Re: [PATCH v2 00/26] mm/fbatch: drain lru_add_drain() and _all()

[Alen Karnil <alen.karnil@ideasonboard.com>]

Hi Hugh,

On 09/09/2026 10:39, Hugh Dickins wrote:
> [PATCH v2 00/26] mm/fbatch: drain lru_add_drain() and _all()
> 
> This series was prompted by lru_add_drain_all() appearing in watchdog
> backtraces: not to blame, but blocked on an unresponsive CPU to run
> its workqueue. lru_add_drain_all() is a heavyweight operation, which
> we often hope to avoid by a much lighter lru_add_drain(); but even
> those local drains can aggravate the lruvec lock contention which
> per-cpu fbatches are intended to ease.
> 
> Now let the folios on the lru_add and other per-cpu fbatches remain
> there, isolatable, with PG_lru set and refcount unraised, just like
> when on an actual LRU. Then many calls to lru_add_drain() and _all()
> can be removed.
> 
> It's something I've wanted to do for years, tried several times, but
> only hit on a good way to do it a few weeks ago: now it's uncomfortable
> to watch others wrestling with the drainage, while I'm sitting on this.
> 
> The key idea came from the "if (!folio_test_clear_lru(folio)) continue;".
> If we don't mind missing to take an action on rare occasions, then maybe
> we won't mind taking an action on the wrong folio on rare occasions, so
> long as it is a folio consenting to PG_lru rules.
> 
> No new locking, but relies on folio_try_get() and folio_test_clear_lru()
> even on the lru_add fbatch; with use of bits not set in aligned pointers,
> and some try_cmpxchg()ing.  Speculative references to folios are already
> accepted: this adds another source of them.
> 
> Performance? You (and the bots) tell me. I'm considering this as a
> cleanup, to make life easier for developers. I expect that some loads
> will show improvement, but also expect some disappointments (perhaps
> I go too far against lru_cache_disable()? or not far enough).
> 
> The timing is not good: I'm taking three weeks off today; but it's best
> to get this out in the open early. If it's welcome in principle for 7.4,
> but changes needed, maybe someone else can step up to shepherd it
> through; otherwise, I can take it up again for 7.5.
> 
> Rebased and retested on Linus's v7.3-rc2 tree, after adding two mm.git
> dependencies which did not get into rc2, but can be expected in rc3:
> Shakeel's "mm/mlock: use the IRQ-safe accessor for NR_MLOCK in
> __munlock_folio()" and Ackerley's "mm/folio: EXPORT_SYMBOL_FOR_KVM(
> lru_cache_drain_for_folio)". Those two are now in mm-hotfixes-stable
> (e14a34548064 and 7891fbb9512f respectively: thanks to Andrew) as of
> mm-everything-2026-09-09-06-32, and the series applied and briefly
> tested over 09-09's earlier mm-everything, so I'm giving a hopeful
> but not entirely honest base-commit below.
Can confirm it cleanly applies and runs on 7.3 
('c03c1a14507dfaf8b856c2608f9ac216021fc222')>
> v1 posted on 24 August had 25 patches, with 26/25 added later:
> Link: https://lore.kernel.org/linux-mm/14a16945-529b-8bc0-ab38-3ea97e54e223@google.com/
> 
> 01/26 mm/fbatch: remove !CONFIG_SMP special case of folio_activate()
>    v2: added Ack from David, R-b Vlastimil
> 02/26 mm/fbatch: allow folios_put_refs() to skip xa_is_value() entries
>    v2: added Ack from David, R-b Vlastimil
> 03/26 mm/fbatch: temporarily disable lazyfree and mlock+munlock batching
>    v2: added R-b Vlastimil
> 04/26 mm/fbatch: lru bit set, no extra ref, while folio on per-cpu fbatch
>    v2: added comment on Bit 1 of page->lru.next per Kiryl and Matthew
>        READ/WRITE_ONCE smp_mb__before_atomic per 07/25 discussion with Kiryl
>        fixed (commented out) BUG_ON in lru_add_del_folio()
> 05/26 mm/fbatch: lru_add_del_folio()+folio_add_lru() after clear_lru()
> 06/26 mm/fbatch: fbatch_drain_lazyfree(onstack fbatch) before ptl unlock
