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From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     Michal Hocko <mhocko@suse.com>, Hugh Dickins <hughd@google.com>,
        Shakeel Butt <shakeelb@google.com>,
        Roman Gushchin <guro@fb.com>, linux-mm@kvack.org,
        cgroups@vger.kernel.org, linux-kernel@vger.kernel.org,
        kernel-team@fb.com
Subject: [PATCH] mm: page_counter: mitigate consequences of a page_counter underflow
Date:   Thu,  8 Apr 2021 10:31:55 -0400
Message-Id: <20210408143155.2679744-1-hannes@cmpxchg.org>
MIME-Version: 1.0
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Precedence: bulk

When the unsigned page_counter underflows, even just by a few pages, a
cgroup will not be able to run anything afterwards and trigger the OOM
killer in a loop.

Underflows shouldn't happen, but when they do in practice, we may just
be off by a small amount that doesn't interfere with the normal
operation - consequences don't need to be that dire.

Reset the page_counter to 0 upon underflow. We'll issue a warning that
the accounting will be off and then try to keep limping along.

[ We used to do this with the original res_counter, where it was a
  more straight-forward correction inside the spinlock section. I
  didn't carry it forward into the lockless page counters for
  simplicity, but it turns out this is quite useful in practice. ]

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
 mm/page_counter.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/mm/page_counter.c b/mm/page_counter.c
index c6860f51b6c6..7d83641eb86b 100644
--- a/mm/page_counter.c
+++ b/mm/page_counter.c
@@ -52,9 +52,13 @@ void page_counter_cancel(struct page_counter *counter, unsigned long nr_pages)
 	long new;
 
 	new = atomic_long_sub_return(nr_pages, &counter->usage);
-	propagate_protected_usage(counter, new);
 	/* More uncharges than charges? */
-	WARN_ON_ONCE(new < 0);
+	if (WARN_ONCE(new < 0, "page_counter underflow: %ld nr_pages=%lu\n",
+		      new, nr_pages)) {
+		new = 0;
+		atomic_long_set(&counter->usage, new);
+	}
+	propagate_protected_usage(counter, new);
 }
 
 /**
-- 
2.31.1