From: Jakub Kicinski kuba@kernel.org Subject: mm, memcg: do not high throttle allocators based on wraparound

If a cgroup violates its memory.high constraints, we may end up unduly penalising it. For example, for the following hierarchy:

A: max high, 20 usage A/B: 9 high, 10 usage A/C: max high, 10 usage

We would end up doing the following calculation below when calculating high delay for A/B:

A/B: 10 - 9 = 1... A: 20 - PAGE_COUNTER_MAX = 21, so set max_overage to 21.

This gets worse with higher disparities in usage in the parent.

I have no idea how this disappeared from the final version of the patch, but it is certainly Not Good(tm). This wasn't obvious in testing because, for a simple cgroup hierarchy with only one child, the result is usually roughly the same. It's only in more complex hierarchies that things go really awry (although still, the effects are limited to a maximum of 2 seconds in schedule_timeout_killable at a maximum).

[chris@chrisdown.name: changelog] Link: http://lkml.kernel.org/r/20200331152424.GA1019937@chrisdown.name Fixes: e26733e0d0ec ("mm, memcg: throttle allocators based on ancestral memory.high") Signed-off-by: Jakub Kicinski kuba@kernel.org Signed-off-by: Chris Down chris@chrisdown.name Acked-by: Michal Hocko mhocko@suse.com Cc: Johannes Weiner hannes@cmpxchg.org Cc: stable@vger.kernel.org [5.4.x] Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/memcontrol.c | 3 +++ 1 file changed, 3 insertions(+)

--- a/mm/memcontrol.c~mm-memcg-do-not-high-throttle-allocators-based-on-wraparound +++ a/mm/memcontrol.c @@ -2336,6 +2336,9 @@ static unsigned long calculate_high_dela usage = page_counter_read(&memcg->memory); high = READ_ONCE(memcg->high);

+ if (usage <= high) + continue; + /* * Prevent division by 0 in overage calculation by acting as if * it was a threshold of 1 page _