[PATCH v3 0/3] writeback: fix race between cgroup_writeback_umount() and inode_switch_wbs()

[PATCH v3 0/3] writeback: fix race between cgroup_writeback_umount() and inode_switch_wbs()

[Baokun Li]

Hi all,

Changes since v2:
 * Collect RVB from Jan Kara. (Thanks for your review!)
 * Patch 3: switch to wake_up_var() / wait_var_event() to drain
   s_isw_nr_in_flight. (Suggested by Christian Brauner and Sashiko)
 * Polish comments and changelogs.

Changes since v1:
 * Use a simple RCU-based fix (patch 1) that is easy to backport to
   older kernels; the per-sb refcount optimization is split out as a
   separate performance patch (patch 3). (Suggested by Jan Kara)

v1: https://patch.msgid.link/20260513094829.867648-1-libaokun@linux.alibaba.com
v2: https://patch.msgid.link/20260517142147.3354909-1-libaokun@linux.alibaba.com

======

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, it returns early -- leaving a held inode
reference that blocks evict_inodes() and a later iput() that hits
freed percpu counters.

Patch 1 closes the race by extending the RCU read-side critical
section to cover the window from inode_prepare_wbs_switch() through
wb_queue_isw(), and adding synchronize_rcu() in the umount path so
that all in-flight switchers complete queueing before
flush_workqueue() runs.  rcu_barrier() is intentionally retained so
the same hunk applies cleanly to stable trees that still queue
switches via queue_rcu_work().

Patch 2 removes the now-dead rcu_barrier() that was left over from
the queue_rcu_work() era (replaced by plain queue_work() in commit
e1b849cfa6b6 "writeback: Avoid contention on wb->list_lock when
switching inodes").  This is mainline-only.

Patch 3 replaces the global synchronize_rcu()/flush_workqueue() pair
with a per-sb counter (s_isw_nr_in_flight) plus three small helpers
(cgroup_writeback_pin / cgroup_writeback_unpin /
cgroup_writeback_drain), eliminating the global serialization
penalty.  This also reverts the RCU extension from patch 1 since the
per-sb counter makes it unnecessary.

Performance
-----------

Measured on a 16 vCPU QEMU guest, all kernels share the same .config.
Background load: 4 ext4 superblocks each running

  while :; do
      mkdir /sys/fs/cgroup/<tag>-tmp$N
      ( echo $BASHPID > <tag>-tmp$N/cgroup.procs
        dd if=/dev/zero of=$mp/burner bs=4k count=256 conv=notrunc \
	   oflag=sync)
      rmdir /sys/fs/cgroup/<tag>-tmp$N
  done

This drives both inode_switch_wbs() (different cgroups writing the
same inode) and cleanup_offline_cgwb() (dying memcgs), keeping the
global isw_nr_in_flight non-zero throughout the run.  Latencies are
wall-clock around umount(8) on a separate target sb; only the target
sb's umount is measured.

Four kernels are compared at each step of the series:

  base       pre-fix mainline
  +race      base + patch 1 (race fix, keeps rcu_barrier)
  +rmbarrier +race + patch 2 (drop rcu_barrier)
  +persb     +rmbarrier + patch 3 (per-sb counter)

Target sb runs its own cgwb churn:

                p50      p95      p99      max
  base         99.7 ms 112.9 ms 112.9 ms 127.2 ms
  +race       110.2 ms 153.8 ms 153.8 ms 160.4 ms
  +rmbarrier   67.6 ms  88.3 ms  88.3 ms  96.8 ms
  +persb        7.9 ms  10.0 ms  10.0 ms  10.1 ms

Idle target umount under cross-sb cgwb-switch pressure:

                p50      p95      p99      max
  base         92.0 ms 123.5 ms 136.5 ms 141.3 ms
  +race       118.8 ms 154.6 ms 164.7 ms 165.3 ms
  +rmbarrier   62.7 ms  95.4 ms 108.1 ms 108.6 ms
  +persb        5.3 ms   6.9 ms   7.4 ms   7.4 ms

8 concurrent umounts of idle sbs under the same pressure:

                p50      p95      p99      max
  base        137.5 ms 166.9 ms 166.9 ms 171.3 ms
  +race       162.2 ms 183.9 ms 183.9 ms 217.0 ms
  +rmbarrier   61.3 ms  99.5 ms  99.5 ms 113.7 ms
  +persb        8.1 ms   9.1 ms   9.1 ms   9.5 ms

A no-pressure baseline run (no background load) measures ~5 ms p50
across all four kernels, validating that the methodology has no
systematic bias.

In-kernel cgroup_writeback_umount() cumulative cost across the same
run (bpftrace, ~340 calls covering all four scenarios):

                                cgroup_writeback_umount() time
  base                          21240 ms total  (~62 ms / call)
  +race      (rcu_barrier+sync) 24966 ms total  (~73 ms / call)
  +rmbarrier (synchronize_rcu)  12371 ms total  (~36 ms / call)
  +persb     (per-sb counter)    1.37 ms total  ( ~4 us / call)

Under +persb the wait_var_event() condition is true on entry
whenever the target sb has nothing in flight, so synchronize_rcu()
and flush_workqueue() are never called on this path.

Notes:

  - Patch 1 adds ~10-27 ms p50 over base by introducing
    synchronize_rcu().  This is the cost of closing the race
    correctly and is paid by stable backports as well.
  - Patch 2 ("drop rcu_barrier()") was expected to be a pure cleanup
    on mainline, but actually removes a real wait: rcu_barrier()
    drains call_rcu() callbacks from *all* subsystems, and the
    cgroup teardown path keeps that pipeline busy under this
    workload.  Removing it cuts ~43-101 ms p50 on top of patch 1.
  - Patch 3 (per-sb counter) replaces the global wait entirely; the
    target sb no longer waits for activity on unrelated sbs,
    recovering near-baseline latency in all three scenarios.

Comments and questions are, as always, welcome.

Thanks,
Baokun

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

 fs/fs-writeback.c              | 81 ++++++++++++++++++++++------------
 include/linux/fs/super_types.h |  8 ++++
 2 files changed, 62 insertions(+), 27 deletions(-)

--
2.43.7

[PATCH v3 1/3] writeback: fix race between cgroup_writeback_umount() and inode_switch_wbs()

[Baokun Li]

When a container exits, the following BUG_ON() is occasionally triggered:

==================================================================
 VFS: Busy inodes after unmount of sdb (ext4)
 ------------[ cut here ]------------
 kernel BUG at fs/super.c:695!
 CPU: 3 PID: 6 Comm: containerd-shim Tainted: G OE K 6.6 #1
 pstate: 63400009 (nZCv daif +PAN -UAO +TCO +DIT -SSBS BTYPE=--)
 pc : generic_shutdown_super+0xf0/0x100
 lr : generic_shutdown_super+0xf0/0x100
 Call trace:
  generic_shutdown_super+0xf0/0x100
  kill_block_super+0x20/0x48
  ext4_kill_sb+0x28/0x60
  deactivate_locked_super+0x54/0x130
  deactivate_super+0x84/0xa0
  cleanup_mnt+0xa4/0x140
  __cleanup_mnt+0x18/0x28
  task_work_run+0x78/0xe0
  do_notify_resume+0x204/0x240
==================================================================

