On Tue, Aug 13, 2019 at 02:27:52PM -0700, Andrew Morton wrote:

On Mon, 12 Aug 2019 15:29:10 -0700 Roman Gushchin guro@fb.com wrote:

...

Percpu caching of local vmstats with the conditional propagation by the cgroup tree leads to an accumulation of errors on non-leaf levels.

Let's imagine two nested memory cgroups A and A/B. Say, a process belonging to A/B allocates 100 pagecache pages on the CPU 0. The percpu cache will spill 3 times, so that 32*3=96 pages will be accounted to A/B and A atomic vmstat counters, 4 pages will remain in the percpu cache.

Imagine A/B is nearby memory.max, so that every following allocation triggers a direct reclaim on the local CPU. Say, each such attempt will free 16 pages on a new cpu. That means every percpu cache will have -16 pages, except the first one, which will have 4 - 16 = -12. A/B and A atomic counters will not be touched at all.

Now a user removes A/B. All percpu caches are freed and corresponding vmstat numbers are forgotten. A has 96 pages more than expected.

As memory cgroups are created and destroyed, errors do accumulate. Even 1-2 pages differences can accumulate into large numbers.

To fix this issue let's accumulate and propagate percpu vmstat values before releasing the memory cgroup. At this point these numbers are stable and cannot be changed.

Since on cpu hotplug we do flush percpu vmstats anyway, we can iterate only over online cpus.

Fixes: 42a300353577 ("mm: memcontrol: fix recursive statistics correctness & scalabilty")

Is this not serious enough for a cc:stable?

I hope the "Fixes" tag will work, but yeah, my bad, cc:stable is definitely a good idea here.

Added stable@ to cc.

Thanks!