From: Shakeel Butt <shakeel.butt@linux.dev>
To: Johannes Weiner <hannes@cmpxchg.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Michal Hocko <mhocko@suse.com>,
Roman Gushchin <roman.gushchin@linux.dev>,
Muchun Song <muchun.song@linux.dev>,
Qi Zheng <qi.zheng@linux.dev>,
Meta kernel team <kernel-team@meta.com>,
linux-mm@kvack.org, cgroups@vger.kernel.org,
linux-kernel@vger.kernel.org,
Karl Erik Hofseth <karl.e.hofseth@opoint.com>,
stable@vger.kernel.org
Subject: Re: [PATCH v2] memcg: keep folio's objcg same as its node
Date: Thu, 6 Aug 2026 13:03:30 -0700 [thread overview]
Message-ID: <anToOuVZ-xF6onE6@linux.dev> (raw)
In-Reply-To: <anTfChWlK998-7sY@cmpxchg.org>
On Thu, Aug 06, 2026 at 03:22:50PM -0400, Johannes Weiner wrote:
> Hm, it's still not quite right.
>
> When there is a root mismatch, we cannot fall back to committing the
> old objcg. That would reintroduce Karl's issue. Not as broadly as
> before, but can still happen if the memcg tree died up to the root.
>
> So we have to commit to the new objcg, always.
>
> What obj_cgroup_is_root() then comes down to is whether uncharge will
> balance the page counters or not.
>
> mem_cgroup_replace_folio() gets a new charge for the new page. We can
> just conditionalize that right away on whether the new objcg will
> actually uncharge.
>
> mem_cgroup_migrate() currently trades the charge, but that won't work
> if the new objcg is root and won't uncharge. So we have to settle it
> right then and there.
>
> This?
Thanks a lot Johannes, this looks much better. I will incorporate this and run
through the reproducer before sending out the v3.
prev parent reply other threads:[~2026-08-06 20:03 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-08-06 16:58 [PATCH v2] memcg: keep folio's objcg same as its node Shakeel Butt
2026-08-06 18:15 ` Johannes Weiner
2026-08-06 19:22 ` Johannes Weiner
2026-08-06 20:03 ` Shakeel Butt [this message]
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