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.orgSigned-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>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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);
 }
 
 /**