The root cause is a race between cgroup_writeback_umount() and
inode_switch_wbs()/cleanup_offline_cgwb(). There is a window between
inode_prepare_wbs_switch() returning true and the subsequent
wb_queue_isw() call. Following is the process that triggers the issue:

      CPU A (umount)           |          CPU B (writeback)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                                 inode_switch_wbs/cleanup_offline_cgwb
                                  atomic_inc(&isw_nr_in_flight)
                                  inode_prepare_wbs_switch
                                   -> passes SB_ACTIVE check
                                   __iget(inode)
 generic_shutdown_super
  sb->s_flags &= ~SB_ACTIVE
  cgroup_writeback_umount(sb)
   smp_mb()
   atomic_read(&isw_nr_in_flight)
   rcu_barrier()
    -> no pending RCU callbacks
   flush_workqueue(isw_wq)
    -> nothing queued, returns
  evict_inodes(sb)
   -> Inode skipped as isw still holds a ref.
  sop->put_super(sb)
   /* destroys percpu counters */
  -> VFS: Busy inodes after unmount!
                                  wb_queue_isw()
                                   queue_work(isw_wq, ...)
                                  /* later in work function */
                                  inode_switch_wbs_work_fn
                                   process_inode_switch_wbs
                                    iput() -> evict
                                     percpu_counter_dec() // UAF!

Fix this by extending the RCU read-side critical section in
inode_switch_wbs() and cleanup_offline_cgwb() to cover from
inode_prepare_wbs_switch() through wb_queue_isw().  Since there is
no sleep in this window, rcu_read_lock() can be used.  Then add a
synchronize_rcu() in cgroup_writeback_umount() before the existing
rcu_barrier(), so that all in-flight switchers that have passed the
SB_ACTIVE check have completed queue_work() before flush_workqueue()
is called.

The existing rcu_barrier() is intentionally retained so this fix can
be backported unchanged to stable kernels (5.10.y, 6.6.y, ...) that
still queue switches via queue_rcu_work(). It is a no-op on current
mainline (since commit e1b849cfa6b6 ("writeback: Avoid contention on
wb->list_lock when switching inodes")) and is removed in a follow-up
patch.

Fixes: a1a0e23e4903 ("writeback: flush inode cgroup wb switches instead of pinning super_block")
Cc: stable@vger.kernel.org
Suggested-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/all/mxnjq2l6guusfchvauxr3v7c4bwjasybxlleqbbh4efloeqspz@iqylk76ohufz
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Baokun Li <libaokun@linux.alibaba.com>
---
 fs/fs-writeback.c | 31 +++++++++++++++++++++++++++++--
 1 file changed, 29 insertions(+), 2 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index a65694cbfe68..6766de9f9d75 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -660,12 +660,19 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id)
 
 	atomic_inc(&isw_nr_in_flight);
 
-	/* find and pin the new wb */
+	/*
+	 * Paired with synchronize_rcu() in cgroup_writeback_umount():
+	 * holding rcu_read_lock across inode_prepare_wbs_switch()
+	 * (covering the SB_ACTIVE check and the inode grab) and
+	 * wb_queue_isw() ensures synchronize_rcu() cannot return until
+	 * the work is queued, so the subsequent flush_workqueue() will
+	 * wait for the switch.
+	 */
 	rcu_read_lock();
+	/* find and pin the new wb */
 	memcg_css = css_from_id(new_wb_id, &memory_cgrp_subsys);
 	if (memcg_css && !css_tryget(memcg_css))
 		memcg_css = NULL;
-	rcu_read_unlock();
 	if (!memcg_css)
 		goto out_free;
 
@@ -681,9 +688,11 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id)
 
 	trace_inode_switch_wbs_queue(inode->i_wb, new_wb, 1);
 	wb_queue_isw(new_wb, isw);
+	rcu_read_unlock();
 	return;
 
 out_free:
+	rcu_read_unlock();
 	atomic_dec(&isw_nr_in_flight);
 	if (new_wb)
 		wb_put(new_wb);
@@ -741,6 +750,14 @@ bool cleanup_offline_cgwb(struct bdi_writeback *wb)
 		new_wb = &wb->bdi->wb; /* wb_get() is noop for bdi's wb */
 
 	nr = 0;
+	/*
+	 * Paired with synchronize_rcu() in cgroup_writeback_umount().
+	 * Holding rcu_read_lock across the SB_ACTIVE check, the inode grab
+	 * and wb_queue_isw() ensures synchronize_rcu() cannot return until
+	 * the work is queued, so the subsequent flush_workqueue() will wait
+	 * for the switch.
+	 */
+	rcu_read_lock();
 	spin_lock(&wb->list_lock);
 	/*
 	 * In addition to the inodes that have completed writeback, also switch
@@ -758,6 +775,7 @@ bool cleanup_offline_cgwb(struct bdi_writeback *wb)
 
 	/* no attached inodes? bail out */
 	if (nr == 0) {
+		rcu_read_unlock();
 		atomic_dec(&isw_nr_in_flight);
 		wb_put(new_wb);
 		kfree(isw);
@@ -766,6 +784,7 @@ bool cleanup_offline_cgwb(struct bdi_writeback *wb)
 
 	trace_inode_switch_wbs_queue(wb, new_wb, nr);
 	wb_queue_isw(new_wb, isw);
+	rcu_read_unlock();
 
 	return restart;
 }
@@ -1221,6 +1240,14 @@ void cgroup_writeback_umount(struct super_block *sb)
 	smp_mb();
 
 	if (atomic_read(&isw_nr_in_flight)) {
+		/*
+		 * Paired with rcu_read_lock() in inode_switch_wbs() and
+		 * cleanup_offline_cgwb().  synchronize_rcu() waits for any
+		 * in-flight switcher that already passed the SB_ACTIVE check
+		 * to finish queueing its work, so flush_workqueue() below
+		 * will then drain it.
+		 */
+		synchronize_rcu();
 		/*
 		 * Use rcu_barrier() to wait for all pending callbacks to
 		 * ensure that all in-flight wb switches are in the workqueue.
-- 
2.43.7

[PATCH v3 2/3] writeback: drop now-unnecessary rcu_barrier() in cgroup_writeback_umount()

[Baokun Li]

Commit e1b849cfa6b6 ("writeback: Avoid contention on wb->list_lock when
switching inodes") replaced the queue_rcu_work() based scheduling of
inode wb switches with a plain queue_work().  Since then no switcher
goes through call_rcu(), so rcu_barrier() in cgroup_writeback_umount()
has no callbacks of its own to wait for.  It still drains unrelated
call_rcu() callbacks from other subsystems on busy systems, which
incidentally slows umount down; drop it.

