[patch] mm, oom: stop reclaiming if GFP_ATOMIC will start failing soon

[David Rientjes]

If GFP_ATOMIC allocations will start failing soon because the amount of
free memory is substantially under per-zone min watermarks, it is better
to oom kill a process rather than continue to reclaim.

This intends to significantly reduce the number of page allocation
failures that are encountered when the demands of user and atomic
allocations overwhelm the ability of reclaim to keep up.  We can see this
with a high ingress of networking traffic where memory allocated in irq
context can overwhelm the ability to reclaim fast enough such that user
memory consistently loops.  In that case, we have reclaimable memory, and
reclaiming is successful, but we've fully depleted memory reserves that
are allowed for non-blockable allocations.

Commit 400e22499dd9 ("mm: don't warn about allocations which stall for
too long") removed evidence of user allocations stalling because of this,
but the situation can apply anytime we get "page allocation failures"
where reclaim is happening but per-zone min watermarks are starved:

Node 0 Normal free:87356kB min:221984kB low:416984kB high:611984kB active_anon:123009936kB inactive_anon:67647652kB active_file:429612kB inactive_file:209980kB unevictable:112348kB writepending:260kB present:198180864kB managed:195027624kB mlocked:81756kB kernel_stack:24040kB pagetables:11460kB bounce:0kB free_pcp:940kB local_pcp:96kB free_cma:0kB
lowmem_reserve[]: 0 0 0 0
Node 1 Normal free:105616kB min:225568kB low:423716kB high:621864kB active_anon:122124196kB inactive_anon:74112696kB active_file:39172kB inactive_file:103696kB unevictable:204480kB writepending:180kB present:201326592kB managed:198174372kB mlocked:204480kB kernel_stack:11328kB pagetables:3680kB bounce:0kB free_pcp:1140kB local_pcp:0kB free_cma:0kB
lowmem_reserve[]: 0 0 0 0

Without this patch, there is no guarantee that user memory allocations
will ever be successful when non-blockable allocations overwhelm the
ability to get above per-zone min watermarks.

This doesn't solve page allocation failures entirely since it's a
preemptive measure based on watermarks that requires concurrent blockable
allocations to trigger the oom kill.  To complete solve page allocation
failures, it would be possible to do the same watermark check for non-
blockable allocations and then queue a worker to asynchronously oom kill
if it finds watermarks to be sufficiently low as well.

Signed-off-by: David Rientjes <rientjes@xxxxxxxxxx>
---
 mm/page_alloc.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4372,11 +4372,21 @@ should_reclaim_retry(gfp_t gfp_mask, unsigned order,
 					ac->nodemask) {
 		unsigned long available;
 		unsigned long reclaimable;
+		unsigned long free;
 		unsigned long min_wmark = min_wmark_pages(zone);
 		bool wmark;
 
+		free = zone_page_state_snapshot(zone, NR_FREE_PAGES);
+		/*
+		 * If this zone is approaching the point where even order-0
+		 * GFP_ATOMIC allocations will fail, stop considering reclaim.
+		 */
+		if (!__zone_watermark_ok(zone, 0, min_wmark, ac_classzone_idx(ac),
+					 alloc_flags | ALLOC_HIGH, free))
+			continue;
+
 		available = reclaimable = zone_reclaimable_pages(zone);
-		available += zone_page_state_snapshot(zone, NR_FREE_PAGES);
+		available += free;
 
 		/*
 		 * Would the allocation succeed if we reclaimed all