From: Johannes Weiner <hannes@xxxxxxxxxxx> 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@xxxxxxxxxxx Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx> Acked-by: Michal Hocko <mhocko@xxxxxxxx> Acked-by: Chris Down <chris@xxxxxxxxxxxxxx> Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx> Cc: Hugh Dickins <hughd@xxxxxxxxxx> Cc: Roman Gushchin <guro@xxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- 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); } /** _
