From e14a3454806468b086fe2e4ca2e1bff95b528531 Mon Sep 17 00:00:00 2001 From: Shakeel Butt Date: Tue, 1 Sep 2026 11:01:09 -0700 Subject: [PATCH] mm/mlock: use the IRQ-safe accessor for NR_MLOCK in __munlock_folio() NR_MLOCK is updated from interrupt context. __free_pages_prepare() clears a stray PG_mlocked and adjusts NR_MLOCK, and a folio can reach it with the flag still set from a bio completion handler: __free_pages_ok+0x6af/0x7a0 __bio_release_pages+0xde/0x260 __iomap_dio_bio_end_io+0x16e/0x1a0 blk_update_request+0x14b/0x3d0 blk_mq_end_request+0x18/0x30 blk_done_softirq+0x49/0x60 The folio gets there like this. A MAP_SHARED file mapping is mlocked, so its page cache folios carry PG_mlocked, and an O_DIRECT write sourced from that mapping GUP-pins those same folios. munlock() then runs mlock_vma_pages_range(), which clears VM_LOCKED before walking the page tables to munlock each folio. A concurrent hole punch reaches the folio through the rmap (i_mmap_rwsem, not mmap_lock) and can land inside that window: __folio_remove_rmap() -> munlock_vma_folio() sees VM_LOCKED already clear, so it neither queues the folio on the mlock batch nor takes a reference, and the pte it clears makes the pending mlock_pte_range() walk skip the folio at its !pte_present() check. filemap_remove_folio() then drops the page cache reference, leaving the bio's pin as the last one, released from the completion handler above. So __zone_stat_mod_folio() here needs interrupts disabled, not merely preemption, and __munlock_folio() has a path where they are not: when the folio has already been taken off the LRU by somebody else the function jumps straight to the counter update without taking the lruvec lock. The read-modify-write of the per-CPU NR_MLOCK diff can then be interrupted by the softirq above, and one of the two decrements is lost, leaving Mlocked in /proc/meminfo permanently overstated. Use zone_stat_mod_folio(). mod_zone_state()'s this_cpu_try_cmpxchg() is atomic against a same-CPU interrupt and retries, and on the path where the lruvec lock is held its cost is negligible next to the lock itself. The UNEVICTABLE_PG* events are deliberately left on the __ accessors: they occupy different vm_event_states slots from the UNEVICTABLE_PGCLEARED that __free_pages_prepare() bumps, and nothing updates those two from interrupt context. Link: https://lore.kernel.org/20260901180109.3797944-1-shakeel.butt@linux.dev Fixes: 2fbb0c10d1e8 ("mm/munlock: mlock_page() munlock_page() batch by pagevec") Signed-off-by: Shakeel Butt Reported-by: syzbot+cd2073ee6d958a8d0fcd@syzkaller.appspotmail.com Closes: https://lore.kernel.org/linux-mm/6a931c5a.08e933ee.dbf97.0093.GAE@google.com/ Acked-by: Hugh Dickins Cc: Jann Horn Cc: Liam R. Howlett Cc: Lorenzo Stoakes Cc: Matthew Wilcox (Oracle) Cc: Pedro Falcato Cc: Vlastimil Babka Cc: Signed-off-by: Andrew Morton --- mm/mlock.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/mm/mlock.c b/mm/mlock.c index efa6716e4dfb..39215a3eab1f 100644 --- a/mm/mlock.c +++ b/mm/mlock.c @@ -141,7 +141,7 @@ static struct lruvec *__munlock_folio(struct folio *folio, struct lruvec *lruvec munlock: if (folio_test_clear_mlocked(folio)) { - __zone_stat_mod_folio(folio, NR_MLOCK, -nr_pages); + zone_stat_mod_folio(folio, NR_MLOCK, -nr_pages); if (isolated || !folio_test_unevictable(folio)) __count_vm_events(UNEVICTABLE_PGMUNLOCKED, nr_pages); else