On Fri, Apr 24, 2020 at 5:06 AM Roman Gushchin guro@fb.com wrote:
On Thu, Apr 23, 2020 at 02:16:29AM -0400, Yafang Shao wrote:
This patch is an improvement of a previous version[1], as the previous version is not easy to understand. This issue persists in the newest kernel, I have to resend the fix. As the implementation is changed, I drop Roman's ack from the previous version.
Here's the explanation of this issue. memory.{low,min} won't take effect if the to-be-reclaimed memcg is the sc->target_mem_cgroup, that can also be proved by the implementation in mem_cgroup_protected(), see bellow, mem_cgroup_protected if (memcg == root) [2] return MEMCG_PROT_NONE;
But this rule is ignored in mem_cgroup_protection(), which will read memory.{emin, elow} as the protection whatever the memcg is.
How would this issue happen? Because in mem_cgroup_protected() we forget to clear the memory.{emin, elow} if the memcg is target_mem_cgroup [2].
An example to illustrate this issue. root_mem_cgroup / A memory.max: 1024M memory.min: 512M memory.current: 800M ('current' must be greater than 'min') Once kswapd starts to reclaim memcg A, it assigns 512M to memory.emin of A. Then kswapd stops. As a result of it, the memory values of A will be, root_mem_cgroup / A memory.max: 1024M memory.min: 512M memory.current: 512M (approximately) memory.emin: 512M
Then a new workload starts to run in memcg A, and it will trigger memcg relcaim in A soon. As memcg A is the target_mem_cgroup of this reclaimer, so it return directly without touching memory.{emin, elow}.[2] The memory values of A will be, root_mem_cgroup / A memory.max: 1024M memory.min: 512M memory.current: 1024M (approximately) memory.emin: 512M Then this memory.emin will be used in mem_cgroup_protection() to get the scan count, which is obvoiusly a wrong scan count.
[1]. https://lore.kernel.org/linux-mm/20200216145249.6900-1-laoar.shao@gmail.com/
Fixes: 9783aa9917f8 ("mm, memcg: proportional memory.{low,min} reclaim") Cc: Chris Down chris@chrisdown.name Cc: Roman Gushchin guro@fb.com Cc: stable@vger.kernel.org Signed-off-by: Yafang Shao laoar.shao@gmail.com
include/linux/memcontrol.h | 13 +++++++++++-- mm/vmscan.c | 4 ++-- 2 files changed, 13 insertions(+), 4 deletions(-)
diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h index d275c72c4f8e..114cfe06bf60 100644 --- a/include/linux/memcontrol.h +++ b/include/linux/memcontrol.h @@ -344,12 +344,20 @@ static inline bool mem_cgroup_disabled(void) return !cgroup_subsys_enabled(memory_cgrp_subsys); }
-static inline unsigned long mem_cgroup_protection(struct mem_cgroup *memcg, +static inline unsigned long mem_cgroup_protection(struct mem_cgroup *root,
struct mem_cgroup *memcg, bool in_low_reclaim)I'd rename "root" to "target", maybe it will make the whole thing more clear.
That would make it better. I will change it.
I'll think a bit more about it, but at the first glance the patch looks sane to me.
Thanks!