linux-kernel - Re: [PATCH 1/4] memcg: use mod_node_page

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Message-ID: <aYtbevHEwx_3fn0Q@linux.dev>
Date: Tue, 10 Feb 2026 08:29:58 -0800
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Dev Jain <dev.jain@....com>
Cc: Harry Yoo <harry.yoo@...cle.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, Johannes Weiner <hannes@...xchg.org>, 
	Michal Hocko <mhocko@...nel.org>, Roman Gushchin <roman.gushchin@...ux.dev>, 
	Muchun Song <muchun.song@...ux.dev>, Qi Zheng <qi.zheng@...ux.dev>, Vlastimil Babka <vbabka@...e.cz>, 
	linux-mm@...ck.org, cgroups@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Meta kernel team <kernel-team@...a.com>
Subject: Re: [PATCH 1/4] memcg: use mod_node_page_state to update stats

On Tue, Feb 10, 2026 at 01:08:49PM +0530, Dev Jain wrote:
[...]
> 
> >>
> > Oh so it is arm64 specific issue. I tested on x86-64 machine and it solves
> > the little regression it had before. So, on arm64 all this_cpu_ops i.e. without
> > double underscore, uses LL/SC instructions. 
> >
> > Need more thought on this. 
> >
> >>> Also can you confirm whether my analysis of the regression was correct?
> >>>  Because if it was, then this diff looks wrong - AFAIU preempt_disable()
> >>>  won't stop an irq handler from interrupting the execution, so this
> >>>  will introduce a bug for code paths running in irq context.
> >>>
> >> I was worried about the correctness too, but this_cpu_add() is safe
> >> against IRQs and so the stat will be _eventually_ consistent?
> >>
> >> Ofc it's so confusing! Maybe I'm the one confused.
> > Yeah there is no issue with proposed patch as it is making the function
> > re-entrant safe.
> 
> Ah yes, this_cpu_add() does the addition in one shot without read-modify-write.
> 
> I am still puzzled whether the original patch was a bug fix or an optimization.

The original patch was a cleanup patch. The memcg stats update functions
were already irq/nmi safe without disabling irqs and that patch did the
same for the numa stats. Though it seems like that is causing regression
for arm64 as this_cpu* ops are expensive on arm64. 

> The patch description says that node stat updation uses irq unsafe interface.
> Therefore, we had foo() calling __foo() nested with local_irq_save/restore. But
> there were code paths which directly called __foo() - so, your patch fixes a bug right

No, those places were already disabling irqs and should be fine.

I am working on adding batched stats update functionality in the hope
that will fix the regression.
>