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@xxxxxxxxxxx>
---
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
