linux-kernel - Re: [PATCH] mm: remove redundant check in handle_mm

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Date:   Tue, 7 Mar 2023 02:48:53 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Haifeng Xu <haifeng.xu@...pee.com>
Cc:     David Hildenbrand <david@...hat.com>, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: remove redundant check in handle_mm_fault

On Tue, Mar 07, 2023 at 10:36:55AM +0800, Haifeng Xu wrote:
> On 2023/3/6 21:49, David Hildenbrand wrote:
> > On 06.03.23 03:49, Haifeng Xu wrote:
> >> mem_cgroup_oom_synchronize() has checked whether current memcg_in_oom is
> >> set or not, so remove the check in handle_mm_fault().
> > 
> > "mem_cgroup_oom_synchronize() will returned immediately if memcg_in_oom is not set, so remove the check from handle_mm_fault()".
> > 
> > However, that requires now always an indirect function call -- do we care about dropping that optimization?
> > 
> > 
> 
> If memcg_in_oom is set, we will check it twice, one is from handle_mm_fault(), the other is from mem_cgroup_oom_synchronize(). That seems a bit redundant.
> 
> if memcg_in_oom is not set, mem_cgroup_oom_synchronize() returns directly. Though it's an indirect function call, but the time spent can be negligible 
> compare to the whole mm user falut preocess. And that won't cause stack overflow error.

I suggest you measure it.