From 22f7496f0b901249f23c5251eb8a10aae126b909 Mon Sep 17 00:00:00 2001 From: Yafang Shao Date: Thu, 6 Aug 2020 23:22:01 -0700 Subject: mm, memcg: avoid stale protection values when cgroup is above protection Patch series "mm, memcg: memory.{low,min} reclaim fix & cleanup", v4. This series contains a fix for a edge case in my earlier protection calculation patches, and a patch to make the area overall a little more robust to hopefully help avoid this in future. This patch (of 2): A cgroup can have both memory protection and a memory limit to isolate it from its siblings in both directions - for example, to prevent it from being shrunk below 2G under high pressure from outside, but also from growing beyond 4G under low pressure. Commit 9783aa9917f8 ("mm, memcg: proportional memory.{low,min} reclaim") implemented proportional scan pressure so that multiple siblings in excess of their protection settings don't get reclaimed equally but instead in accordance to their unprotected portion. During limit reclaim, this proportionality shouldn't apply of course: there is no competition, all pressure is from within the cgroup and should be applied as such. Reclaim should operate at full efficiency. However, mem_cgroup_protected() never expected anybody to look at the effective protection values when it indicated that the cgroup is above its protection. As a result, a query during limit reclaim may return stale protection values that were calculated by a previous reclaim cycle in which the cgroup did have siblings. When this happens, reclaim is unnecessarily hesitant and potentially slow to meet the desired limit. In theory this could lead to premature OOM kills, although it's not obvious this has occurred in practice. Workaround the problem by special casing reclaim roots in mem_cgroup_protection. These memcgs are never participating in the reclaim protection because the reclaim is internal. We have to ignore effective protection values for reclaim roots because mem_cgroup_protected might be called from racing reclaim contexts with different roots. Calculation is relying on root -> leaf tree traversal therefore top-down reclaim protection invariants should hold. The only exception is the reclaim root which should have effective protection set to 0 but that would be problematic for the following setup: Let's have global and A's reclaim in parallel: | A (low=2G, usage = 3G, max = 3G, children_low_usage = 1.5G) |\ | C (low = 1G, usage = 2.5G) B (low = 1G, usage = 0.5G) for A reclaim we have B.elow = B.low C.elow = C.low For the global reclaim A.elow = A.low B.elow = min(B.usage, B.low) because children_low_usage <= A.elow C.elow = min(C.usage, C.low) With the effective values resetting we have A reclaim A.elow = 0 B.elow = B.low C.elow = C.low and global reclaim could see the above and then B.elow = C.elow = 0 because children_low_usage > A.elow Which means that protected memcgs would get reclaimed. In future we would like to make mem_cgroup_protected more robust against racing reclaim contexts but that is likely more complex solution than this simple workaround. [hannes@cmpxchg.org - large part of the changelog] [mhocko@suse.com - workaround explanation] [chris@chrisdown.name - retitle] Fixes: 9783aa9917f8 ("mm, memcg: proportional memory.{low,min} reclaim") Signed-off-by: Yafang Shao Signed-off-by: Chris Down Signed-off-by: Andrew Morton Acked-by: Michal Hocko Acked-by: Johannes Weiner Acked-by: Chris Down Acked-by: Roman Gushchin Link: http://lkml.kernel.org/r/cover.1594638158.git.chris@chrisdown.name Link: http://lkml.kernel.org/r/044fb8ecffd001c7905d27c0c2ad998069fdc396.1594638158.git.chris@chrisdown.name Signed-off-by: Linus Torvalds --- mm/memcontrol.c | 8 ++++++++ mm/vmscan.c | 3 ++- 2 files changed, 10 insertions(+), 1 deletion(-) (limited to 'mm') diff --git a/mm/memcontrol.c b/mm/memcontrol.c index b4cb1bb61d46..c610617bb19a 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -6605,6 +6605,14 @@ enum mem_cgroup_protection mem_cgroup_protected(struct mem_cgroup *root, if (!root) root = root_mem_cgroup; + + /* + * Effective values of the reclaim targets are ignored so they + * can be stale. Have a look at mem_cgroup_protection for more + * details. + * TODO: calculation should be more robust so that we do not need + * that special casing. + */ if (memcg == root) return MEMCG_PROT_NONE; diff --git a/mm/vmscan.c b/mm/vmscan.c index 2ac43664aba4..9f0811d24255 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -2331,7 +2331,8 @@ out: unsigned long protection; lruvec_size = lruvec_lru_size(lruvec, lru, sc->reclaim_idx); - protection = mem_cgroup_protection(memcg, + protection = mem_cgroup_protection(sc->target_mem_cgroup, + memcg, sc->memcg_low_reclaim); if (protection) { -- cgit v1.2.3-58-ga151