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

[John Hubbard]

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@xxxxxxxxxx>
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@xxxxxxxxxx>

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@xxxxxxxxxx>
Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>
---
 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