Re: [BUG]: mm/vmscan.c: shrink_slab does not work correctly with memcg disabled via commandline

[Michal Hocko]

Cc few more people

On Thu 01-08-19 15:42:50, Jan Hadrava wrote:
> There seems to be a bug in mm/vmscan.c shrink_slab function when kernel is
> compilled with CONFIG_MEMCG=y and it is then disabled at boot with commandline
> parameter cgroup_disable=memory. SLABs are then not getting shrinked if the
> system memory is consumed by userspace.

This looks similar to http://lkml.kernel.org/r/1563385526-20805-1-git-send-email-yang.shi@xxxxxxxxxxxxxxxxx
although the culprit commit has been identified to be different. Could
you try it out please? Maybe we need more fixes.

keeping the rest of the email for the reference

> This issue is present in linux-stable 4.19 and all newer lines.
>     (tested on git tags v5.3-rc2 v5.2.5 v5.1.21 v4.19.63)
> And it is no not present in 4.14.135 (v4.14.135).
> 
> Git bisect is pointing to commit:
> 	b0dedc49a2daa0f44ddc51fbf686b2ef012fccbf
> 
> Particulary the last hunk seems to be causing it:
> 
> @@ -585,13 +657,7 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
>                         .memcg = memcg,
>                 };
> 
> -               /*
> -                * If kernel memory accounting is disabled, we ignore
> -                * SHRINKER_MEMCG_AWARE flag and call all shrinkers
> -                * passing NULL for memcg.
> -                */
> -               if (memcg_kmem_enabled() &&
> -                   !!memcg != !!(shrinker->flags & SHRINKER_MEMCG_AWARE))
> +               if (!!memcg != !!(shrinker->flags & SHRINKER_MEMCG_AWARE))
>                         continue;
> 
>                 if (!(shrinker->flags & SHRINKER_NUMA_AWARE))
> 
> Following commit aeed1d325d429ac9699c4bf62d17156d60905519
> deletes conditional continue (and so it fixes the problem). But it is creating
> similar issue few lines earlier:
> 
> @@ -644,7 +642,7 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
>         struct shrinker *shrinker;
>         unsigned long freed = 0;
> 
> -       if (memcg && !mem_cgroup_is_root(memcg))
> +       if (!mem_cgroup_is_root(memcg))
>                 return shrink_slab_memcg(gfp_mask, nid, memcg, priority);
> 
>         if (!down_read_trylock(&shrinker_rwsem))
> @@ -657,9 +655,6 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
>                         .memcg = memcg,
>                 };
> 
> -               if (!!memcg != !!(shrinker->flags & SHRINKER_MEMCG_AWARE))
> -                       continue;
> -
>                 if (!(shrinker->flags & SHRINKER_NUMA_AWARE))
>                         sc.nid = 0;
> 
> 
> How was the bisection done:
> 
>  - Compile kernel with x86_64_defconfig + CONFIG_MEMCG=y
>  - Boot VM with cgroup_disable=memory and filesystem with 500k Inodes and run
>    simple script on it:
>    - Observe number of active objects of ext4_inode_cache
>      --> around 400, but anything under 1000 was accepted by the bisect script
> 
>    - Call `find / > /dev/null`
>    - Again observe number of active objects of ext4_inode_cache
>      --> around 7000, but anything over 6000 was accepted by the script
> 
>    - Consume whole memory by simple program `while(1){ malloc(1); }` until it
>      gets killed by oom-killer.
>    - Again observe number of active objects of ext4_inode_cache
>      --> around 7000, script threshold: >= 6000 --> bug is there
>      --> around 100, script threshold <= 1000 --> bug not present
> 
> Real-world appearance:
> 
> We encountered this issue after upgrading kernel from 4.9 to 4.19 on our backup
> server. (Debian Stretch userspace, upgrade to Debian Buster distribution kernel
> or custom build 4.19.60.) The server has around 12 M of used inodes and only
> 4 GB of RAM. Whenever we run the backup, memory gets quickly consumed by kernel
> SLABs (mainly ext4_inode_cache). Userspace starts receiving a lot of hits by
> oom-killer after that so the server is completly unusable until reboot.
> 
> We just removed the cgroup_disable=memory parameter on our server. Memory
> footprint of memcg is significantly smaller then it used to be in the time we
> started using this parameter. But i still think that mentioned behaviour is a
> bug and should be fixed.
> 
> By the way, it seems like the raspberrypi kernel was fighting this issue as well:
> 	https://github.com/raspberrypi/linux/issues/2829
> If I'm reading correctly: they disabled memcg via commandline due to some
> memory leaks. Month later: they hit this issue and reenabled memcg.
> 
> 
> Thanks,
> Jan

-- 
Michal Hocko
SUSE Labs