[PATCH v3 3/3] writeback: use a per-sb counter to drain inode wb switches at umount
From: Baokun Li
Date: Mon May 18 2026 - 10:00:15 EST
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@xxxxxxxxxx>
Link: https://lore.kernel.org/r/177910456953.488929.2169908940676707307.b4-review@b4
Signed-off-by: Baokun Li <libaokun@xxxxxxxxxxxxxxxxx>
---
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