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
  <IRQ>
  __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: 2fbb0c10d1 ("mm/munlock: mlock_page() munlock_page() batch by pagevec")
Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
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 <hughd@google.com>
Cc: Jann Horn <jannh@google.com>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Pedro Falcato <pfalcato@suse.de>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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