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e14a345480
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:
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| arch | ||
| block | ||
| certs | ||
| crypto | ||
| Documentation | ||
| drivers | ||
| fs | ||
| include | ||
| init | ||
| io_uring | ||
| ipc | ||
| kernel | ||
| lib | ||
| LICENSES | ||
| mm | ||
| net | ||
| rust | ||
| samples | ||
| scripts | ||
| security | ||
| sound | ||
| tools | ||
| usr | ||
| virt | ||
| .clang-format | ||
| .clippy.toml | ||
| .cocciconfig | ||
| .editorconfig | ||
| .get_maintainer.ignore | ||
| .gitattributes | ||
| .gitignore | ||
| .mailmap | ||
| .pylintrc | ||
| .rustfmt.toml | ||
| COPYING | ||
| CREDITS | ||
| Kbuild | ||
| Kconfig | ||
| MAINTAINERS | ||
| Makefile | ||
| README | ||
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