[093/163] mm/page_counter.c: fix protection usage propagation
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Message ID 20200807062218.U7OzBph9t%akpm@linux-foundation.org
State New
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Series
  • [001/163] mm/memory.c: avoid access flag update TLB flush for retried page fault
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Commit Message

Andrew Morton Aug. 7, 2020, 6:22 a.m. UTC
From: Michal Koutný <mkoutny@suse.com>
Subject: mm/page_counter.c: fix protection usage propagation

When workload runs in cgroups that aren't directly below root cgroup and
their parent specifies reclaim protection, it may end up ineffective.

The reason is that propagate_protected_usage() is not called in all
hierarchy up.  All the protected usage is incorrectly accumulated in the
workload's parent.  This means that siblings_low_usage is overestimated
and effective protection underestimated.  Even though it is transitional
phenomenon (uncharge path does correct propagation and fixes the wrong
children_low_usage), it can undermine the intended protection
unexpectedly.

We have noticed this problem while seeing a swap out in a descendant of a
protected memcg (intermediate node) while the parent was conveniently
under its protection limit and the memory pressure was external to that
hierarchy.  Michal has pinpointed this down to the wrong
siblings_low_usage which led to the unwanted reclaim.

The fix is simply updating children_low_usage in respective ancestors also
in the charging path.

Link: http://lkml.kernel.org/r/20200803153231.15477-1-mhocko@kernel.org
Fixes: 230671533d64 ("mm: memory.low hierarchical behavior")
Signed-off-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Michal Hocko <mhocko@suse.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Roman Gushchin <guro@fb.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: <stable@vger.kernel.org>	[4.18+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/page_counter.c |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

Patch
diff mbox series

--- a/mm/page_counter.c~mm-fix-protection-usage-propagation
+++ a/mm/page_counter.c
@@ -72,7 +72,7 @@  void page_counter_charge(struct page_cou
 		long new;
 
 		new = atomic_long_add_return(nr_pages, &c->usage);
-		propagate_protected_usage(counter, new);
+		propagate_protected_usage(c, new);
 		/*
 		 * This is indeed racy, but we can live with some
 		 * inaccuracy in the watermark.
@@ -116,7 +116,7 @@  bool page_counter_try_charge(struct page
 		new = atomic_long_add_return(nr_pages, &c->usage);
 		if (new > c->max) {
 			atomic_long_sub(nr_pages, &c->usage);
-			propagate_protected_usage(counter, new);
+			propagate_protected_usage(c, new);
 			/*
 			 * This is racy, but we can live with some
 			 * inaccuracy in the failcnt.
@@ -125,7 +125,7 @@  bool page_counter_try_charge(struct page
 			*fail = c;
 			goto failed;
 		}
-		propagate_protected_usage(counter, new);
+		propagate_protected_usage(c, new);
 		/*
 		 * Just like with failcnt, we can live with some
 		 * inaccuracy in the watermark.