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, 4 insertions, 4 deletions

diff --git a/lib/test_printf.c b/lib/test_printf.c
index d34dc636b81c..46b4e6c414a3 100644
--- a/lib/test_printf.c
+++ b/lib/test_printf.c
@@ -674,17 +674,17 @@ 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);