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

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Message-ID: <2025091216-purveyor-prior-2a81@gregkh>
Date: Fri, 12 Sep 2025 14:45:49 +0200
From: Greg KH <gregkh@...uxfoundation.org>
To: Andrew Guerrero <ajgja@...zon.com>
Cc: 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 Mon, Sep 08, 2025 at 09:09:00PM +0000, Andrew Guerrero wrote:
> 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.

Testing is key :)

> 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.

Perhaps just do that for now if you all are hitting this issue?  It
seems to be the only report I've seen so far.

thanks,

greg k-h