Re: [PATCH 4/3] mm: drop MMF_OOM_SKIP from exit_mmap

[Michal Hocko]

On Thu 09-12-21 08:24:04, Suren Baghdasaryan wrote:
> On Thu, Dec 9, 2021 at 1:12 AM Michal Hocko <mhocko@xxxxxxxx> wrote:
> >
> > Do we want this on top?
> 
> As we discussed in this thread
> https://lore.kernel.org/all/YY4snVzZZZYhbigV@xxxxxxxxxxxxxx,
> __oom_reap_task_mm in exit_mmap allows oom-reaper/process_mrelease to
> unmap pages in parallel with exit_mmap without blocking each other.
> Removal of __oom_reap_task_mm from exit_mmap prevents this parallelism
> and has a negative impact on performance. So the conclusion of that
> thread I thought was to keep that part. My understanding is that we
> also wanted to remove MMF_OOM_SKIP as a follow-up patch but
> __oom_reap_task_mm would stay.

OK, then we were talking past each other, I am afraid. I really wanted
to get rid of this oom specific stuff from exit_mmap. It was there out
of necessity. With a proper locking we can finally get rid of the crud.
As I've said previously oom reaping has never been a hot path.

If we really want to optimize this path then I would much rather see a
generic solution which would allow to move the write lock down after
unmap_vmas. That would require oom reaper to be able to handle mlocked
memory.
-- 
Michal Hocko
SUSE Labs