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

[Suren Baghdasaryan <surenb@google.com>]

On Mon, Mar 23, 2026 at 10:02 AM Lorenzo Stoakes (Oracle)
<ljs@kernel.org> wrote:
>
> On Mon, Mar 23, 2026 at 09:26:02AM -0700, Suren Baghdasaryan wrote:
> > 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.
>
> I don't feel that strongly about it :>)
>
> >
> > Thanks for the review!
>
> No problem! Working through your vma killable series atm also :)

Thanks!
I will need to respin v5 after Sashiko found a couple of legit issues
at [1] but I'll wait for you to post your feedback before sending it.

[1] https://sashiko.dev/#/patchset/20260322054309.898214-1-surenb@google.com

>
> >
> > >
> > > Thanks, Lorenzo
>
> Cheers, Lorenzo