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

John Hubbard
2026-04-07T22:45:05Z

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>
---
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.
+ */
+ need_drain = vma->vm_flags & VM_LOCKED;
+
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