deletions

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 18ca33a1945d..0cfad30fb44c 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4105,13 +4105,14 @@ 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,
@@ -5076,14 +5077,6 @@ __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;