Fixes: e1b849cfa6b6 ("writeback: Avoid contention on wb->list_lock when switching inodes")
Signed-off-by: Baokun Li <libaokun@linux.alibaba.com>
---
 fs/fs-writeback.c | 5 -----
 1 file changed, 5 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 6766de9f9d75..325a30cc35bf 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -1248,11 +1248,6 @@ void cgroup_writeback_umount(struct super_block *sb)
 		 * will then drain it.
 		 */
 		synchronize_rcu();
-		/*
-		 * Use rcu_barrier() to wait for all pending callbacks to
-		 * ensure that all in-flight wb switches are in the workqueue.
-		 */
-		rcu_barrier();
 		flush_workqueue(isw_wq);
 	}
 }
-- 
2.43.7

Re: [PATCH v3 2/3] writeback: drop now-unnecessary rcu_barrier() in cgroup_writeback_umount()

[Jan Kara]

On Mon 18-05-26 21:53:48, Baokun Li wrote:
> Commit e1b849cfa6b6 ("writeback: Avoid contention on wb->list_lock when
> switching inodes") replaced the queue_rcu_work() based scheduling of
> inode wb switches with a plain queue_work().  Since then no switcher
> goes through call_rcu(), so rcu_barrier() in cgroup_writeback_umount()
> has no callbacks of its own to wait for.  It still drains unrelated
> call_rcu() callbacks from other subsystems on busy systems, which
> incidentally slows umount down; drop it.
> 
> Fixes: e1b849cfa6b6 ("writeback: Avoid contention on wb->list_lock when switching inodes")
> Signed-off-by: Baokun Li <libaokun@linux.alibaba.com>

I've already replied to previous version but anyway: feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

								Honza

> ---
>  fs/fs-writeback.c | 5 -----
>  1 file changed, 5 deletions(-)
> 
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index 6766de9f9d75..325a30cc35bf 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -1248,11 +1248,6 @@ void cgroup_writeback_umount(struct super_block *sb)
>  		 * will then drain it.
>  		 */
>  		synchronize_rcu();
> -		/*
> -		 * Use rcu_barrier() to wait for all pending callbacks to
> -		 * ensure that all in-flight wb switches are in the workqueue.
> -		 */
> -		rcu_barrier();
>  		flush_workqueue(isw_wq);
>  	}
>  }
> -- 
> 2.43.7
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

Re: [PATCH v3 2/3] writeback: drop now-unnecessary rcu_barrier() in cgroup_writeback_umount()

[Baokun Li]

On 2026/5/20 16:46, Jan Kara wrote:
> On Mon 18-05-26 21:53:48, Baokun Li wrote:
>> Commit e1b849cfa6b6 ("writeback: Avoid contention on wb->list_lock when
>> switching inodes") replaced the queue_rcu_work() based scheduling of
>> inode wb switches with a plain queue_work().  Since then no switcher
>> goes through call_rcu(), so rcu_barrier() in cgroup_writeback_umount()
>> has no callbacks of its own to wait for.  It still drains unrelated
>> call_rcu() callbacks from other subsystems on busy systems, which
>> incidentally slows umount down; drop it.
>>
>> Fixes: e1b849cfa6b6 ("writeback: Avoid contention on wb->list_lock when switching inodes")
>> Signed-off-by: Baokun Li <libaokun@linux.alibaba.com>
> I've already replied to previous version but anyway: feel free to add:
>
> Reviewed-by: Jan Kara <jack@suse.cz>
>
> 								Honza


Hi Honza,

Thank you for your review!

Sorry for the rushed v3 — your Reviewed-by on v2 came in right after
I hit send, so I missed picking it up. I'll carry it forward in v4.


Thanks,
Baokun


>> ---
>>  fs/fs-writeback.c | 5 -----
>>  1 file changed, 5 deletions(-)
>>
>> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
>> index 6766de9f9d75..325a30cc35bf 100644
>> --- a/fs/fs-writeback.c
>> +++ b/fs/fs-writeback.c
>> @@ -1248,11 +1248,6 @@ void cgroup_writeback_umount(struct super_block *sb)
>>  		 * will then drain it.
>>  		 */
>>  		synchronize_rcu();
>> -		/*
>> -		 * Use rcu_barrier() to wait for all pending callbacks to
>> -		 * ensure that all in-flight wb switches are in the workqueue.
>> -		 */
>> -		rcu_barrier();
>>  		flush_workqueue(isw_wq);
>>  	}
>>  }
>> -- 
>> 2.43.7
>>

[PATCH v3 3/3] writeback: use a per-sb counter to drain inode wb switches at umount

[Baokun Li]

Tracking in-flight inode wb switches with a single global counter
(isw_nr_in_flight) plus a synchronize_rcu() based wait in
cgroup_writeback_umount() forces every umount to take a global hit
whenever any other superblock on the system has wb switches in flight,
even if the superblock being unmounted has none of its own.

Replace the global synchronize_rcu()/flush_workqueue() pair with a
per-sb counter, s_isw_nr_in_flight, plus three small helpers:

  - cgroup_writeback_pin(sb)   - increment counter
  - cgroup_writeback_unpin(sb) - decrement and wake drainer if last
  - cgroup_writeback_drain(sb) - wait for counter to reach zero

The wiring is:

  - inode_prepare_wbs_switch() pins before checking SB_ACTIVE and
    grabbing the inode; failure paths unpin before returning.  A
    lockless SB_ACTIVE check at the top of the function lets us skip
    the atomic_inc/smp_mb dance once SB_ACTIVE has been cleared (it
    is monotonic and never set back).
  - process_inode_switch_wbs() unpins after the matching iput().
  - cgroup_writeback_umount() drains the per-sb counter via
    wait_var_event().

The smp_mb() pair between inode_prepare_wbs_switch() and
cgroup_writeback_umount() keeps the SB_ACTIVE / counter ordering:
either the umounter sees a non-zero counter and waits, or the
switcher sees SB_ACTIVE cleared and aborts before grabbing the
inode.

The global isw_nr_in_flight is left in place, since it is still used
to throttle in-flight switches via WB_FRN_MAX_IN_FLIGHT.

The rcu_read_lock() extension in inode_switch_wbs() and
cleanup_offline_cgwb() that the race fix added is no longer needed
and is reverted; the synchronize_rcu() that the race fix added to
cgroup_writeback_umount() is dropped as well.

The following numbers were measured on a 16 vCPU QEMU guest with 4
background superblocks each churning "create memcg -> write 1 MiB ->
rmdir memcg" to keep the global isw_nr_in_flight non-zero.  Latencies
are wall-clock around umount(8); only the target sb's umount is
measured.

Target sb runs its own cgwb churn:

                              p50      p95      p99      max
  global synchronize_rcu()   67.6 ms  88.3 ms  88.3 ms  96.8 ms
  per-sb counter (this)       7.9 ms  10.0 ms  10.0 ms  10.1 ms

Idle target umount latency under cross-sb cgwb-switch pressure:

                              p50      p95      p99      max
  global synchronize_rcu()   62.7 ms  95.4 ms 108.1 ms 108.6 ms
  per-sb counter (this)       5.3 ms   6.9 ms   7.4 ms   7.4 ms
  no-pressure baseline        4.9 ms   5.9 ms   6.3 ms   6.7 ms

8 concurrent umounts of idle sbs under the same pressure:

                              p50      p95      max
  global synchronize_rcu()   61.3 ms  99.5 ms 113.7 ms
  per-sb counter (this)       8.1 ms   9.1 ms   9.5 ms

In-kernel cgroup_writeback_umount() time across the same run
(bpftrace, ~340 calls covering all scenarios):

  global synchronize_rcu()    12371 ms total (~36 ms / call)
  per-sb counter (this)        1.37 ms total ( ~4 us / call)

