Merge tag 'vfs-7.2-rc1.writeback' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs

Pull vfs writeback updates from Christian Brauner:

 - 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.

* tag 'vfs-7.2-rc1.writeback' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs:
  mm: kick writeback flusher for IOCB_DONTCACHE with targeted dirty tracking
  mm: track DONTCACHE dirty pages per bdi_writeback
  mm: preserve PG_dropbehind flag during folio split
  writeback: use a per-sb counter to drain inode wb switches at umount
  writeback: drop now-unnecessary rcu_barrier() in cgroup_writeback_umount()
  writeback: fix race between cgroup_writeback_umount() and inode_switch_wbs()

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