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

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Date:   Tue, 14 Mar 2023 12:40:10 +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 14, 2023 at 06:29:24PM +0800, Haifeng Xu wrote:
> 
> 
> On 2023/3/14 17:09, David Hildenbrand wrote:
> > On 14.03.23 09:05, Haifeng Xu wrote:
> >>
> >>
> >> On 2023/3/8 17:13, David Hildenbrand wrote:
> >>> On 08.03.23 10:03, Haifeng Xu wrote:
> >>>>
> >>>>
> >>>> On 2023/3/7 10:48, Matthew Wilcox wrote:
> >>>>> 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.
> >>>>
> >>>> test steps:
> >>>> 1) Run command: ./mmap_anon_test(global alloc, so the memcg_in_oom is not set)
> >>>> 2) Calculate the quotient of cost time and page-fault counts, run 10 rounds and average the results.
> >>>>
> >>>> The test result shows that whether using indirect function call or not, the time spent in user fault
> >>>> is almost the same, about 2.3ms.
> >>>
> >>> I guess most of the benchmark time is consumed by allocating fresh pages in your test (also, why exactly do you use MAP_SHARED?).
> >>>
> >>> Is 2.3ms the total time for writing to that 1GiB of memory or how did you derive that number? Posting both results would be cleaner (with more digits ;) ).
> >>>
> >>
> >> Hi Daivd, the details of test result were posted last week. Do you have any suggestions or more concerns about this change?
> > 
> > No, I guess it really doesn't matter performance wise.
> > 
> > One valid question would be: why perform this change at all? The redundancy doesn't seem to harm performance either.
> > 
> > If the change would obviously improve code readability it would be easy to justify. I'm not convinced, that is the case, but maybe for others.
> 
> Yes, this change doesn't optimize performance, just improve the code readability. 
> It seems that nobody ack this change, should I change the commit message and resend this patch?

I don't see the point of this patch.  Just drop it.