Re: [PATCH RESEND v2 1/2] mm: drop oom code from exit_mmap

[Matthew Wilcox]

On Wed, Jun 01, 2022 at 02:47:41PM -0700, Suren Baghdasaryan wrote:
> > Unclear why this patch fiddles with the mm_struct locking in this
> > fashion - changelogging that would have been helpful.
> 
> Yeah, I should have clarified this in the description. Everything up
> to unmap_vmas() can be done under mmap_read_lock and that way
> oom-reaper and process_mrelease can do the unmapping in parallel with
> exit_mmap. That's the reason we take mmap_read_lock, unmap the vmas,
> mark the mm with MMF_OOM_SKIP and take the mmap_write_lock to execute
> free_pgtables. I think maple trees do not change that except there is
> no mm->mmap anymore, so the line at the end of exit_mmap where we
> reset mm->mmap to NULL can be removed (I show that line below).

I don't understand why we _want_ unmapping to proceed in parallel?  Is it
so urgent to unmap these page tables that we need two processes doing
it at the same time?  And doesn't that just change the contention from
visible (contention on a lock) to invisible (contention on cachelines)?