[GIT PULL 10/16 for v7.2] vfs writeback

[Christian Brauner]

Hey Linus,

/* Summary */

This contains the writeback changes for this cycle:

* Fix a race between cgroup_writeback_umount() and inode_switch_wbs()

  When a container exits, a race between cgroup_writeback_umount() and
  inode_switch_wbs()/cleanup_offline_cgwb() can trigger "VFS: Busy
  inodes after unmount" followed by a use-after-free on percpu
  counters. There is a window between inode_prepare_wbs_switch()
  returning true (having passed the SB_ACTIVE check and grabbed the
  inode) and the subsequent wb_queue_isw() call: if
  cgroup_writeback_umount() observes the global isw_nr_in_flight
  counter as non-zero but flush_workqueue() finds nothing queued yet,
  it returns early - leaving a held inode reference that blocks
  evict_inodes() and a later iput() that hits freed percpu counters.

  The race is closed by covering the window from
  inode_prepare_wbs_switch() through wb_queue_isw() with an RCU
  read-side critical section and synchronizing in the umount path. On
  top of that the now-dead rcu_barrier() left over from the
  queue_rcu_work() era is removed, and the global
  synchronize_rcu()/flush_workqueue() pair is replaced with a per-sb
  in-flight counter plus pin/unpin/drain helpers so umount no longer
  serializes against switch activity on unrelated superblocks.

  Under cgroup writeback churn on a 16 vCPU guest this takes umount
  latency from ~92-138ms p50 down to ~5-8ms p50 and the cumulative
  cost of cgroup_writeback_umount() from ~62ms to ~4us per call. The
  initial race fix is kept separate and minimal so it backports
  cleanly to stable trees that still queue switches via
  queue_rcu_work().

* Improve write performance with RWF_DONTCACHE

  Dirty DONTCACHE pages are now tracked per bdi_writeback so that the
  writeback flusher can be kicked in a targeted fashion for
  IOCB_DONTCACHE writes instead of relying on global writeback, and
  the PG_dropbehind flag is preserved when a folio is split.

/* Testing */

gcc (Debian 14.2.0-19) 14.2.0
Debian clang version 19.1.7 (3+b1)

No build failures or warnings were observed.

/* Conflicts */

Merge conflicts with mainline
=============================

No known conflicts.

Merge conflicts with other trees
================================

The following changes since commit 254f49634ee16a731174d2ae34bc50bd5f45e731:

  Linux 7.1-rc1 (2026-04-26 14:19:00 -0700)

are available in the Git repository at:

  git@xxxxxxxxxxxxxxxxxxx:pub/scm/linux/kernel/git/vfs/vfs tags/vfs-7.2-rc1.writeback

for you to fetch changes up to 0275dc184aa007b260374af6d46fb15741c062a8:

  Merge patch series "mm: improve write performance with RWF_DONTCACHE" (2026-06-04 10:18:25 +0200)

----------------------------------------------------------------
vfs-7.2-rc1.writeback

Please consider pulling these changes from the signed vfs-7.2-rc1.writeback tag.

Thanks!
Christian

----------------------------------------------------------------
Baokun Li (3):
      writeback: fix race between cgroup_writeback_umount() and inode_switch_wbs()
      writeback: drop now-unnecessary rcu_barrier() in cgroup_writeback_umount()
      writeback: use a per-sb counter to drain inode wb switches at umount

Christian Brauner (2):
      Merge patch series "writeback: fix race between cgroup_writeback_umount() and inode_switch_wbs()"
      Merge patch series "mm: improve write performance with RWF_DONTCACHE"

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                | 138 +++++++++++++++++++++++++++++++--------
 include/linux/backing-dev-defs.h |   3 +
 include/linux/fs.h               |   6 +-
 include/linux/fs/super_types.h   |   8 +++
 include/trace/events/writeback.h |   3 +-
 mm/filemap.c                     |  15 ++++-
 mm/huge_memory.c                 |   1 +
 mm/page-writeback.c              |   6 ++
 8 files changed, 147 insertions(+), 33 deletions(-)