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, 8 deletions

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)