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    SubjectRe: [PATCH 1/1] mm: prevent a race between process_mrelease and exit_mmap
    On Mon, Nov 1, 2021 at 1:37 AM Michal Hocko <mhocko@suse.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@suse.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.
    The impact seems quite low IMHO. WDYT?

    > --
    > Michal Hocko
    > SUSE Labs

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