Suggested-by: Christian Brauner <brauner@kernel.org>
Link: https://lore.kernel.org/r/177910456953.488929.2169908940676707307.b4-review@b4
Signed-off-by: Baokun Li <libaokun@linux.alibaba.com>
---
 fs/fs-writeback.c              | 87 ++++++++++++++++++----------------
 include/linux/fs/super_types.h |  8 ++++
 2 files changed, 54 insertions(+), 41 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 325a30cc35bf..32fec4b9094e 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -497,6 +497,23 @@ static bool inode_do_switch_wbs(struct inode *inode,
 	return switched;
 }
 
+static inline void cgroup_writeback_pin(struct super_block *sb)
+{
+	atomic_inc(&sb->s_isw_nr_in_flight);
+}
+
+static inline void cgroup_writeback_unpin(struct super_block *sb)
+{
+	if (atomic_dec_and_test(&sb->s_isw_nr_in_flight))
+		wake_up_var(&sb->s_isw_nr_in_flight);
+}
+
+static inline void cgroup_writeback_drain(struct super_block *sb)
+{
+	wait_var_event(&sb->s_isw_nr_in_flight,
+		       !atomic_read(&sb->s_isw_nr_in_flight));
+}
+
 static void process_inode_switch_wbs(struct bdi_writeback *new_wb,
 				     struct inode_switch_wbs_context *isw)
 {
@@ -554,8 +571,12 @@ static void process_inode_switch_wbs(struct bdi_writeback *new_wb,
 		wb_put_many(old_wb, nr_switched);
 	}
 
-	for (inodep = isw->inodes; *inodep; inodep++)
+	for (inodep = isw->inodes; *inodep; inodep++) {
+		struct super_block *sb = (*inodep)->i_sb;
+
 		iput(*inodep);
+		cgroup_writeback_unpin(sb);
+	}
 	wb_put(new_wb);
 	kfree(isw);
 	atomic_dec(&isw_nr_in_flight);
@@ -598,16 +619,19 @@ void inode_switch_wbs_work_fn(struct work_struct *work)
 static bool inode_prepare_wbs_switch(struct inode *inode,
 				     struct bdi_writeback *new_wb)
 {
+	/* Avoid the atomic_inc/smp_mb dance once SB_ACTIVE is gone. */
+	if (!(inode->i_sb->s_flags & SB_ACTIVE))
+		return false;
+
 	/*
-	 * Paired with smp_mb() in cgroup_writeback_umount().
-	 * isw_nr_in_flight must be increased before checking SB_ACTIVE and
-	 * grabbing an inode, otherwise isw_nr_in_flight can be observed as 0
-	 * in cgroup_writeback_umount() and the isw_wq will be not flushed.
+	 * Pairs with smp_mb() in cgroup_writeback_umount(): the umounter either
+	 * sees a non-zero counter and waits, or we see SB_ACTIVE clear below.
 	 */
+	cgroup_writeback_pin(inode->i_sb);
 	smp_mb();
 
 	if (IS_DAX(inode))
-		return false;
+		goto out_dec;
 
 	/* while holding I_WB_SWITCH, no one else can update the association */
 	spin_lock(&inode->i_lock);
@@ -615,13 +639,17 @@ static bool inode_prepare_wbs_switch(struct inode *inode,
 	    inode_state_read(inode) & (I_WB_SWITCH | I_FREEING | I_WILL_FREE) ||
 	    inode_to_wb(inode) == new_wb) {
 		spin_unlock(&inode->i_lock);
-		return false;
+		goto out_dec;
 	}
 	inode_state_set(inode, I_WB_SWITCH);
 	__iget(inode);
 	spin_unlock(&inode->i_lock);
 
 	return true;
+
+out_dec:
+	cgroup_writeback_unpin(inode->i_sb);
+	return false;
 }
 
 static void wb_queue_isw(struct bdi_writeback *wb,
@@ -673,6 +701,7 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id)
 	memcg_css = css_from_id(new_wb_id, &memory_cgrp_subsys);
 	if (memcg_css && !css_tryget(memcg_css))
 		memcg_css = NULL;
+	rcu_read_unlock();
 	if (!memcg_css)
 		goto out_free;
 
@@ -688,11 +717,9 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id)
 
 	trace_inode_switch_wbs_queue(inode->i_wb, new_wb, 1);
 	wb_queue_isw(new_wb, isw);
-	rcu_read_unlock();
 	return;
 
 out_free:
-	rcu_read_unlock();
 	atomic_dec(&isw_nr_in_flight);
 	if (new_wb)
 		wb_put(new_wb);
@@ -750,14 +777,6 @@ bool cleanup_offline_cgwb(struct bdi_writeback *wb)
 		new_wb = &wb->bdi->wb; /* wb_get() is noop for bdi's wb */
 
 	nr = 0;
-	/*
-	 * Paired with synchronize_rcu() in cgroup_writeback_umount().
-	 * Holding rcu_read_lock across the SB_ACTIVE check, the inode grab
-	 * and wb_queue_isw() ensures synchronize_rcu() cannot return until
-	 * the work is queued, so the subsequent flush_workqueue() will wait
-	 * for the switch.
-	 */
-	rcu_read_lock();
 	spin_lock(&wb->list_lock);
 	/*
 	 * In addition to the inodes that have completed writeback, also switch
@@ -775,7 +794,6 @@ bool cleanup_offline_cgwb(struct bdi_writeback *wb)
 
 	/* no attached inodes? bail out */
 	if (nr == 0) {
-		rcu_read_unlock();
 		atomic_dec(&isw_nr_in_flight);
 		wb_put(new_wb);
 		kfree(isw);
@@ -784,7 +802,6 @@ bool cleanup_offline_cgwb(struct bdi_writeback *wb)
 
 	trace_inode_switch_wbs_queue(wb, new_wb, nr);
 	wb_queue_isw(new_wb, isw);
-	rcu_read_unlock();
 
 	return restart;
 }
@@ -1217,39 +1234,27 @@ int cgroup_writeback_by_id(u64 bdi_id, int memcg_id,
 }
 
 /**
- * cgroup_writeback_umount - flush inode wb switches for umount
+ * cgroup_writeback_umount - wait for in-flight inode wb switches on @sb
  * @sb: target super_block
  *
- * This function is called when a super_block is about to be destroyed and
- * flushes in-flight inode wb switches.  An inode wb switch goes through
- * RCU and then workqueue, so the two need to be flushed in order to ensure
- * that all previously scheduled switches are finished.  As wb switches are
- * rare occurrences and synchronize_rcu() can take a while, perform
- * flushing iff wb switches are in flight.
+ * Wait until every inode wb switch that already passed the SB_ACTIVE
+ * check on this superblock has been completed by the worker.  Since
+ * SB_ACTIVE is cleared before this is called, no new switches can start
+ * for @sb, so s_isw_nr_in_flight will monotonically drop to zero.
  */
 void cgroup_writeback_umount(struct super_block *sb)
 {
-
 	if (!(sb->s_bdi->capabilities & BDI_CAP_WRITEBACK))
 		return;
 
