[PATCH v2 3/3] writeback: use a per-sb counter to drain inode wb switches at umount
From: Baokun Li
Date: Sun May 17 2026 - 10:22:54 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:
- inode_prepare_wbs_switch() increments before checking SB_ACTIVE
and grabbing the inode; failure paths decrement before returning.
- process_inode_switch_wbs() decrements after the matching iput().
- cgroup_writeback_umount() waits for the per-sb counter to reach zero.
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 existing 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() introduced by the race fix is no longer
needed and is reverted. The synchronize_rcu() in
cgroup_writeback_umount() is dropped as well.
inode_prepare_wbs_switch() also gains a lockless SB_ACTIVE check at
the top to skip the atomic_inc/smp_mb cost for inodes whose sb is
already being torn down.
The following numbers were measured with 4 background superblocks
each churning "create memcg -> write 1 MiB -> rmdir memcg" to keep
the global isw_nr_in_flight non-zero. A separate idle target sb is
mounted and umounted in a loop (N=100); only its umount latency is
measured.
Idle target umount latency under cross-sb cgwb-switch pressure:
p50 p95 p99 max
global synchronize_rcu() 64.4 ms 95.8 ms 101.4 ms 110.5 ms
per-sb counter (this) 5.3 ms 6.9 ms 7.4 ms 7.7 ms
no-pressure baseline 5.2 ms 5.9 ms 6.0 ms 6.1 ms
8 concurrent umounts of idle sbs under the same pressure:
p50 p95 max
global synchronize_rcu() 57.9 ms 82.1 ms 90.0 ms
per-sb counter (this) 7.5 ms 7.8 ms 8.0 ms
In-kernel cgroup_writeback_umount() time over 286 calls (bpftrace):
global synchronize_rcu() 8717 ms total (~30 ms / call)
per-sb counter (this) 1.16 ms total (~4 us / call)
When s_isw_nr_in_flight is zero the loop body is never entered.
Signed-off-by: Baokun Li <libaokun@xxxxxxxxxxxxxxxxx>
---
fs/fs-writeback.c | 74 ++++++++++++++--------------------
include/linux/fs/super_types.h | 8 ++++
2 files changed, 38 insertions(+), 44 deletions(-)
diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 9ae290547eb2..4282fcfe027b 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -554,8 +554,12 @@ 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);
+ atomic_dec(&sb->s_isw_nr_in_flight);
+ }
wb_put(new_wb);
kfree(isw);
atomic_dec(&isw_nr_in_flight);
@@ -598,16 +602,19 @@ 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.
*/
+ atomic_inc(&inode->i_sb->s_isw_nr_in_flight);
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 +622,17 @@ 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:
+ atomic_dec(&inode->i_sb->s_isw_nr_in_flight);
+ return false;
}
static void wb_queue_isw(struct bdi_writeback *wb,
@@ -665,6 +676,7 @@ 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;
@@ -679,18 +691,10 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id)
isw->inodes[0] = inode;
trace_inode_switch_wbs_queue(inode->i_wb, new_wb, 1);
- /*
- * Paired with synchronize_rcu() in cgroup_writeback_umount().
- * Holding rcu_read_lock across wb_queue_isw() ensures
- * synchronize_rcu() cannot return until the work is queued, so
- * the subsequent flush_workqueue() will wait for the switch.
- */
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);
@@ -748,14 +752,6 @@ 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
@@ -773,7 +769,6 @@ 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);
@@ -782,7 +777,6 @@ 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;
}
@@ -1215,38 +1209,30 @@ 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();
+ while (atomic_read(&sb->s_isw_nr_in_flight)) {
flush_workqueue(isw_wq);
+ cond_resched();
}
}
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