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 8:14 AM Suren Baghdasaryan <surenb@google.com> wrote:
>
> 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!

Discussing how the patch I want to post works for maple trees that
Matthew is working on, I've got a question:

IIUC, according to Michal's post here:
https://lore.kernel.org/all/20170725154514.GN26723@dhcp22.suse.cz,
unmap_vmas() can race with other mmap_lock read holders (including
oom_reap_task_mm()) with no issues.
Maple tree patchset requires rcu read lock or the mmap semaphore be
held (read or write side) when walking the tree, including inside
unmap_vmas(). When asked, he told me that he is not sure why it's
currently "safe" to walk the vma->vm_next list in unmap_vmas() while
another thread is reaping the mm.
Michal (or maybe someone else), could you please clarify why
unmap_vmas() can safely race with oom_reap_task_mm()? Or maybe my
understanding was wrong?
Thanks,
Suren.



>
> > --
> > Michal Hocko
> > SUSE Labs