 	/*
-	 * SB_ACTIVE should be reliably cleared before checking
-	 * isw_nr_in_flight, see generic_shutdown_super().
+	 * Pairs with smp_mb() in inode_prepare_wbs_switch(): we either observe
+	 * a non-zero counter and wait, or the switcher sees SB_ACTIVE clear
+	 * (cleared by generic_shutdown_super()) and bails before grabbing the
+	 * inode.
 	 */
 	smp_mb();
-
-	if (atomic_read(&isw_nr_in_flight)) {
-		/*
-		 * Paired with rcu_read_lock() in inode_switch_wbs() and
-		 * cleanup_offline_cgwb().  synchronize_rcu() waits for any
-		 * in-flight switcher that already passed the SB_ACTIVE check
-		 * to finish queueing its work, so flush_workqueue() below
-		 * will then drain it.
-		 */
-		synchronize_rcu();
-		flush_workqueue(isw_wq);
-	}
+	cgroup_writeback_drain(sb);
 }
 
 static int __init cgroup_writeback_init(void)
diff --git a/include/linux/fs/super_types.h b/include/linux/fs/super_types.h
index 383050e7fdf5..1ab4e2265129 100644
--- a/include/linux/fs/super_types.h
+++ b/include/linux/fs/super_types.h
@@ -274,6 +274,14 @@ struct super_block {
 
 	/* number of fserrors that are being sent to fsnotify/filesystems */
 	refcount_t				s_pending_errors;
+
+#ifdef CONFIG_CGROUP_WRITEBACK
+	/*
+	 * Number of in-flight inode wb switches for this sb.  Drained by
+	 * cgroup_writeback_umount() before tear-down.
+	 */
+	atomic_t				s_isw_nr_in_flight;
+#endif
 } __randomize_layout;
 
 /*
-- 
2.43.7

Re: [PATCH v3 3/3] writeback: use a per-sb counter to drain inode wb switches at umount

[Baokun Li]

Sashiko[1] flagged a stale comment in patch 3 — the rcu_read_lock()

comment still describes the old synchronize_rcu()/flush_workqueue()
scheme which no longer exists after this patch.

I'll fold the following fixup into the next version:

--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -688,16 +688,8 @@ static void inode_switch_wbs(struct inode *inode,
int new_wb_id)
 
        atomic_inc(&isw_nr_in_flight);
 
-       /*
-        * Paired with synchronize_rcu() in cgroup_writeback_umount():
-        * holding rcu_read_lock across inode_prepare_wbs_switch()
-        * (covering the SB_ACTIVE check and the inode grab) and
-        * wb_queue_isw() ensures synchronize_rcu() cannot return until
-        * the work is queued, so the subsequent flush_workqueue() will
-        * wait for the switch.
-        */
-       rcu_read_lock();
        /* find and pin the new wb */
+       rcu_read_lock();
        memcg_css = css_from_id(new_wb_id, &memory_cgrp_subsys);
        if (memcg_css && !css_tryget(memcg_css))
                memcg_css = NULL;

In no hurry to resend — further review is welcome.

[1]
https://sashiko.dev/#/patchset/20260518135349.1187628-1-libaokun%40linux.alibaba.com


