Re: [PATCH] shmem, memcg: enable memcg aware shrinker

[Greg Thelen <gthelen@google.com>]

Yang Shi <shy828301@gmail.com> wrote:

> On Sun, May 31, 2020 at 8:22 PM Greg Thelen <gthelen@google.com> wrote:
>>
>> Since v4.19 commit b0dedc49a2da ("mm/vmscan.c: iterate only over charged
>> shrinkers during memcg shrink_slab()") a memcg aware shrinker is only
>> called when the per-memcg per-node shrinker_map indicates that the
>> shrinker may have objects to release to the memcg and node.
>>
>> shmem_unused_huge_count and shmem_unused_huge_scan support the per-tmpfs
>> shrinker which advertises per memcg and numa awareness.  The shmem
>> shrinker releases memory by splitting hugepages that extend beyond
>> i_size.
>>
>> Shmem does not currently set bits in shrinker_map.  So, starting with
>> b0dedc49a2da, memcg reclaim avoids calling the shmem shrinker under
>> pressure.  This leads to undeserved memcg OOM kills.
>> Example that reliably sees memcg OOM kill in unpatched kernel:
>>   FS=/tmp/fs
>>   CONTAINER=/cgroup/memory/tmpfs_shrinker
>>   mkdir -p $FS
>>   mount -t tmpfs -o huge=always nodev $FS
>>   # Create 1000 MB container, which shouldn't suffer OOM.
>>   mkdir $CONTAINER
>>   echo 1000M > $CONTAINER/memory.limit_in_bytes
>>   echo $BASHPID >> $CONTAINER/cgroup.procs
>>   # Create 4000 files.  Ideally each file uses 4k data page + a little
>>   # metadata.  Assume 8k total per-file, 32MB (4000*8k) should easily
>>   # fit within container's 1000 MB.  But if data pages use 2MB
>>   # hugepages (due to aggressive huge=always) then files consume 8GB,
>>   # which hits memcg 1000 MB limit.
>>   for i in {1..4000}; do
>>     echo . > $FS/$i
>>   done
>
> It looks all the inodes which have tail THP beyond i_size are on one
> single list, then the shrinker actually just splits the first
> nr_to_scan inodes. But since the list is not memcg aware, so it seems
> it may split the THPs which are not charged to the victim memcg and
> the victim memcg still may suffer from pre-mature oom, right?

Correct.  shmem_unused_huge_shrink() is not memcg aware.  In response to
memcg pressure it will split the post-i_size tails of nr_to_scan tmpfs
inodes regardless of if they're charged to the under-pressure memcg.
do_shrink_slab() looks like it'll repeatedly call
shmem_unused_huge_shrink().  So it will split tails of many inodes.  So
I think it'll avoid the oom by over shrinking.  This is not ideal.  But
it seems better than undeserved oom kill.

I think the solution (as Kirill Tkhai suggested) a memcg-aware index
would solve both:
1) avoid premature oom by registering shrinker to responding to memcg
   pressure
2) avoid shrinking/splitting inodes unrelated to the under-pressure
   memcg

I can certainly look into that (thanks Kirill for the pointers).  In the
short term I'm still interested in avoiding premature OOMs with the
original thread (i.e. restore pre-4.19 behavior to shmem shrinker for
memcg pressure).  I plan to test and repost v2.