+ mm-page_counter-mitigate-consequences-of-a-page_counter-underflow.patch added to -mm tree

+ mm-page_counter-mitigate-consequences-of-a-page_counter-underflow.patch added to -mm tree

[akpm]

The patch titled
     Subject: mm: page_counter: mitigate consequences of a page_counter underflow
has been added to the -mm tree.  Its filename is
     mm-page_counter-mitigate-consequences-of-a-page_counter-underflow.patch

This patch should soon appear at
    https://ozlabs.org/~akpm/mmots/broken-out/mm-page_counter-mitigate-consequences-of-a-page_counter-underflow.patch
and later at
    https://ozlabs.org/~akpm/mmotm/broken-out/mm-page_counter-mitigate-consequences-of-a-page_counter-underflow.patch

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------------------------------------------------------
From: Johannes Weiner <hannes@cmpxchg.org>
Subject: mm: page_counter: mitigate consequences of a page_counter underflow

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. ]

Link: https://lkml.kernel.org/r/20210408143155.2679744-1-hannes@cmpxchg.org
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Chris Down <chris@chrisdown.name>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Roman Gushchin <guro@fb.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/page_counter.c |    8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

--- a/mm/page_counter.c~mm-page_counter-mitigate-consequences-of-a-page_counter-underflow
+++ a/mm/page_counter.c
@@ -52,9 +52,13 @@ void page_counter_cancel(struct page_cou
 	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);
 }
 
 /**
_

Patches currently in -mm which might be from hannes@cmpxchg.org are

mm-page-writeback-simplify-memcg-handling-in-test_clear_page_writeback.patch
mm-memcontrol-fix-cpuhotplug-statistics-flushing.patch
mm-memcontrol-kill-mem_cgroup_nodeinfo.patch
mm-memcontrol-privatize-memcg_page_state-query-functions.patch
cgroup-rstat-support-cgroup1.patch
cgroup-rstat-punt-root-level-optimization-to-individual-controllers.patch
mm-memcontrol-switch-to-rstat.patch
mm-memcontrol-switch-to-rstat-fix-2.patch
mm-memcontrol-consolidate-lruvec-stat-flushing.patch
kselftests-cgroup-update-kmem-test-for-new-vmstat-implementation.patch
mm-page_counter-mitigate-consequences-of-a-page_counter-underflow.patch