[PATCH v7 0/3] mm: improve write performance with RWF_DONTCACHE

[Jeff Layton <jlayton@kernel.org>]

This patch series is intended to improve write performance with
RWF_DONTCACHE. This version fixes additional stat accounting issues
found during review: integer promotion on 32-bit, cgroup writeback
domain migration, folio split flag preservation, and a UAF that could
occur in filemap_dontcache_kick_writeback().

Because there are substantive changes in this set, I've dropped the
R-b's. Please resend them if you're OK with this version.

Christian, please consider these for v7.2.

Thanks,

Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
Changes in v7:
- Fix integer promotion bug on 32-bit: store folio_nr_pages() in signed
  long before negating, matching existing patterns in page-writeback.c
- Transfer WB_DONTCACHE_DIRTY in inode_do_switch_wbs() alongside
  WB_RECLAIMABLE so the stat migrates with the inode on cgroup wb switch
- Preserve PG_dropbehind in __split_folio_to_order() so tail folios
  retain the flag and are properly accounted when cleaned
- Take reference to wb around unlocked_inode_to_wb_end() to avoid UAF
- Link to v6: https://lore.kernel.org/r/20260505-dontcache-v6-0-66463805dd6a@kernel.org

Changes in v6:
- Use atomic folio_test_clear_dropbehind() in __filemap_get_folio_mpol()
  to prevent double-decrement of WB_DONTCACHE_DIRTY by concurrent readers
- Add mapping_can_writeback() guard before decrementing WB_DONTCACHE_DIRTY
  in __filemap_get_folio_mpol() to match the increment path
- Use wb_stat_sum() instead of wb_stat() in wb_check_start_dontcache() so
  small writes below the percpu batch threshold are visible to the flusher
- Use test_and_clear_bit for WB_start_dontcache before starting writeback
  to prevent lost wakeups from concurrent DONTCACHE writers
- Move wb_wakeup() outside the unlocked_inode_to_wb_begin/end section in
  filemap_dontcache_kick_writeback() to avoid spin_unlock_irq() re-enabling
  interrupts while the i_pages xa_lock is held during cgroup writeback switch
- Drop Reviewed-by tags due to substantive changes
- Link to v5: https://lore.kernel.org/r/20260504-dontcache-v5-0-4103e58bb377@kernel.org

Changes in v5:
- Flesh out comment over filemap_dontcache_kick_writeback()
- Drop testcases from posting
- Link to v4: https://lore.kernel.org/r/20260501-dontcache-v4-0-5d5e6dc71cb3@kernel.org

Changes in v4:
- Track DONTCACHE dirty pages per bdi_writeback
- New benchmark for competing buffered and dontcache writers
- New benchmark replicating Jens' original 32 concurrent writer test
- Link to v3: https://lore.kernel.org/r/20260426-dontcache-v3-0-79eb37da9547@kernel.org

Changes in v3:
- Track dirty DONTCACHE pages in the VM
- Have flusher write back a proportional number of pages after DONTCACHE write
- Link to v2: https://lore.kernel.org/r/20260408-dontcache-v2-0-948dec1e756b@kernel.org

Changes in v2:
- kick flusher thread instead of initiating writeback inline
- add mechanism to run 'perf lock' around the testcases
- Link to v1: https://lore.kernel.org/r/20260401-dontcache-v1-0-1f5746fab47a@kernel.org

---
Jeff Layton (3):
      mm: preserve PG_dropbehind flag during folio split
      mm: track DONTCACHE dirty pages per bdi_writeback
      mm: kick writeback flusher for IOCB_DONTCACHE with targeted dirty tracking

 fs/fs-writeback.c                | 67 ++++++++++++++++++++++++++++++++++++++++
 include/linux/backing-dev-defs.h |  3 ++
 include/linux/fs.h               |  6 ++--
 include/trace/events/writeback.h |  3 +-
 mm/filemap.c                     | 15 +++++++--
 mm/huge_memory.c                 |  1 +
 mm/page-writeback.c              |  6 ++++
 7 files changed, 94 insertions(+), 7 deletions(-)
---
base-commit: 7e2326f4275c11652e1fdaae11de06159fef1d90
change-id: 20260401-dontcache-5811efd7eaf3

Best regards,
-- 
Jeff Layton <jlayton@kernel.org>