Thanks,
Baokun

On 2026/5/18 21:53, Baokun Li wrote:
> Tracking in-flight inode wb switches with a single global counter
> (isw_nr_in_flight) plus a synchronize_rcu() based wait in
> cgroup_writeback_umount() forces every umount to take a global hit
> whenever any other superblock on the system has wb switches in flight,
> even if the superblock being unmounted has none of its own.
>
> Replace the global synchronize_rcu()/flush_workqueue() pair with a
> per-sb counter, s_isw_nr_in_flight, plus three small helpers:
>
>   - cgroup_writeback_pin(sb)   - increment counter
>   - cgroup_writeback_unpin(sb) - decrement and wake drainer if last
>   - cgroup_writeback_drain(sb) - wait for counter to reach zero
>
> The wiring is:
>
>   - inode_prepare_wbs_switch() pins before checking SB_ACTIVE and
>     grabbing the inode; failure paths unpin before returning.  A
>     lockless SB_ACTIVE check at the top of the function lets us skip
>     the atomic_inc/smp_mb dance once SB_ACTIVE has been cleared (it
>     is monotonic and never set back).
>   - process_inode_switch_wbs() unpins after the matching iput().
>   - cgroup_writeback_umount() drains the per-sb counter via
>     wait_var_event().
>
> The smp_mb() pair between inode_prepare_wbs_switch() and
> cgroup_writeback_umount() keeps the SB_ACTIVE / counter ordering:
> either the umounter sees a non-zero counter and waits, or the
> switcher sees SB_ACTIVE cleared and aborts before grabbing the
> inode.
>
> The global isw_nr_in_flight is left in place, since it is still used
> to throttle in-flight switches via WB_FRN_MAX_IN_FLIGHT.
>
> The rcu_read_lock() extension in inode_switch_wbs() and
> cleanup_offline_cgwb() that the race fix added is no longer needed
> and is reverted; the synchronize_rcu() that the race fix added to
> cgroup_writeback_umount() is dropped as well.
>
> The following numbers were measured on a 16 vCPU QEMU guest with 4
> background superblocks each churning "create memcg -> write 1 MiB ->
> rmdir memcg" to keep the global isw_nr_in_flight non-zero.  Latencies
> are wall-clock around umount(8); only the target sb's umount is
> measured.
>
> Target sb runs its own cgwb churn:
>
>                               p50      p95      p99      max
>   global synchronize_rcu()   67.6 ms  88.3 ms  88.3 ms  96.8 ms
>   per-sb counter (this)       7.9 ms  10.0 ms  10.0 ms  10.1 ms
>
> Idle target umount latency under cross-sb cgwb-switch pressure:
>
>                               p50      p95      p99      max
>   global synchronize_rcu()   62.7 ms  95.4 ms 108.1 ms 108.6 ms
>   per-sb counter (this)       5.3 ms   6.9 ms   7.4 ms   7.4 ms
>   no-pressure baseline        4.9 ms   5.9 ms   6.3 ms   6.7 ms
>
> 8 concurrent umounts of idle sbs under the same pressure:
>
>                               p50      p95      max
>   global synchronize_rcu()   61.3 ms  99.5 ms 113.7 ms
>   per-sb counter (this)       8.1 ms   9.1 ms   9.5 ms
>
> In-kernel cgroup_writeback_umount() time across the same run
> (bpftrace, ~340 calls covering all scenarios):
>
>   global synchronize_rcu()    12371 ms total (~36 ms / call)
>   per-sb counter (this)        1.37 ms total ( ~4 us / call)
>
> Suggested-by: Christian Brauner <brauner@kernel.org>
> Link: https://lore.kernel.org/r/177910456953.488929.2169908940676707307.b4-review@b4
> Signed-off-by: Baokun Li <libaokun@linux.alibaba.com>
> ---
>  fs/fs-writeback.c              | 87 ++++++++++++++++++----------------
>  include/linux/fs/super_types.h |  8 ++++
>  2 files changed, 54 insertions(+), 41 deletions(-)
>
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index 325a30cc35bf..32fec4b9094e 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -497,6 +497,23 @@ static bool inode_do_switch_wbs(struct inode *inode,
>  	return switched;
>  }
>  
> +static inline void cgroup_writeback_pin(struct super_block *sb)
> +{
> +	atomic_inc(&sb->s_isw_nr_in_flight);
> +}
> +
> +static inline void cgroup_writeback_unpin(struct super_block *sb)
> +{
> +	if (atomic_dec_and_test(&sb->s_isw_nr_in_flight))
> +		wake_up_var(&sb->s_isw_nr_in_flight);
> +}
> +
> +static inline void cgroup_writeback_drain(struct super_block *sb)
> +{
> +	wait_var_event(&sb->s_isw_nr_in_flight,
> +		       !atomic_read(&sb->s_isw_nr_in_flight));
> +}
> +
>  static void process_inode_switch_wbs(struct bdi_writeback *new_wb,
>  				     struct inode_switch_wbs_context *isw)
>  {
> @@ -554,8 +571,12 @@ static void process_inode_switch_wbs(struct bdi_writeback *new_wb,
>  		wb_put_many(old_wb, nr_switched);
>  	}
>  
> -	for (inodep = isw->inodes; *inodep; inodep++)
> +	for (inodep = isw->inodes; *inodep; inodep++) {
> +		struct super_block *sb = (*inodep)->i_sb;
> +
>  		iput(*inodep);
> +		cgroup_writeback_unpin(sb);
> +	}
>  	wb_put(new_wb);
>  	kfree(isw);
>  	atomic_dec(&isw_nr_in_flight);
> @@ -598,16 +619,19 @@ void inode_switch_wbs_work_fn(struct work_struct *work)
>  static bool inode_prepare_wbs_switch(struct inode *inode,
>  				     struct bdi_writeback *new_wb)
>  {
> +	/* Avoid the atomic_inc/smp_mb dance once SB_ACTIVE is gone. */
> +	if (!(inode->i_sb->s_flags & SB_ACTIVE))
> +		return false;
> +
>  	/*
> -	 * Paired with smp_mb() in cgroup_writeback_umount().
> -	 * isw_nr_in_flight must be increased before checking SB_ACTIVE and
> -	 * grabbing an inode, otherwise isw_nr_in_flight can be observed as 0
> -	 * in cgroup_writeback_umount() and the isw_wq will be not flushed.
> +	 * Pairs with smp_mb() in cgroup_writeback_umount(): the umounter either
> +	 * sees a non-zero counter and waits, or we see SB_ACTIVE clear below.
>  	 */
> +	cgroup_writeback_pin(inode->i_sb);
>  	smp_mb();
>  
>  	if (IS_DAX(inode))
> -		return false;
> +		goto out_dec;
>  
>  	/* while holding I_WB_SWITCH, no one else can update the association */
>  	spin_lock(&inode->i_lock);
> @@ -615,13 +639,17 @@ static bool inode_prepare_wbs_switch(struct inode *inode,
>  	    inode_state_read(inode) & (I_WB_SWITCH | I_FREEING | I_WILL_FREE) ||
>  	    inode_to_wb(inode) == new_wb) {
>  		spin_unlock(&inode->i_lock);
> -		return false;
> +		goto out_dec;
>  	}
>  	inode_state_set(inode, I_WB_SWITCH);
>  	__iget(inode);
>  	spin_unlock(&inode->i_lock);
>  
>  	return true;
> +
> +out_dec:
> +	cgroup_writeback_unpin(inode->i_sb);
> +	return false;
>  }
>  
>  static void wb_queue_isw(struct bdi_writeback *wb,
> @@ -673,6 +701,7 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id)
>  	memcg_css = css_from_id(new_wb_id, &memory_cgrp_subsys);
>  	if (memcg_css && !css_tryget(memcg_css))
>  		memcg_css = NULL;
> +	rcu_read_unlock();
>  	if (!memcg_css)
>  		goto out_free;
>  
> @@ -688,11 +717,9 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id)
>  
>  	trace_inode_switch_wbs_queue(inode->i_wb, new_wb, 1);
>  	wb_queue_isw(new_wb, isw);
> -	rcu_read_unlock();
>  	return;
>  
>  out_free:
> -	rcu_read_unlock();
>  	atomic_dec(&isw_nr_in_flight);
>  	if (new_wb)
>  		wb_put(new_wb);
> @@ -750,14 +777,6 @@ bool cleanup_offline_cgwb(struct bdi_writeback *wb)
>  		new_wb = &wb->bdi->wb; /* wb_get() is noop for bdi's wb */
>  
>  	nr = 0;
> -	/*
> -	 * Paired with synchronize_rcu() in cgroup_writeback_umount().
> -	 * Holding rcu_read_lock across the SB_ACTIVE check, the inode grab
> -	 * and wb_queue_isw() ensures synchronize_rcu() cannot return until
> -	 * the work is queued, so the subsequent flush_workqueue() will wait
> -	 * for the switch.
> -	 */
> -	rcu_read_lock();
>  	spin_lock(&wb->list_lock);
>  	/*
>  	 * In addition to the inodes that have completed writeback, also switch
> @@ -775,7 +794,6 @@ bool cleanup_offline_cgwb(struct bdi_writeback *wb)
>  
>  	/* no attached inodes? bail out */
>  	if (nr == 0) {
> -		rcu_read_unlock();
>  		atomic_dec(&isw_nr_in_flight);
>  		wb_put(new_wb);
>  		kfree(isw);
> @@ -784,7 +802,6 @@ bool cleanup_offline_cgwb(struct bdi_writeback *wb)
>  
>  	trace_inode_switch_wbs_queue(wb, new_wb, nr);
>  	wb_queue_isw(new_wb, isw);
> -	rcu_read_unlock();
>  
>  	return restart;
>  }
> @@ -1217,39 +1234,27 @@ int cgroup_writeback_by_id(u64 bdi_id, int memcg_id,
>  }
>  
>  /**
> - * cgroup_writeback_umount - flush inode wb switches for umount
> + * cgroup_writeback_umount - wait for in-flight inode wb switches on @sb
>   * @sb: target super_block
>   *
> - * This function is called when a super_block is about to be destroyed and
> - * flushes in-flight inode wb switches.  An inode wb switch goes through
> - * RCU and then workqueue, so the two need to be flushed in order to ensure
> - * that all previously scheduled switches are finished.  As wb switches are
> - * rare occurrences and synchronize_rcu() can take a while, perform
> - * flushing iff wb switches are in flight.
> + * Wait until every inode wb switch that already passed the SB_ACTIVE
> + * check on this superblock has been completed by the worker.  Since
> + * SB_ACTIVE is cleared before this is called, no new switches can start
> + * for @sb, so s_isw_nr_in_flight will monotonically drop to zero.
>   */
>  void cgroup_writeback_umount(struct super_block *sb)
>  {
> -
>  	if (!(sb->s_bdi->capabilities & BDI_CAP_WRITEBACK))
>  		return;
>  
>  	/*
> -	 * SB_ACTIVE should be reliably cleared before checking
> -	 * isw_nr_in_flight, see generic_shutdown_super().
> +	 * Pairs with smp_mb() in inode_prepare_wbs_switch(): we either observe
> +	 * a non-zero counter and wait, or the switcher sees SB_ACTIVE clear
> +	 * (cleared by generic_shutdown_super()) and bails before grabbing the
> +	 * inode.
>  	 */
>  	smp_mb();
> -
> -	if (atomic_read(&isw_nr_in_flight)) {
> -		/*
> -		 * Paired with rcu_read_lock() in inode_switch_wbs() and
> -		 * cleanup_offline_cgwb().  synchronize_rcu() waits for any
> -		 * in-flight switcher that already passed the SB_ACTIVE check
> -		 * to finish queueing its work, so flush_workqueue() below
> -		 * will then drain it.
> -		 */
> -		synchronize_rcu();
> -		flush_workqueue(isw_wq);
> -	}
> +	cgroup_writeback_drain(sb);
>  }
>  
>  static int __init cgroup_writeback_init(void)
> diff --git a/include/linux/fs/super_types.h b/include/linux/fs/super_types.h
> index 383050e7fdf5..1ab4e2265129 100644
> --- a/include/linux/fs/super_types.h
> +++ b/include/linux/fs/super_types.h
> @@ -274,6 +274,14 @@ struct super_block {
>  
>  	/* number of fserrors that are being sent to fsnotify/filesystems */
>  	refcount_t				s_pending_errors;
> +
> +#ifdef CONFIG_CGROUP_WRITEBACK
> +	/*
> +	 * Number of in-flight inode wb switches for this sb.  Drained by
> +	 * cgroup_writeback_umount() before tear-down.
> +	 */
> +	atomic_t				s_isw_nr_in_flight;
> +#endif
>  } __randomize_layout;
>  
>  /*

