[PATCH 4.4 32/74] writeback: flush inode cgroup wb switches instead of pinning super_block

Greg Kroah-Hartman
2016-03-08T00:18:12Z

From: Tejun Heo <tj@kernel.org>

commit a1a0e23e49037c23ea84bc8cc146a03584d13577 upstream.

If cgroup writeback is in use, inodes can be scheduled for
asynchronous wb switching. Before 5ff8eaac1636 ("writeback: keep
superblock pinned during cgroup writeback association switches"), this
could race with umount leading to super_block being destroyed while
inodes are pinned for wb switching. 5ff8eaac1636 fixed it by bumping
s_active while wb switches are in flight; however, this allowed
in-flight wb switches to make umounts asynchronous when the userland
expected synchronosity - e.g. fsck immediately following umount may
fail because the device is still busy.

This patch removes the problematic super_block pinning and instead
makes generic_shutdown_super() flush in-flight wb switches. wb
switches are now executed on a dedicated isw_wq so that they can be
flushed and isw_nr_in_flight keeps track of the number of in-flight wb
switches so that flushing can be avoided in most cases.

v2: Move cgroup_writeback_umount() further below and add MS_ACTIVE
check in inode_switch_wbs() as Jan an Al suggested.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Tahsin Erdogan <tahsin@google.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Link: http://lkml.kernel.org/g/CAAeU0aNCq7LGODvVGRU-oU_o-6enii5ey0p1c26D1ZzYwkDc5A@mail.gmail.com
Fixes: 5ff8eaac1636 ("writeback: keep superblock pinned during cgroup writeback association switches")
Reviewed-by: Jan Kara <jack@suse.cz>
Tested-by: Tahsin Erdogan <tahsin@google.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
fs/fs-writeback.c | 54 ++++++++++++++++++++++++++++++++++------------
fs/super.c | 1
include/linux/writeback.h | 5 ++++
3 files changed, 47 insertions(+), 13 deletions(-)

--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -223,6 +223,9 @@ static void wb_wait_for_completion(struc
#define WB_FRN_HIST_MAX_SLOTS (WB_FRN_HIST_THR_SLOTS / 2 + 1)
/* one round can affect upto 5 slots */

+static atomic_t isw_nr_in_flight = ATOMIC_INIT(0);
+static struct workqueue_struct *isw_wq;
+
void __inode_attach_wb(struct inode *inode, struct page *page)
{
struct backing_dev_info *bdi = inode_to_bdi(inode);
@@ -317,7 +320,6 @@ static void inode_switch_wbs_work_fn(str
struct inode_switch_wbs_context *isw =
container_of(work, struct inode_switch_wbs_context, work);
struct inode *inode = isw->inode;
- struct super_block *sb = inode->i_sb;
struct address_space *mapping = inode->i_mapping;
struct bdi_writeback *old_wb = inode->i_wb;
struct bdi_writeback *new_wb = isw->new_wb;
@@ -424,8 +426,9 @@ skip_switch:
wb_put(new_wb);

iput(inode);
- deactivate_super(sb);
kfree(isw);
+
+ atomic_dec(&isw_nr_in_flight);
}

static void inode_switch_wbs_rcu_fn(struct rcu_head *rcu_head)
@@ -435,7 +438,7 @@ static void inode_switch_wbs_rcu_fn(stru

/* needs to grab bh-unsafe locks, bounce to work item */
INIT_WORK(&isw->work, inode_switch_wbs_work_fn);
- schedule_work(&isw->work);
+ queue_work(isw_wq, &isw->work);
}

/**
@@ -471,20 +474,20 @@ static void inode_switch_wbs(struct inod

/* while holding I_WB_SWITCH, no one else can update the association */
spin_lock(&inode->i_lock);
-
- if (inode->i_state & (I_WB_SWITCH | I_FREEING) ||
- inode_to_wb(inode) == isw->new_wb)
- goto out_unlock;
-
- if (!atomic_inc_not_zero(&inode->i_sb->s_active))
- goto out_unlock;
-
+ if (!(inode->i_sb->s_flags & MS_ACTIVE) ||
+ inode->i_state & (I_WB_SWITCH | I_FREEING) ||
+ inode_to_wb(inode) == isw->new_wb) {
+ spin_unlock(&inode->i_lock);
+ goto out_free;
+ }
inode->i_state |= I_WB_SWITCH;
spin_unlock(&inode->i_lock);

ihold(inode);
isw->inode = inode;

+ atomic_inc(&isw_nr_in_flight);
+
/*
* In addition to synchronizing among switchers, I_WB_SWITCH tells
* the RCU protected stat update paths to grab the mapping's
@@ -494,8 +497,6 @@ static void inode_switch_wbs(struct inod
call_rcu(&isw->rcu_head, inode_switch_wbs_rcu_fn);
return;

-out_unlock:
- spin_unlock(&inode->i_lock);
out_free:
if (isw->new_wb)
wb_put(isw->new_wb);
@@ -849,6 +850,33 @@ restart:
wb_put(last_wb);
}

+/**
+ * cgroup_writeback_umount - flush inode wb switches for umount
+ *
+ * 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.
+ */
+void cgroup_writeback_umount(void)
+{
+ if (atomic_read(&isw_nr_in_flight)) {
+ synchronize_rcu();
+ flush_workqueue(isw_wq);
+ }
+}
+
+static int __init cgroup_writeback_init(void)
+{
+ isw_wq = alloc_workqueue("inode_switch_wbs", 0, 0);
+ if (!isw_wq)
+ return -ENOMEM;
+ return 0;
+}
+fs_initcall(cgroup_writeback_init);
+
#else /* CONFIG_CGROUP_WRITEBACK */

static struct bdi_writeback *
--- a/fs/super.c
+++ b/fs/super.c
@@ -415,6 +415,7 @@ void generic_shutdown_super(struct super
sb->s_flags &= ~MS_ACTIVE;

fsnotify_unmount_inodes(sb);
+ cgroup_writeback_umount();

evict_inodes(sb);

--- a/include/linux/writeback.h
+++ b/include/linux/writeback.h
@@ -198,6 +198,7 @@ void wbc_attach_and_unlock_inode(struct
void wbc_detach_inode(struct writeback_control *wbc);
void wbc_account_io(struct writeback_control *wbc, struct page *page,
size_t bytes);
+void cgroup_writeback_umount(void);

/**
* inode_attach_wb - associate an inode with its wb
@@ -301,6 +302,10 @@ static inline void wbc_account_io(struct
{
}

+static inline void cgroup_writeback_umount(void)
+{
+}
+
#endif /* CONFIG_CGROUP_WRITEBACK */

/*