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

[Suren Baghdasaryan <surenb@google.com>]

On Thu, Jun 2, 2022 at 6:39 AM Matthew Wilcox <willy@infradead.org> wrote:
>
> 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)?

It's important for process_madvise() syscall not to be blocked by a
potentially lower priority task doing exit_mmap. I've seen such
priority inversion happening when the dying process is running on a
little core taking its time while a high-priority task is waiting in
the syscall while there is no reason for them to block each other.

>