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Date:   Thu, 8 Apr 2021 17:18:41 +0100
From:   Chris Down <chris@chrisdown.name>
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        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: Re: [PATCH] mm: page_counter: mitigate consequences of a
 page_counter underflow
Message-ID: <YG8s4X9AB1GoxvYA@chrisdown.name>
References: <20210408143155.2679744-1-hannes@cmpxchg.org>
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Johannes Weiner writes:
>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>

Acked-by: Chris Down <chris@chrisdown.name>

>---
> 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
>
>