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

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Date: Mon, 6 May 2024 21:18:16 +0200
From: David Hildenbrand <david@...hat.com>
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>, Roman Gushchin <roman.gushchin@...ux.dev>,
 Muchun Song <muchun.song@...ux.dev>, linux-mm@...ck.org,
 cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
 syzbot+9319a4268a640e26b72b@...kaller.appspotmail.com
Subject: Re: [PATCH] mm: do not update memcg stats for
 NR_{FILE/SHMEM}_PMDMAPPED

On 06.05.24 20:52, Yosry Ahmed wrote:
> On Mon, May 6, 2024 at 11:35 AM David Hildenbrand <david@...hat.com> wrote:
>>
>> On 06.05.24 19:00, Yosry Ahmed wrote:
>>> Do not use __lruvec_stat_mod_folio() when updating NR_FILE_PMDMAPPED and
>>> NR_SHMEM_PMDMAPPED as these stats are not maintained per-memcg. Use
>>> __mod_node_page_state() instead, which updates the global per-node stats
>>> only.
>>
>> What's the effect of this? IIUC, it's been that way forever, no?
> 
> Yes, but it has been the case that all the NR_VM_EVENT_ITEMS stats
> were maintained per-memcg, although some of those fields are not
> exposed anywhere.
> 
> Shakeel recently added commit14e0f6c957e39 ("memcg: reduce memory for
> the lruvec and memcg stats"), which changed this such that we only
> maintain the stats we actually expose per-memcg (via a translation
> table).

Valuable information we should add to the patch description :)

> 
> He also added commit 514462bbe927b ("memcg: warn for unexpected events
> and stats"), which warns if we try to update a stat per-memcg that we
> do not maintain per-memcg (i.e. the warning firing here). The goal is
> to make sure the translation table has all the stats it needs to have.
> 
> Both of these commits were just merged today into mm-stable, hence the
> need for the fix now. It is the warning working as intended. No Fixes
> or CC stable are needed, but if necessary I would think:

WARN* should usually be "Fixes:"d, because WARN* expresses a condition 
that shouldn't be happening.

Documentation/process/coding-style.rst contains details.

> 
> Fixes: 514462bbe927b ("memcg: warn for unexpected events and stats")
> 
> , because without the warning, the stat update will just be ignored.
> So if anything the warning should have been added *after* this was
> fixed up.

Ideally, yes. But if it's in mm-stable, we usually can no longer 
reshuffle patches (commit IDs stable).

-- 
Cheers,

David / dhildenb