From: Matt Fleming <mfleming@cloudflare.com>

Under memory pressure it's possible for GFP_ATOMIC order-0 allocations
to fail even though free pages are available in the highatomic reserves.
GFP_ATOMIC allocations cannot trigger unreserve_highatomic_pageblock()
since it's only run from reclaim.

Given that such allocations will pass the watermarks in
__zone_watermark_unusable_free(), it makes sense to fallback to
highatomic reserves the same way that ALLOC_OOM can.

This fixes order-0 page allocation failures observed on Cloudflare's
fleet when handling network packets:

  kswapd1: page allocation failure: order:0, mode:0x820(GFP_ATOMIC),
  nodemask=(null),cpuset=/,mems_allowed=0-7
  CPU: 10 PID: 696 Comm: kswapd1 Kdump: loaded Tainted: G           O 6.6.43-CUSTOM #1
  Hardware name: MACHINE
  Call Trace:
   <IRQ>
   dump_stack_lvl+0x3c/0x50
   warn_alloc+0x13a/0x1c0
   __alloc_pages_slowpath.constprop.0+0xc9d/0xd10
   __alloc_pages+0x327/0x340
   __napi_alloc_skb+0x16d/0x1f0
   bnxt_rx_page_skb+0x96/0x1b0 [bnxt_en]
   bnxt_rx_pkt+0x201/0x15e0 [bnxt_en]
   __bnxt_poll_work+0x156/0x2b0 [bnxt_en]
   bnxt_poll+0xd9/0x1c0 [bnxt_en]
   __napi_poll+0x2b/0x1b0
   bpf_trampoline_6442524138+0x7d/0x1000
   __napi_poll+0x5/0x1b0
   net_rx_action+0x342/0x740
   handle_softirqs+0xcf/0x2b0
   irq_exit_rcu+0x6c/0x90
   sysvec_apic_timer_interrupt+0x72/0x90
   </IRQ>

Fixes: 1d91df85f399 ("mm/page_alloc: handle a missing case for memalloc_nocma_{save/restore} APIs")
Suggested-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: linux-mm@kvack.org
Cc: stable@vger.kernel.org # v5.9+
Link: https://lore.kernel.org/all/CAGis_TWzSu=P7QJmjD58WWiu3zjMTVKSzdOwWE8ORaGytzWJwQ@mail.gmail.com/
Signed-off-by: Matt Fleming <mfleming@cloudflare.com>
---
 mm/page_alloc.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

v2: Update comment and add Fixes, Reviewed-by, and Cc: stable tags.

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 8afab64814dc..94a2ffe28008 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2893,12 +2893,12 @@ struct page *rmqueue_buddy(struct zone *preferred_zone, struct zone *zone,
 			page = __rmqueue(zone, order, migratetype, alloc_flags);
 
 			/*
-			 * If the allocation fails, allow OOM handling access
-			 * to HIGHATOMIC reserves as failing now is worse than
-			 * failing a high-order atomic allocation in the
-			 * future.
+			 * If the allocation fails, allow OOM handling and
+			 * order-0 (atomic) allocs access to HIGHATOMIC
+			 * reserves as failing now is worse than failing a
+			 * high-order atomic allocation in the future.
 			 */
-			if (!page && (alloc_flags & ALLOC_OOM))
+			if (!page && (alloc_flags & (ALLOC_OOM|ALLOC_NON_BLOCK)))
 				page = __rmqueue_smallest(zone, order, MIGRATE_HIGHATOMIC);
 
 			if (!page) {
-- 
2.34.1