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

From: Michal Hocko <[email-masked]>
Date: Wed, 17 Jun 2020 15:59:51 +0200

On Wed 17-06-20 19:07:20, Naresh Kamboju wrote:
> On Thu, 21 May 2020 at 22:04, Michal Hocko <[email-masked]> wrote:
> >
> > On Thu 21-05-20 11:55:16, Michal Hocko wrote:
> > > On Wed 20-05-20 20:09:06, Chris Down wrote:
> > > > Hi Naresh,
> > > >
> > > > Naresh Kamboju writes:
> > > > > As a part of investigation on this issue LKFT teammate Anders Roxell
> > > > > git bisected the problem and found bad commit(s) which caused this problem.
> > > > >
> > > > > The following two patches have been reverted on next-20200519 and retested the
> > > > > reproducible steps and confirmed the test case mkfs -t ext4 got PASS.
> > > > > ( invoked oom-killer is gone now)
> > > > >
> > > > > Revert "mm, memcg: avoid stale protection values when cgroup is above
> > > > > protection"
> > > > >    This reverts commit 23a53e1c02006120f89383270d46cbd040a70bc6.
> > > > >
> > > > > Revert "mm, memcg: decouple e{low,min} state mutations from protection
> > > > > checks"
> > > > >    This reverts commit 7b88906ab7399b58bb088c28befe50bcce076d82.
> > > >
> > > > Thanks Anders and Naresh for tracking this down and reverting.
> > > >
> > > > I'll take a look tomorrow. I don't see anything immediately obviously wrong
> > > > in either of those commits from a (very) cursory glance, but they should
> > > > only be taking effect if protections are set.
> > >
> > > Agreed. If memory.{low,min} is not used then the patch should be
> > > effectively a nop.
> >
> > I was staring into the code and do not see anything.  Could you give the
> > following debugging patch a try and see whether it triggers?
> >
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index cc555903a332..df2e8df0eb71 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -2404,6 +2404,8 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
> >                          * sc->priority further than desirable.
> >                          */
> >                         scan = max(scan, SWAP_CLUSTER_MAX);
> > +
> > +                       trace_printk("scan:%lu protection:%lu\n", scan, protection);
> >                 } else {
> >                         scan = lruvec_size;
> >                 }
> > @@ -2648,6 +2650,7 @@ static void shrink_node_memcgs(pg_data_t *pgdat, struct scan_control *sc)
> >                 mem_cgroup_calculate_protection(target_memcg, memcg);
> >
> >                 if (mem_cgroup_below_min(memcg)) {
> > +                       trace_printk("under min:%lu emin:%lu\n", memcg->memory.min, memcg->memory.emin);
> >                         /*
> >                          * Hard protection.
> >                          * If there is no reclaimable memory, OOM.
> > @@ -2660,6 +2663,7 @@ static void shrink_node_memcgs(pg_data_t *pgdat, struct scan_control *sc)
> >                          * there is an unprotected supply
> >                          * of reclaimable memory from other cgroups.
> >                          */
> > +                       trace_printk("under low:%lu elow:%lu\n", memcg->memory.low, memcg->memory.elow);
> >                         if (!sc->memcg_low_reclaim) {
> >                                 sc->memcg_low_skipped = 1;
> >                                 continue;
> 
> As per your suggestions on debugging this problem,
> trace_printk is replaced with printk and applied to your patch on top of the
> problematic kernel and here is the test output and link.
> 
> 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
> 
> Full test log link,
> https://lkft.validation.linaro.org/scheduler/job/1497412#L1451

Thanks a lot. So it is clear that mem_cgroup_below_min got confused and
reported protected cgroup. Both effective and real limits are 0 so there
is no garbage in them. The problem is in mem_cgroup_below_* and it is
quite obvious.

We are doing the following
+static inline bool mem_cgroup_below_min(struct mem_cgroup *memcg)
+{
+       if (mem_cgroup_disabled())
+               return false;
+
+       return READ_ONCE(memcg->memory.emin) >=
+               page_counter_read(&memcg->memory);
+}

and it makes some sense. Except for the root memcg where we do not
account any memory. Adding if (mem_cgroup_is_root(memcg)) return false;
should do the trick. The same is the case for mem_cgroup_below_low.
Could you give it a try please just to confirm?
-- 
Michal Hocko
SUSE Labs