Re: [PATCH v2] mm: do not update memcg stats for NR_{FILE/SHMEM}_PMDMAPPED

[Roman Gushchin <roman.gushchin@linux.dev>]

On Mon, May 06, 2024 at 07:29:24PM +0000, Yosry Ahmed wrote:
> Previously, all NR_VM_EVENT_ITEMS stats were maintained per-memcg,
> although some of those fields are not exposed anywhere. Commit
> 14e0f6c957e39 ("memcg: reduce memory for the lruvec and memcg stats")
> changed this such that we only maintain the stats we actually expose
> per-memcg via a translation table.
> 
> Additionally, commit 514462bbe927b ("memcg: warn for unexpected events
> and stats") added a warning if a per-memcg stat update is attempted for
> a stat that is not in the translation table. The warning started firing
> for the NR_{FILE/SHMEM}_PMDMAPPED stat updates in the rmap code. These
> stats are not maintained per-memcg, and hence are not in the translation
> table.
> 
> Do not use __lruvec_stat_mod_folio() when updating NR_FILE_PMDMAPPED and
> NR_SHMEM_PMDMAPPED. Use __mod_node_page_state() instead, which updates
> the global per-node stats only.
> 
> Reported-by: syzbot+9319a4268a640e26b72b@syzkaller.appspotmail.com
> Closes: https://lore.kernel.org/lkml/0000000000001b9d500617c8b23c@google.com
> Fixes: 514462bbe927 ("memcg: warn for unexpected events and stats")
> Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
> Signed-off-by: Yosry Ahmed <yosryahmed@google.com>

Reviewed-by: Roman Gushchin <roman.gushchin@linux.dev>

Thanks!