Re: mm: mkfs.ext4 invoked oom-killer on i386 - pagecache_get_page

Naresh Kamboju
2020-06-17T15:53:05Z

On Wed, 17 Jun 2020 at 19:41, Michal Hocko <mhocko@kernel.org> wrote:
>
> [Our emails have crossed]
>
> On Wed 17-06-20 14:57:58, Chris Down wrote:
> > Naresh Kamboju writes:
> > > mkfs -t ext4 /dev/disk/by-id/ata-TOSHIBA_MG04ACA100N_Y8RQK14KF6XF
> > > mke2fs 1.43.8 (1-Jan-2018)
> > > Creating filesystem with 244190646 4k blocks and 61054976 inodes
> > > Filesystem UUID: 7c380766-0ed8-41ba-a0de-3c08e78f1891
> > > Superblock backups stored on blocks:
> > > 32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632, 2654208,
> > > 4096000, 7962624, 11239424, 20480000, 23887872, 71663616, 78675968,
> > > 102400000, 214990848
> > > Allocating group tables: 0/7453 done
> > > Writing inode tables: 0/7453 done
> > > Creating journal (262144 blocks): [ 51.544525] under min:0 emin:0
> > > [ 51.845304] under min:0 emin:0
> > > [ 51.848738] under min:0 emin:0
> > > [ 51.858147] under min:0 emin:0
> > > [ 51.861333] under min:0 emin:0
> > > [ 51.862034] under min:0 emin:0
> > > [ 51.862442] under min:0 emin:0
> > > [ 51.862763] under min:0 emin:0
> >
> > Thanks, this helps a lot. Somehow we're entering mem_cgroup_below_min even
> > when min/emin is 0 (which should indeed be the case if you haven't set them
> > in the hierarchy).
> >
> > My guess is that page_counter_read(&memcg->memory) is 0, which means
> > mem_cgroup_below_min will return 1.
>
> Yes this is the case because this is likely the root memcg which skips
> all charges.
>
> > However, I don't know for sure why that should then result in the OOM killer
> > coming along. My guess is that since this memcg has 0 pages to scan anyway,
> > we enter premature OOM under some conditions. I don't know why we wouldn't
> > have hit that with the old version of mem_cgroup_protected that returned
> > MEMCG_PROT_* members, though.
>
> Not really. There is likely no other memcg to reclaim from and assuming
> min limit protection will result in no reclaimable memory and thus the
> OOM killer.
>
> > Can you please try the patch with the `>=` checks in mem_cgroup_below_min
> > and mem_cgroup_below_low changed to `>`? If that fixes it, then that gives a
> > strong hint about what's going on here.
>
> This would work but I believe an explicit check for the root memcg would
> be easier to spot the reasoning.

May I request you to send debugging or proposed fix patches here.
I am happy to do more testing.

FYI,
Here is my repository for testing.
git: https://github.com/nareshkamboju/linux/tree/printk
branch: printk

- Naresh