Re: [PATCH 1/1] mm: prevent a race between process_mrelease and exit_mmap

[Suren Baghdasaryan <surenb@google.com>]

On Tue, Nov 2, 2021 at 12:58 AM Michal Hocko <mhocko@suse.com> wrote:
>
> On Mon 01-11-21 08:44:58, Suren Baghdasaryan wrote:
> [...]
> > 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?
>
> Results being very noisy is what I recall as well. Thanks!

I believe, despite the noise, the percentiles show that overall we do
not noticeably regress the exit path by taking mmap_lock
unconditionally.
If there are no objections, I would like to post a patchset which
implements unconditional locking in exit_mmap() and process_madvise()
calling __oom_reap_task_mm() under protection of read mmap_lock.
Thanks!

> --
> Michal Hocko
> SUSE Labs