linux-kernel - Re: [PATCH] mm: memcontrol: fix memcg accounting during cpu hotplug

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Message-ID: <20250908210900.24088-1-ajgja@amazon.com>
Date: Mon, 8 Sep 2025 21:09:00 +0000
From: Andrew Guerrero <ajgja@...zon.com>
To: <gregkh@...uxfoundation.org>
CC: <ajgja@...zon.com>, <akpm@...ux-foundation.org>,
	<cgroups@...r.kernel.org>, <gunnarku@...zon.com>, <guro@...com>,
	<hannes@...xchg.org>, <roman.gushchin@...ux.dev>,
	<linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>, <mhocko@...nel.org>,
	<shakeel.butt@...ux.dev>, <muchun.song@...ux.dev>, <stable@...r.kernel.org>,
	<vdavydov.dev@...il.com>
Subject: Re: [PATCH] mm: memcontrol: fix memcg accounting during cpu hotplug

On 2025-09-07 13:10 UTC, Greg KH wrote:
> On Sat, Sep 06, 2025 at 03:21:08AM +0000, Andrew Guerrero wrote:
> > This patch is intended for the 5.10 longterm release branch. It will not apply
> > cleanly to mainline and is inadvertantly fixed by a larger series of changes in 
> > later release branches:
> > a3d4c05a4474 ("mm: memcontrol: fix cpuhotplug statistics flushing").
> 
> Why can't we take those instead?
> 
> > In 5.15, the counter flushing code is completely removed. This may be another
> > viable option here too, though it's a larger change.
> 
> If it's not needed anymore, why not just remove it with the upstream
> commits as well?

Yeah, my understanding is the typical flow is to pull commits from upstream into
stable branches. However, I'm not confident I know the the answer to "which
upstream commits?" To get started,

`git log -L :memcg_hotplug_cpu_dead:mm/memcontrol.c linux-5.10.y..linux-5.15.y`

tells me that the upstream changes to pull are:

- https://lore.kernel.org/all/20210209163304.77088-1-hannes@cmpxchg.org/T/#u
- https://lore.kernel.org/all/20210716212137.1391164-1-shakeelb@google.com/T/#u

However, these are substantial features that "fix" the issue indirectly by
transitioning the memcg accounting system over to rstats. I can pick these 10
upstream commits, but I'm worried I may overlook some additional patches from
5.15.y that need to go along with them. I may need some guidance if we go this
route.

Another reasonable option is to take neither route. We can maintain this patch
internally and then drop it once we upgrade to a new kernel version.

Let me know how you would like to proceed.

Thanks!
Andrew