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()
c17fdf62ae