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

Michal Hocko
2021-11-01T08:37:56Z

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.

[...]

> > 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.
--
Michal Hocko
SUSE Labs