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

[Haakon Bugge <haakon.bugge@oracle.com>]

> On 9 Apr 2026, at 20:03, Lorenzo Stoakes <ljs@kernel.org> wrote:
> 
> 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);
> 

I think we all agree that the stress-ng test case is synthetic. I evaluated John's patch as I understood that was requested, and the outcome was, merely, as expected.

The fio case is more interesting, as, if my runs make sense, it improves IOPS by ~20% and avoid threads being stuck at termination. But, I am not intimate with fio, so take that part as a grain of salt.


Thxs, Håkon