On Sat, Dec 28, 2019 at 09:45:11AM +0800, Yafang Shao wrote:
On Sat, Dec 28, 2019 at 7:49 AM Roman Gushchin guro@fb.com wrote:
On Fri, Dec 27, 2019 at 07:43:53AM -0500, Yafang Shao wrote:
memory.{emin, elow} are set in mem_cgroup_protected(), and the values of them won't be changed until next recalculation in this function. After either or both of them are set, the next reclaimer to relcaim this memcg may be a different reclaimer, e.g. this memcg is also the root memcg of the new reclaimer, and then in mem_cgroup_protection() in get_scan_count() the old values of them will be used to calculate scan count, that is not proper. We should reset them to zero in this case.
Here's an example of this issue.
root_mem_cgroup / A memory.max=1024M memory.min=512M memory.current=800MOnce kswapd is waked up, it will try to scan all MEMCGs, including this A, and it will assign memory.emin of A with 512M. After that, A may reach its hard limit(memory.max), and then it will do memcg reclaim. Because A is the root of this reclaimer, so it will not calculate its memory.emin. So the memory.emin is the old value 512M, and then this old value will be used in mem_cgroup_protection() in get_scan_count() to get the scan count. That is not proper.
Fixes: 9783aa9917f8 ("mm, memcg: proportional memory.{low,min} reclaim") Signed-off-by: Yafang Shao laoar.shao@gmail.com Cc: Chris Down chris@chrisdown.name Cc: Roman Gushchin guro@fb.com Cc: stable@vger.kernel.org
mm/memcontrol.c | 11 ++++++++++- 1 file changed, 10 insertions(+), 1 deletion(-)
diff --git a/mm/memcontrol.c b/mm/memcontrol.c index 601405b..bb3925d 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -6287,8 +6287,17 @@ enum mem_cgroup_protection mem_cgroup_protected(struct mem_cgroup *root,
if (!root) root = root_mem_cgroup;
if (memcg == root)
if (memcg == root) {/** Reset memory.(emin, elow) for reclaiming the memcg* itself.*/if (memcg != root_mem_cgroup) {memcg->memory.emin = 0;memcg->memory.elow = 0;}I'm sorry, that didn't bring it from scratch, but I doubt that zeroing effecting protection is correct. Imagine a simple config: a large cgroup subtree with memory.max set on the top level. Reaching this limit doesn't mean that all protection configuration inside the tree can be ignored.
No, they won't be ignored. Pls. see the logic in mem_cgroup_protected(), it will re-calculate all its children's effective min and low.
Ah, you're right. I forgot about this if (parent == root) goto exit;
which saves elow/emin from being truncated to 0. Sorry.
Please, feel free to add Acked-by: Roman Gushchin guro@fb.com
Thanks!