> 07/26 mm/fbatch: LRU_NEXT_ACTIVATE bit to optimize folio_activate()
>    v2: hunk in lru_add_del_folio() updated according to mods in 04/26
> 08/26 mm/fbatch: replace mlock_new_folio() by __folio_add_lru(,mlockit)
>    v2: updated 04/26 Bit 1 comment on folio_add_lru() to __folio_add_lru()
> 09/26 mm/fbatch: restore mlock+munlock batching, without extra ref
>    v2: rediffed around Shakeel's NR_MLOCK fix, fixed unknowingly in v1
> 10/26 mm/fbatch: remove several uses of mlock_drain_local()
> 11/26 mm/fbatch: remove migration's PAGE_WAS_MLOCKED lru_add_drain()
>    v2: conversely, use !folio_evictable() in isolate_migratepages_block()
> 12/26 mm/fbatch: remove percpu_pvec_drained and folios_put()
> 13/26 mm/fbatch: no lru_add drain to collect_longterm_unpinnable_folios()
>    v2: revert David's rc1 lru_cache_drain_for_folio() from mm/folio.c,
>        and Ackerley's EXPORT for KVM; but keep an inline stub in swap.h
> 14/26 mm/fbatch: no lru_add_drain() nor _all() for memfd_wait_for_pins()
> 15/26 mm/fbatch: remove shake_folio() shake_page() from memory-failure
>    v2: added Ack from Miaohe
> 16/26 mm/fbatch: remove lru_cache_disable() from NUMA folio migration
> 17/26 mm/fbatch: no lru_cache_disable() in __alloc_contig_migrate_range()
> 18/26 mm/fbatch: remove lru_add_drain() and _all() calls from various
> 19/26 mm/fbatch: vm/stat_refresh include lru_add_drain() on each cpu
> 20/26 s390/fbatch: no lru_add_drain_all() in s390_wiggle_split_folio()
> 21/26 block/fbatch: no lru_add_drain_all() in invalidate_bdev()
> 22/26 fs/fbatch: drop_caches invalidate_bh_lrus() not lru_add_drain_all()
> 23/26 fs,mm/fbatch: use invalidate_bh_lrus() not invalidate_bh_lrus_cpu()
> 24/26 fs,mm/fbatch: lru_cache_disable() keep off buffer_head lrus only
> 25/26 mm/fbatch: move lru_add_drain_all() declaration to mm/internal.h
> 26/26 mm/fbatch: drop reference inside the loop when draining
> 
>   Documentation/mm/unevictable-lru.rst |   2 +-
>   arch/s390/kernel/uv.c                |   1 -
>   block/bdev.c                         |   1 -
>   fs/buffer.c                          |  40 +--
>   fs/drop_caches.c                     |   4 +-
>   include/linux/buffer_head.h          |   8 +-
>   include/linux/folio_batch.h          |   7 +-
>   include/linux/huge_mm.h              |   6 +-
>   include/linux/mm.h                   |  18 --
>   include/linux/mm_inline.h            |  29 ++
>   include/linux/mm_types.h             |  14 +-
>   include/linux/swap.h                 |  23 +-
>   mm/compaction.c                      |  27 +-
>   mm/fadvise.c                         |  17 +-
>   mm/folio.c                           | 436 +++++++++------------------
>   mm/gup.c                             |   9 -
>   mm/huge_memory.c                     |  16 +-
>   mm/hwpoison-inject.c                 |   1 -
>   mm/internal.h                        |  15 +-
>   mm/khugepaged.c                      |  11 -
>   mm/ksm.c                             |  12 -
>   mm/madvise.c                         |   9 +-
>   mm/memfd.c                           |   6 +-
>   mm/memory-failure.c                  |  38 +--
>   mm/memory.c                          |  14 +-
>   mm/memory_hotplug.c                  |   4 +
>   mm/mempolicy.c                       |   7 -
>   mm/migrate.c                         |  13 +-
>   mm/migrate_device.c                  |   9 -
>   mm/mlock.c                           | 211 +++++++------
>   mm/page_alloc.c                      |   3 -
>   mm/rmap.c                            |   4 -
>   mm/shmem.c                           |   2 -
>   mm/truncate.c                        |  17 +-
>   mm/vmscan.c                          |  16 +-
>   mm/vmstat.c                          |   1 +
>   36 files changed, 388 insertions(+), 663 deletions(-)
> 
> base-commit: 7891fbb9512f127826e1d5dbf380ee212bd15eb0
> 
> Hugh

-- 
Thanks,
Alen Karnil