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

[Suren Baghdasaryan]

On Mon, Nov 1, 2021 at 8:44 AM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
>
> On Mon, Nov 1, 2021 at 1:37 AM Michal Hocko <mhocko@xxxxxxxx> wrote:
> >
> > On Fri 29-10-21 09:07:39, Suren Baghdasaryan wrote:
> > > On Fri, Oct 29, 2021 at 6:03 AM Michal Hocko <mhocko@xxxxxxxx> 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@xxxxxxxxxxxxxx:
>
> 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@xxxxxxxxxxxxxx/
> 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