[PATCH v5 0/5] mm: batch TLB flushing for dirty folios in vmscan

[Zhang Peng <zippermonkey@icloud.com>]

This series introduces batch TLB flushing optimization for dirty folios
during memory reclaim, aiming to reduce IPI overhead on multi-core systems.

Background
----------
Currently, when performing pageout in memory reclaim, try_to_unmap_flush_dirty()
is called for each dirty folio individually. On multi-core systems, this causes
frequent IPIs which can significantly impact performance.

Approach
--------
This patch series accumulates dirty folios into batches and performs a single
TLB flush for the entire batch, rather than flushing for each individual folio.

Changes
-------
Patch 1: Extract the folio activation block at activate_locked into
         folio_activate_locked().
Patch 2: Extract the folio-freeing path (buffer release, lazyfree,
         __remove_mapping, folio_batch drain) into folio_free().
Patch 3: Extract the pageout() dispatch state machine into pageout_one().
Patch 4: Extract the TTU setup and try_to_unmap() block into folio_try_unmap().
Patch 5: Implement batch TLB flushing logic. Dirty folios are accumulated in
         batches and a single TLB flush is performed for each batch before
         calling pageout.

Testing
-------
The benchmark script uses stress-ng to compare TLB shootdown behavior before and
after this patch. It constrains a stress-ng workload via memcg to force reclaim
through shrink_folio_list(), reporting TLB shootdowns and IPIs.

Core benchmark command: stress-ng --vm 16 --vm-bytes 2G --vm-keep --timeout 60

==========================================================================
                 batch_dirty_tlb_flush Benchmark Results
==========================================================================
  Kernel: 7.0.0-rc1+   CPUs: 16
  MemTotal: 31834M   SwapTotal: 8191M
  memcg limit: 512M   alloc: 2G   workers: 16   duration: 60s
--------------------------------------------------------------------------
Metric                 Before        After             Delta (abs / %)
--------------------------------------------------------------------------
bogo ops/s             28238.63      35833.97          +7595.34 (+26.9%)
TLB shootdowns         55428953      17621697          -37807256 (-68.2%)
Function call IPIs     34073695      14498768          -19574927 (-57.4%)
pgscan_anon (pages)    52856224      60252894          7396670 (+14.0%)
pgsteal_anon (pages)   29004962      34054753          5049791 (+17.4%)
--------------------------------------------------------------------------

Suggested-by: Kairui Song <kasong@tencent.com>
Signed-off-by: Zhang Peng <bruzzhang@tencent.com>
---
Changes in v5 (addressing David Hildenbrand's review on v4):
- Patch 1: Drop the nr_pages parameter; use folio_nr_pages() inside
  folio_activate_locked(). Add VM_WARN_ON_ONCE_FOLIO(!folio_test_locked)
  at the top of the function. Turn the VM_BUG_ON_FOLIO(folio_test_active)
  into VM_WARN_ON_ONCE_FOLIO and move it to the top.
- Patch 2: Rename folio_free() to folio_try_reclaim_free() to make the
  reclaim-specific role clear and justify the bool return. Make nr_pages
  const. Add VM_WARN_ON_ONCE_FOLIO(folio_ref_count, folio) at the free_it:
  label. Drop the now-redundant "swapped out as a whole" comment and the
  "else continue" after goto keep_locked.
- Patch 3: Rename pageout_one() to folio_try_pageout() for consistency
  with the other helpers. Fix continuation-line alignment.
- Patch 4: Fix continuation-line alignment.
- Patch 5: Replace the in-place folio_batch_reinit() + walk pattern
  with a plain local array; @fbatch is now only read during the walk
  and cleared once afterwards, so its invariant is never temporarily
  broken. Add a comment documenting why each recheck (writeback, mapped,
  dma_pinned) is required.
- Link to v4: https://lore.kernel.org/r/20260525-batch-tlb-flush-v4-0-83789d6abc00@icloud.com

Changes in v4 (addressing Barry Song's review on v3):
- Drop the "track reclaimed pages in reclaim_stat" patch; keep
  shrink_folio_list() returning nr_reclaimed directly. Avoids touching
  the function signature and its MGLRU evict_folios() and
  reclaim_clean_pages_from_list() callers in this series.
- Rename folio_active_bounce() to folio_activate_locked(). The new name
  reflects the precondition (the folio is locked) that callers care about.
- Split the folio_free()/pageout_one() extraction into two patches;
  make pageout_one() return bool so shrink_folio_list() can see whether
  the folio was reclaimed or kept.
- Move the !folio_mapped() check out of folio_try_unmap() into the
  caller, so folio_try_unmap() is only invoked for mapped folios.
- Link to v3: https://lore.kernel.org/r/20260410-batch-tlb-flush-v3-0-ff0b9d3a351a@icloud.com

Changes in v3:
- Patch 5: Replace folio_test_lru() condition check with
  VM_WARN_ON_FOLIO assertion, as PG_lru should never be set for
  isolated folios
- Patch 5: Add comment explaining folio_batch reuse-in-place
  technique in pageout_batch()
- Patch 5: Rewrite comment above folio_unlock() to explain why the
  folio is unlocked while batching
- Link to v2: https://lore.kernel.org/r/20260326-batch-tlb-flush-v2-0-403e523325c4@icloud.com

Changes in v2:
- Fix incorrect comment about page_ref_freeze
- Add folio_maybe_dma_pinned() check in pageout_batch()
- Link to v1: https://lore.kernel.org/r/20260309-batch-tlb-flush-v1-0-eb8fed7d1a9e@icloud.com

---
Zhang Peng (5):
      mm/vmscan: introduce folio_activate_locked() helper
      mm/vmscan: extract folio_free() from shrink_folio_list()
      mm/vmscan: extract pageout_one() from shrink_folio_list()
      mm/vmscan: extract folio unmap logic into folio_try_unmap()
      mm/vmscan: flush TLB for every 31 folios evictions

 mm/vmscan.c | 455 ++++++++++++++++++++++++++++++++++++++----------------------
 1 file changed, 293 insertions(+), 162 deletions(-)
---
base-commit: d0b709f436b2788a10407624688ab8327c5ce18d
change-id: 20260309-batch-tlb-flush-893f0e56b496

Best regards,
-- 
Zhang Peng <zippermonkey@icloud.com>