Re: [RFC] mm: stress-ng --mremap triggers severe lruvec lock contention in populate/unmap paths

[Lorenzo Stoakes <ljs@kernel.org>]

On Tue, Apr 07, 2026 at 05:35:18PM -0700, Hugh Dickins wrote:
> On Tue, 7 Apr 2026, John Hubbard wrote:
> > On 4/7/26 1:09 PM, Joseph Salisbury wrote:
> > > Hello,
> > >
> > > I would like to ask for feedback on an MM performance issue triggered by
> > > stress-ng's mremap stressor:
> > >
> > > stress-ng --mremap 8192 --mremap-bytes 4K --timeout 30 --metrics-brief
> > >
> > > This was first investigated as a possible regression from 0ca0c24e3211
> > > ("mm: store zero pages to be swapped out in a bitmap"), but the current
> > > evidence suggests that commit is mostly exposing an older problem for
> > > this workload rather than directly causing it.
> > >
> >
> > Can you try this out? (Adding Hugh to Cc.)
> >
> > From: John Hubbard <jhubbard@nvidia.com>
> > Date: Tue, 7 Apr 2026 15:33:47 -0700
> > Subject: [PATCH] mm/gup: skip lru_add_drain() for non-locked populate
> > X-NVConfidentiality: public
> > Cc: John Hubbard <jhubbard@nvidia.com>
> >
> > populate_vma_page_range() calls lru_add_drain() unconditionally after
> > __get_user_pages(). With high-frequency single-page MAP_POPULATE/munmap
> > cycles at high thread counts, this forces a lruvec->lru_lock acquire
> > per page, defeating per-CPU folio_batch batching.
> >
> > The drain was added by commit ece369c7e104 ("mm/munlock: add
> > lru_add_drain() to fix memcg_stat_test") for VM_LOCKED populate, where
> > unevictable page stats must be accurate after faulting. Non-locked VMAs
> > have no such requirement. Skip the drain for them.
> >
> > Cc: Hugh Dickins <hughd@google.com>
> > Signed-off-by: John Hubbard <jhubbard@nvidia.com>
>
> Thanks for the Cc.  I'm not convinced that we should be making such a
> change, just to avoid the stress that an avowed stresstest is showing;
> but can let others debate that - and, need it be said, I have no
> problem with Joseph trying your patch.

Yeah, the test case (as said by others also) is rather synthetic, and it's a
test designed to saturate, if not I/O throttled by swap then we hammer the
populate path. It feels like a micro-optimisation for something that is not (at
least not yet demonstrated to be) an actual problem.

stress-ng is not a benchmarking tool per se, it's designed to eek out bugs.

So really we need to see a real-world case I think.

>
> I tend to stand by my comment in that commit, that it's not just for
> VM_LOCKED: I believe it's in everyone's interest that a bulk faulting
> interface like populate_vma_page_range() or faultin_vma_page_range()
> should drain its local pagevecs at the end, to save others sometimes
> needing the much more expensive lru_add_drain_all().

I mean yeah, but I guess anywhere that _really_ needs to be sure of the drain
has to do an lru_add_drain_all(), because it'd be fragile to rely on
lru_add_drain()'s being done at the right time?

>
> But lru_add_drain() and lru_add_drain_all(): there's so much to be
> said and agonized over there  They've distressed me for years, and
> are a hot topic for us at present.  But I won't be able to contribute
> more on that subject, not this week.

Yeah they do feel rather delicate... :) sometimes you _really do_ need to know
everything's drained. But other times it feels a bit whack-a-mole.

I also do agree it makes sense to drain locally after a batch operation.

It all comes down to whether this manifests in a real-world case, at which point
maybe this is a more useful change?

>
> Hugh
>
> > ---
> >  mm/gup.c | 13 ++++++++++++-
> >  1 file changed, 12 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/gup.c b/mm/gup.c
> > index 8e7dc2c6ee73..2dd5de1cb5b9 100644
> > --- a/mm/gup.c
> > +++ b/mm/gup.c
> > @@ -1816,6 +1816,7 @@ long populate_vma_page_range(struct vm_area_struct *vma,
> >  	struct mm_struct *mm = vma->vm_mm;
> >  	unsigned long nr_pages = (end - start) / PAGE_SIZE;
> >  	int local_locked = 1;
> > +	bool need_drain;
> >  	int gup_flags;
> >  	long ret;
> >
> > @@ -1857,9 +1858,19 @@ long populate_vma_page_range(struct vm_area_struct *vma,
> >  	 * We made sure addr is within a VMA, so the following will
> >  	 * not result in a stack expansion that recurses back here.
> >  	 */
> > +	/*
> > +	 * Read VM_LOCKED before __get_user_pages(), which may drop
> > +	 * mmap_lock when FOLL_UNLOCKABLE is set, after which the vma
> > +	 * must not be accessed. The read is stable: mmap_lock is held
> > +	 * for read here, so mlock() (which needs the write lock)
> > +	 * cannot change VM_LOCKED concurrently.
> > +	 */

BTW, not to nitpick (OK, maybe to nitpick :) this comments feels a bit
redundant. Maybe useful to note that the lock might be dropped (but you don't
indicate why it's OK to still assume state about the VMA), and it's a known
thing that you need a VMA write lock to alter flags, if we had to comment this
each time mm would be mostly comments :)

So if you want a comment here I'd say something like 'the lock might be dropped
due to FOLL_UNLOCKABLE, but that's ok, we would simply end up doing a redundant
drain in this case'.

But I'm not sure it's needed?

> > +	need_drain = vma->vm_flags & VM_LOCKED;

Please use the new VMA flag interface :)

	need_drain = vma_test(VMA_LOCKED_BIT);

> > +
> >  	ret = __get_user_pages(mm, start, nr_pages, gup_flags,
> >  			       NULL, locked ? locked : &local_locked);
> > -	lru_add_drain();
> > +	if (need_drain)
> > +		lru_add_drain();
> >  	return ret;
> >  }
> >
> >
> > base-commit: 3036cd0d3328220a1858b1ab390be8b562774e8a
> > --
> > 2.53.0
> >
> >
> > thanks,
> > --
> > John Hubbard
>

Cheers, Lorenzo