linux-kernel - Re: [PATCH 1/1] mm: prevent a race between process_mrelease and exit

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Message-ID: <CAJuCfpGgi49C6fT7D8yV7evmzFVqZS2qvSQRHZkvNruc43ZmFQ@mail.gmail.com>
Date:   Mon, 1 Nov 2021 12:59:53 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        David Rientjes <rientjes@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <guro@...com>, Rik van Riel <riel@...riel.com>,
        Minchan Kim <minchan@...nel.org>,
        Christian Brauner <christian@...uner.io>,
        Christoph Hellwig <hch@...radead.org>,
        Oleg Nesterov <oleg@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Jann Horn <jannh@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Florian Weimer <fweimer@...hat.com>,
        Jan Engelhardt <jengelh@...i.de>,
        Linux API <linux-api@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kernel-team <kernel-team@...roid.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [PATCH 1/1] mm: prevent a race between process_mrelease and exit_mmap

On Mon, Nov 1, 2021 at 8:44 AM Suren Baghdasaryan <surenb@...gle.com> wrote:
>
> On Mon, Nov 1, 2021 at 1:37 AM Michal Hocko <mhocko@...e.com> wrote:
> >
> > On Fri 29-10-21 09:07:39, Suren Baghdasaryan wrote:
> > > On Fri, Oct 29, 2021 at 6:03 AM Michal Hocko <mhocko@...e.com> wrote:
> > [...]
> > > > Well, I still do not see why that is a problem. This syscall is meant to
> > > > release the address space not to do it fast.
> > >
> > > It's the same problem for a userspace memory reaper as for the
> > > oom-reaper. The goal is to release the memory of the victim and to
> > > quickly move on to the next one if needed.
> >
> > The purpose of the oom_reaper is to _guarantee_ a forward progress. It
> > doesn't have to be quick or optimized for speed.
>
> Fair enough. Then the same guarantees should apply to userspace memory
> reapers. I think you clarified that well in your replies in
> https://lore.kernel.org/all/20170725154514.GN26723@dhcp22.suse.cz:
>
> Because there is no _guarantee_ that the final __mmput will release
> the memory in finite time. And we cannot guarantee that longterm.
> ...
> __mmput calls into exit_aio and that can wait for completion and there
> is no way to guarantee this will finish in finite time.
>
> >
> > [...]
> >
> > > > Btw. the above code will not really tell you much on a larger machine
> > > > unless you manage to trigger mmap_sem contection. Otherwise you are
> > > > measuring the mmap_sem writelock fast path and that should be really
> > > > within a noise comparing to the whole address space destruction time. If
> > > > that is not the case then we have a real problem with the locking...
> > >
> > > My understanding of that discussion is that the concern was that even
> > > taking uncontended mmap_sem writelock would regress the exit path.
> > > That was what I wanted to confirm. Am I misreading it?
> >
> > No, your reading match my recollection. I just think that code
> > robustness in exchange of a rw semaphore write lock fast path is a
> > reasonable price to pay even if that has some effect on micro
> > benchmarks.
>
> I'm with you on this one, that's why I wanted to measure the price we
> would pay. Below are the test results:
>
> Test: https://lore.kernel.org/all/20170725142626.GJ26723@dhcp22.suse.cz/
> Compiled: gcc -O2 -static test.c -o test
> Test machine: 128 core / 256 thread 2x AMD EPYC 7B12 64-Core Processor
> (family 17h)
>
> baseline (Linus master, f31531e55495ca3746fb895ffdf73586be8259fa)
> p50 (median)   87412
> p95                  168210
> p99                  190058
> average           97843.8
> stdev               29.85%
>
> unconditional mmap_write_lock in exit_mmap (last column is the change
> from the baseline)
> p50 (median)   88312     +1.03%
> p95                  170797   +1.54%
> p99                  191813   +0.92%
> average           97659.5  -0.19%
> stdev               32.41%
>
> unconditional mmap_write_lock in exit_mmap + Matthew's patch (last
> column is the change from the baseline)
> p50 (median)   88807      +1.60%
> p95                  167783     -0.25%
> p99                  187853     -1.16%
> average           97491.4    -0.36%
> stdev               30.61%
>
> stdev is quite high in all cases, so the test is very noisy.

Need to clarify that what I called here "stdev" is actually stdev /
average in %.

> The impact seems quite low IMHO. WDYT?
>
> > --
> > Michal Hocko
> > SUSE Labs