Re: [PATCH v3 3/3] writeback: use a per-sb counter to drain inode wb switches at umount

[Jan Kara]

On Mon 18-05-26 21:53:49, Baokun Li wrote:
> Tracking in-flight inode wb switches with a single global counter
> (isw_nr_in_flight) plus a synchronize_rcu() based wait in
> cgroup_writeback_umount() forces every umount to take a global hit
> whenever any other superblock on the system has wb switches in flight,
> even if the superblock being unmounted has none of its own.
> 
> Replace the global synchronize_rcu()/flush_workqueue() pair with a
> per-sb counter, s_isw_nr_in_flight, plus three small helpers:
> 
>   - cgroup_writeback_pin(sb)   - increment counter
>   - cgroup_writeback_unpin(sb) - decrement and wake drainer if last
>   - cgroup_writeback_drain(sb) - wait for counter to reach zero
> 
> The wiring is:
> 
>   - inode_prepare_wbs_switch() pins before checking SB_ACTIVE and
>     grabbing the inode; failure paths unpin before returning.  A
>     lockless SB_ACTIVE check at the top of the function lets us skip
>     the atomic_inc/smp_mb dance once SB_ACTIVE has been cleared (it
>     is monotonic and never set back).
>   - process_inode_switch_wbs() unpins after the matching iput().
>   - cgroup_writeback_umount() drains the per-sb counter via
>     wait_var_event().
> 
> The smp_mb() pair between inode_prepare_wbs_switch() and
> cgroup_writeback_umount() keeps the SB_ACTIVE / counter ordering:
> either the umounter sees a non-zero counter and waits, or the
> switcher sees SB_ACTIVE cleared and aborts before grabbing the
> inode.
> 
> The global isw_nr_in_flight is left in place, since it is still used
> to throttle in-flight switches via WB_FRN_MAX_IN_FLIGHT.
> 
> The rcu_read_lock() extension in inode_switch_wbs() and
> cleanup_offline_cgwb() that the race fix added is no longer needed
> and is reverted; the synchronize_rcu() that the race fix added to
> cgroup_writeback_umount() is dropped as well.
> 
> The following numbers were measured on a 16 vCPU QEMU guest with 4
> background superblocks each churning "create memcg -> write 1 MiB ->
> rmdir memcg" to keep the global isw_nr_in_flight non-zero.  Latencies
> are wall-clock around umount(8); only the target sb's umount is
> measured.
> 
> Target sb runs its own cgwb churn:
> 
>                               p50      p95      p99      max
>   global synchronize_rcu()   67.6 ms  88.3 ms  88.3 ms  96.8 ms
>   per-sb counter (this)       7.9 ms  10.0 ms  10.0 ms  10.1 ms
> 
> Idle target umount latency under cross-sb cgwb-switch pressure:
> 
>                               p50      p95      p99      max
>   global synchronize_rcu()   62.7 ms  95.4 ms 108.1 ms 108.6 ms
>   per-sb counter (this)       5.3 ms   6.9 ms   7.4 ms   7.4 ms
>   no-pressure baseline        4.9 ms   5.9 ms   6.3 ms   6.7 ms
> 
> 8 concurrent umounts of idle sbs under the same pressure:
> 
>                               p50      p95      max
>   global synchronize_rcu()   61.3 ms  99.5 ms 113.7 ms
>   per-sb counter (this)       8.1 ms   9.1 ms   9.5 ms
> 
> In-kernel cgroup_writeback_umount() time across the same run
> (bpftrace, ~340 calls covering all scenarios):
> 
>   global synchronize_rcu()    12371 ms total (~36 ms / call)
>   per-sb counter (this)        1.37 ms total ( ~4 us / call)
> 
> Suggested-by: Christian Brauner <brauner@kernel.org>
> Link: https://lore.kernel.org/r/177910456953.488929.2169908940676707307.b4-review@b4
> Signed-off-by: Baokun Li <libaokun@linux.alibaba.com>

