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

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Date: Mon, 6 May 2024 13:30:23 -0700
From: Roman Gushchin <roman.gushchin@...ux.dev>
To: Yosry Ahmed <yosryahmed@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Shakeel Butt <shakeel.butt@...ux.dev>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...nel.org>,
	Muchun Song <muchun.song@...ux.dev>,
	David Hildenbrand <david@...hat.com>, linux-mm@...ck.org,
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
	syzbot+9319a4268a640e26b72b@...kaller.appspotmail.com
Subject: Re: [PATCH v2] mm: do not update memcg stats for
 NR_{FILE/SHMEM}_PMDMAPPED

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@...kaller.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@...ux.dev>
> Signed-off-by: Yosry Ahmed <yosryahmed@...gle.com>

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

Thanks!