Re: [PATCH 1/1] mm/vmscan: prevent MGLRU reclaim from pinning address space

[Suren Baghdasaryan <surenb@google.com>]

On Mon, Mar 23, 2026 at 6:43 AM Lorenzo Stoakes (Oracle) <ljs@kernel.org> wrote:
>
> On Sun, Mar 22, 2026 at 12:08:43AM -0700, Suren Baghdasaryan wrote:
> > When shrinking lruvec, MGLRU pins address space before walking it.
> > This is excessive since all it needs for walking the page range is
> > a stable mm_struct to be able to take and release mmap_read_lock and
> > a stable mm->mm_mt tree to walk. This address space pinning results
> > in delays when releasing the memory of a dying process. This also
> > prevents mm reapers (both in-kernel oom-reaper and userspace
> > process_mrelease()) from doing their job during MGLRU scan because
> > they check task_will_free_mem() which will yield negative result due
> > to the elevated mm->mm_users.
> >
> > Replace unnecessary address space pinning with mm_struct pinning by
> > replacing mmget/mmput with mmgrab/mmdrop calls. mm_mt is contained
> > within mm_struct itself, therefore it won't be freed as long as
> > mm_struct is stable and it won't change during the walk because
> > mmap_read_lock is being held.
> >
> > Fixes: bd74fdaea146 ("mm: multi-gen LRU: support page table walks")
> > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
>
> Oh and sorry, missed this - do we need a 'Fixed' for something that's just a
> perf improvement?
>
> This doesn't seem buggy?

Well, if we are pinning something we should not is that a bug? I
didn't CC stable, so I don't expect it to be backported (though if
it's accepted, we will backport it in Android as it causes real
problems with reclaim), however if someone is looking for MGLRU fixes
I would like this patch to be discoverable. If you feel strongly about
it, I'll remove the tag.

Thanks for the review!

>
> Thanks, Lorenzo