Looks good. Feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

								Honza

> ---
>  fs/fs-writeback.c              | 87 ++++++++++++++++++----------------
>  include/linux/fs/super_types.h |  8 ++++
>  2 files changed, 54 insertions(+), 41 deletions(-)
> 
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index 325a30cc35bf..32fec4b9094e 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -497,6 +497,23 @@ static bool inode_do_switch_wbs(struct inode *inode,
>  	return switched;
>  }
>  
> +static inline void cgroup_writeback_pin(struct super_block *sb)
> +{
> +	atomic_inc(&sb->s_isw_nr_in_flight);
> +}
> +
> +static inline void cgroup_writeback_unpin(struct super_block *sb)
> +{
> +	if (atomic_dec_and_test(&sb->s_isw_nr_in_flight))
> +		wake_up_var(&sb->s_isw_nr_in_flight);
> +}
> +
> +static inline void cgroup_writeback_drain(struct super_block *sb)
> +{
> +	wait_var_event(&sb->s_isw_nr_in_flight,
> +		       !atomic_read(&sb->s_isw_nr_in_flight));
> +}
> +
>  static void process_inode_switch_wbs(struct bdi_writeback *new_wb,
>  				     struct inode_switch_wbs_context *isw)
>  {
> @@ -554,8 +571,12 @@ static void process_inode_switch_wbs(struct bdi_writeback *new_wb,
>  		wb_put_many(old_wb, nr_switched);
>  	}
>  
> -	for (inodep = isw->inodes; *inodep; inodep++)
> +	for (inodep = isw->inodes; *inodep; inodep++) {
> +		struct super_block *sb = (*inodep)->i_sb;
> +
>  		iput(*inodep);
> +		cgroup_writeback_unpin(sb);
> +	}
>  	wb_put(new_wb);
>  	kfree(isw);
>  	atomic_dec(&isw_nr_in_flight);
> @@ -598,16 +619,19 @@ void inode_switch_wbs_work_fn(struct work_struct *work)
>  static bool inode_prepare_wbs_switch(struct inode *inode,
>  				     struct bdi_writeback *new_wb)
>  {
> +	/* Avoid the atomic_inc/smp_mb dance once SB_ACTIVE is gone. */
> +	if (!(inode->i_sb->s_flags & SB_ACTIVE))
> +		return false;
> +
>  	/*
> -	 * Paired with smp_mb() in cgroup_writeback_umount().
> -	 * isw_nr_in_flight must be increased before checking SB_ACTIVE and
> -	 * grabbing an inode, otherwise isw_nr_in_flight can be observed as 0
> -	 * in cgroup_writeback_umount() and the isw_wq will be not flushed.
> +	 * Pairs with smp_mb() in cgroup_writeback_umount(): the umounter either
> +	 * sees a non-zero counter and waits, or we see SB_ACTIVE clear below.
>  	 */
> +	cgroup_writeback_pin(inode->i_sb);
>  	smp_mb();
>  
>  	if (IS_DAX(inode))
> -		return false;
> +		goto out_dec;
>  
>  	/* while holding I_WB_SWITCH, no one else can update the association */
>  	spin_lock(&inode->i_lock);
> @@ -615,13 +639,17 @@ static bool inode_prepare_wbs_switch(struct inode *inode,
>  	    inode_state_read(inode) & (I_WB_SWITCH | I_FREEING | I_WILL_FREE) ||
>  	    inode_to_wb(inode) == new_wb) {
>  		spin_unlock(&inode->i_lock);
> -		return false;
> +		goto out_dec;
>  	}
>  	inode_state_set(inode, I_WB_SWITCH);
>  	__iget(inode);
>  	spin_unlock(&inode->i_lock);
>  
>  	return true;
> +
> +out_dec:
> +	cgroup_writeback_unpin(inode->i_sb);
> +	return false;
>  }
>  
>  static void wb_queue_isw(struct bdi_writeback *wb,
> @@ -673,6 +701,7 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id)
>  	memcg_css = css_from_id(new_wb_id, &memory_cgrp_subsys);
>  	if (memcg_css && !css_tryget(memcg_css))
>  		memcg_css = NULL;
> +	rcu_read_unlock();
>  	if (!memcg_css)
>  		goto out_free;
>  
> @@ -688,11 +717,9 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id)
>  
>  	trace_inode_switch_wbs_queue(inode->i_wb, new_wb, 1);
>  	wb_queue_isw(new_wb, isw);
> -	rcu_read_unlock();
>  	return;
>  
>  out_free:
> -	rcu_read_unlock();
>  	atomic_dec(&isw_nr_in_flight);
>  	if (new_wb)
>  		wb_put(new_wb);
> @@ -750,14 +777,6 @@ bool cleanup_offline_cgwb(struct bdi_writeback *wb)
>  		new_wb = &wb->bdi->wb; /* wb_get() is noop for bdi's wb */
>  
>  	nr = 0;
> -	/*
> -	 * Paired with synchronize_rcu() in cgroup_writeback_umount().
> -	 * Holding rcu_read_lock across the SB_ACTIVE check, the inode grab
> -	 * and wb_queue_isw() ensures synchronize_rcu() cannot return until
> -	 * the work is queued, so the subsequent flush_workqueue() will wait
> -	 * for the switch.
> -	 */
> -	rcu_read_lock();
>  	spin_lock(&wb->list_lock);
>  	/*
>  	 * In addition to the inodes that have completed writeback, also switch
> @@ -775,7 +794,6 @@ bool cleanup_offline_cgwb(struct bdi_writeback *wb)
>  
>  	/* no attached inodes? bail out */
>  	if (nr == 0) {
> -		rcu_read_unlock();
>  		atomic_dec(&isw_nr_in_flight);
>  		wb_put(new_wb);
>  		kfree(isw);
> @@ -784,7 +802,6 @@ bool cleanup_offline_cgwb(struct bdi_writeback *wb)
>  
>  	trace_inode_switch_wbs_queue(wb, new_wb, nr);
>  	wb_queue_isw(new_wb, isw);
> -	rcu_read_unlock();
>  
>  	return restart;
>  }
> @@ -1217,39 +1234,27 @@ int cgroup_writeback_by_id(u64 bdi_id, int memcg_id,
>  }
>  
>  /**
> - * cgroup_writeback_umount - flush inode wb switches for umount
> + * cgroup_writeback_umount - wait for in-flight inode wb switches on @sb
>   * @sb: target super_block
>   *
> - * This function is called when a super_block is about to be destroyed and
> - * flushes in-flight inode wb switches.  An inode wb switch goes through
> - * RCU and then workqueue, so the two need to be flushed in order to ensure
> - * that all previously scheduled switches are finished.  As wb switches are
> - * rare occurrences and synchronize_rcu() can take a while, perform
> - * flushing iff wb switches are in flight.
> + * Wait until every inode wb switch that already passed the SB_ACTIVE
> + * check on this superblock has been completed by the worker.  Since
> + * SB_ACTIVE is cleared before this is called, no new switches can start
> + * for @sb, so s_isw_nr_in_flight will monotonically drop to zero.
>   */
>  void cgroup_writeback_umount(struct super_block *sb)
>  {
> -
>  	if (!(sb->s_bdi->capabilities & BDI_CAP_WRITEBACK))
>  		return;
>  
>  	/*
> -	 * SB_ACTIVE should be reliably cleared before checking
> -	 * isw_nr_in_flight, see generic_shutdown_super().
> +	 * Pairs with smp_mb() in inode_prepare_wbs_switch(): we either observe
> +	 * a non-zero counter and wait, or the switcher sees SB_ACTIVE clear
> +	 * (cleared by generic_shutdown_super()) and bails before grabbing the
> +	 * inode.
>  	 */
>  	smp_mb();
> -
> -	if (atomic_read(&isw_nr_in_flight)) {
> -		/*
> -		 * Paired with rcu_read_lock() in inode_switch_wbs() and
> -		 * cleanup_offline_cgwb().  synchronize_rcu() waits for any
> -		 * in-flight switcher that already passed the SB_ACTIVE check
> -		 * to finish queueing its work, so flush_workqueue() below
> -		 * will then drain it.
> -		 */
> -		synchronize_rcu();
> -		flush_workqueue(isw_wq);
> -	}
> +	cgroup_writeback_drain(sb);
>  }
>  
>  static int __init cgroup_writeback_init(void)
> diff --git a/include/linux/fs/super_types.h b/include/linux/fs/super_types.h
> index 383050e7fdf5..1ab4e2265129 100644
> --- a/include/linux/fs/super_types.h
> +++ b/include/linux/fs/super_types.h
> @@ -274,6 +274,14 @@ struct super_block {
>  
>  	/* number of fserrors that are being sent to fsnotify/filesystems */
>  	refcount_t				s_pending_errors;
> +
> +#ifdef CONFIG_CGROUP_WRITEBACK
> +	/*
> +	 * Number of in-flight inode wb switches for this sb.  Drained by
> +	 * cgroup_writeback_umount() before tear-down.
> +	 */
> +	atomic_t				s_isw_nr_in_flight;
> +#endif
>  } __randomize_layout;
>  
>  /*
> -- 
> 2.43.7
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR