[PATCH 0/4] Post merge per-bdi writeback patches

[PATCH 0/4] Post merge per-bdi writeback patches

[Jens Axboe]

Hi,

This is what I currently have queued up for post merge of the
writeback patchset. It contains:

- The bdi work allocation and queue cleanup from Christoph.
- An sb->s_bdi pointer that gets assigned in get_sb_bdev(), OR
  from the fs ->fill_super() if the file system doesn't use
  get_sb_bdev(). It is required for WB_SYNC_ALL writeback. Someone
  more familiar with this code should check that it is done in the
  right place. As usual, I'm unsure of NFS...
- Once the ->s_bdi is in place, we can get rid of bdi_writeback_all()
  for WB_SYNC_ALL. WB_SYNC_ALL now has to use bdi_start_writeback().
  This cleans up bdi_writeback_all() nicely.
- With bdi_writeback_all() only doing WB_SYNC_NONE, we can drop
  the bdi_lock requirement and move to RCU for bdi_list.

Please take a good look and review/comment, thanks! This patchset
is based on top of writeback-v20. It boots and works fine for me.

 fs/btrfs/disk-io.c          |    1 
 fs/fs-writeback.c           |  111 ++++++++----------------------------
 fs/fuse/inode.c             |    2 
 fs/super.c                  |    6 +
 fs/sync.c                   |    9 ++
 fs/ubifs/super.c            |    1 
 include/linux/backing-dev.h |    1 
 include/linux/fs.h          |    1 
 mm/backing-dev.c            |   76 +++++++++++++++++-------
 mm/page-writeback.c         |    8 +-
 10 files changed, 105 insertions(+), 111 deletions(-)

-- 
Jens Axboe

[PATCH 1/4] writeback: merely wakeup flusher thread if work allocation fails for WB_SYNC_NONE

[Jens Axboe]

From: Christoph Hellwig <hch@infradead.org>

Since it's an opportunistic writeback and not a data integrity action,
don't punt to blocking writeback. Just wakeup the thread and it will
flush old data.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
---
 fs/fs-writeback.c |   46 ++++++++++++++--------------------------------
 1 files changed, 14 insertions(+), 32 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index da86ef5..1873fd0 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -75,13 +75,6 @@ static inline void bdi_work_init(struct bdi_work *work,
 	work->state = WS_USED;
 }
 
-static inline void bdi_work_init_on_stack(struct bdi_work *work,
-					  struct writeback_control *wbc)
-{
-	bdi_work_init(work, wbc);
-	work->state |= WS_ONSTACK;
-}
-
 /**
  * writeback_in_progress - determine whether there is writeback in progress
  * @bdi: the device's backing_dev_info structure.
@@ -207,34 +200,23 @@ static struct bdi_work *bdi_alloc_work(struct writeback_control *wbc)
 
 void bdi_start_writeback(struct writeback_control *wbc)
 {
-	const bool must_wait = wbc->sync_mode == WB_SYNC_ALL;
-	struct bdi_work work_stack, *work = NULL;
-
-	if (!must_wait)
-		work = bdi_alloc_work(wbc);
+	/*
+	 * WB_SYNC_NONE is opportunistic writeback. If this allocation fails,
+	 * bdi_queue_work() will wake up the thread and flush old data. This
+	 * should ensure some amount of progress in freeing memory.
+	 */
+	if (wbc->sync_mode != WB_SYNC_ALL) {
+		struct bdi_work *w = bdi_alloc_work(wbc);
 
-	if (!work) {
-		work = &work_stack;
-		bdi_work_init_on_stack(work, wbc);
-	}
+		bdi_queue_work(wbc->bdi, w);
+	} else {
+		struct bdi_work work;
 
-	bdi_queue_work(wbc->bdi, work);
+		bdi_work_init(&work, wbc);
+		work.state |= WS_ONSTACK;
 
-	/*
-	 * If the sync mode is WB_SYNC_ALL, block waiting for the work to
-	 * complete. If not, we only need to wait for the work to be started,
-	 * if we allocated it on-stack. We use the same mechanism, if the
-	 * wait bit is set in the bdi_work struct, then threads will not
-	 * clear pending until after they are done.
-	 *
-	 * Note that work == &work_stack if must_wait is true, so we don't
-	 * need to do call_rcu() here ever, since the completion path will
-	 * have done that for us.
-	 */
-	if (must_wait || work == &work_stack) {
-		bdi_wait_on_work_clear(work);
-		if (work != &work_stack)
-			call_rcu(&work->rcu_head, bdi_work_free);
+		bdi_queue_work(wbc->bdi, &work);
+		bdi_wait_on_work_clear(&work);
 	}
 }
 
-- 
1.6.4.1.207.g68ea

Re: [PATCH 1/4] writeback: merely wakeup flusher thread if work allocation fails for WB_SYNC_NONE

[Christoph Hellwig]

> +	/*
> +	 * WB_SYNC_NONE is opportunistic writeback. If this allocation fails,
> +	 * bdi_queue_work() will wake up the thread and flush old data. This
> +	 * should ensure some amount of progress in freeing memory.
> +	 */
> +	if (wbc->sync_mode != WB_SYNC_ALL) {
> +		struct bdi_work *w = bdi_alloc_work(wbc);
>  
> +		bdi_queue_work(wbc->bdi, w);
> +	} else {
> +		struct bdi_work work;
>  
> +		bdi_work_init(&work, wbc);
> +		work.state |= WS_ONSTACK;
>  
> +		bdi_queue_work(wbc->bdi, &work);
> +		bdi_wait_on_work_clear(&work);

That's even nice than my version, great.

[PATCH 2/4] Assign bdi in super_block

[Jens Axboe]

We do this automatically in get_sb_bdev() from the set_bdev_super()
callback. Filesystems that have their own private backing_dev_info
must assign that in ->fill_super().

Note that ->s_bdi assignment is required for proper writeback!

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
---
 fs/btrfs/disk-io.c |    1 +
 fs/fuse/inode.c    |    2 ++
 fs/super.c         |    6 ++++++
 fs/sync.c          |    9 ++++++++-
 fs/ubifs/super.c   |    1 +
 include/linux/fs.h |    1 +
 6 files changed, 19 insertions(+), 1 deletions(-)

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 15831d5..8b81927 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -1600,6 +1600,7 @@ struct btrfs_root *open_ctree(struct super_block *sb,
 
 	sb->s_blocksize = 4096;
 	sb->s_blocksize_bits = blksize_bits(4096);
+	sb->s_bdi = &fs_info->bdi;
 
 	/*
 	 * we set the i_size on the btree inode to the max possible int.
diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
index 4567db6..e5dbecd 100644
--- a/fs/fuse/inode.c
+++ b/fs/fuse/inode.c
@@ -894,6 +894,8 @@ static int fuse_fill_super(struct super_block *sb, void *data, int silent)
 	if (err)
 		goto err_put_conn;
 
+	sb->s_bdi = &fc->bdi;
+
 	/* Handle umasking inside the fuse code */
 	if (sb->s_flags & MS_POSIXACL)
 		fc->dont_mask = 1;
diff --git a/fs/super.c b/fs/super.c
index 9cda337..b03fea8 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -707,6 +707,12 @@ static int set_bdev_super(struct super_block *s, void *data)
 {
 	s->s_bdev = data;
 	s->s_dev = s->s_bdev->bd_dev;
+
+	/*
+	 * We set the bdi here to the queue backing, file systems can
+	 * overwrite this in ->fill_super()
+	 */
+	s->s_bdi = &bdev_get_queue(s->s_bdev)->backing_dev_info;
 	return 0;
 }
 
diff --git a/fs/sync.c b/fs/sync.c
index 103cc7f..8582c34 100644
--- a/fs/sync.c
+++ b/fs/sync.c
@@ -27,6 +27,13 @@
  */
 static int __sync_filesystem(struct super_block *sb, int wait)
 {
+	/*
+	 * This should be safe, as we require bdi backing to actually
+	 * write out data in the first place
+	 */
+	if (!sb->s_bdi)
+		return 0;
+
 	/* Avoid doing twice syncing and cache pruning for quota sync */
 	if (!wait) {
 		writeout_quota_sb(sb, -1);
@@ -101,7 +108,7 @@ restart:
 		spin_unlock(&sb_lock);
 
 		down_read(&sb->s_umount);
-		if (!(sb->s_flags & MS_RDONLY) && sb->s_root)
+		if (!(sb->s_flags & MS_RDONLY) && sb->s_root && sb->s_bdi)
 			__sync_filesystem(sb, wait);
 		up_read(&sb->s_umount);
 
diff --git a/fs/ubifs/super.c b/fs/ubifs/super.c
index 51763aa..c4af069 100644
--- a/fs/ubifs/super.c
+++ b/fs/ubifs/super.c
@@ -1980,6 +1980,7 @@ static int ubifs_fill_super(struct super_block *sb, void *data, int silent)
 	if (err)
 		goto out_bdi;
 
+	sb->s_bdi = &c->bdi;
 	sb->s_fs_info = c;
 	sb->s_magic = UBIFS_SUPER_MAGIC;
 	sb->s_blocksize = UBIFS_BLOCK_SIZE;
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 26da98f..940f2a8 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1343,6 +1343,7 @@ struct super_block {
 	int			s_nr_dentry_unused;	/* # of dentry on lru */
 
 	struct block_device	*s_bdev;
+	struct backing_dev_info *s_bdi;
 	struct mtd_info		*s_mtd;
 	struct list_head	s_instances;
 	struct quota_info	s_dquot;	/* Diskquota specific options */
-- 
1.6.4.1.207.g68ea

Re: [PATCH 2/4] Assign bdi in super_block

[Christoph Hellwig]

On Fri, Sep 11, 2009 at 01:50:30PM +0200, Jens Axboe wrote:
> We do this automatically in get_sb_bdev() from the set_bdev_super()
> callback. Filesystems that have their own private backing_dev_info
> must assign that in ->fill_super().
> 
> Note that ->s_bdi assignment is required for proper writeback!

Looks good to me.   Can we get rid of BDI_CAP_NO_WRITEBACK with this by
simply not assigning a bdi?

Re: [PATCH 2/4] Assign bdi in super_block

[Jens Axboe]

On Fri, Sep 11 2009, Christoph Hellwig wrote:
> On Fri, Sep 11, 2009 at 01:50:30PM +0200, Jens Axboe wrote:
> > We do this automatically in get_sb_bdev() from the set_bdev_super()
> > callback. Filesystems that have their own private backing_dev_info
> > must assign that in ->fill_super().
> > 
> > Note that ->s_bdi assignment is required for proper writeback!
> 
> Looks good to me.   Can we get rid of BDI_CAP_NO_WRITEBACK with this by
> simply not assigning a bdi?

Good question, I was indeed looking for an sb equivalent of that bdi
flag. And now ->s_bdi being set or not is indeed that. Some of our
generated writeback originates at the bdi level though, so we may or may
not have the sb. From a quick look at fs-writeback.c, it looks feasible
though. I'll try.

Can I take that as an acked-by?

-- 
Jens Axboe

Re: [PATCH 2/4] Assign bdi in super_block

[Christoph Hellwig]

On Fri, Sep 11, 2009 at 08:12:27PM +0200, Jens Axboe wrote:
> Good question, I was indeed looking for an sb equivalent of that bdi
> flag. And now ->s_bdi being set or not is indeed that. Some of our
> generated writeback originates at the bdi level though, so we may or may
> not have the sb. From a quick look at fs-writeback.c, it looks feasible
> though. I'll try.
> 
> Can I take that as an acked-by?

Yes.

[PATCH 3/4] writeback: only use bdi_writeback_all() for WB_SYNC_NONE writeout

[Jens Axboe]

Data integrity writeback must use bdi_start_writeback() and ensure
that wbc->sb and wbc->bdi are set.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
---
 fs/fs-writeback.c |   69 ++++++++++------------------------------------------
 1 files changed, 14 insertions(+), 55 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 1873fd0..5d4bd1c 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -187,7 +187,8 @@ static void bdi_wait_on_work_clear(struct bdi_work *work)
 		    TASK_UNINTERRUPTIBLE);
 }
 
-static struct bdi_work *bdi_alloc_work(struct writeback_control *wbc)
+static void bdi_alloc_queue_work(struct backing_dev_info *bdi,
+				 struct writeback_control *wbc)
 {
 	struct bdi_work *work;
 
@@ -195,7 +196,7 @@ static struct bdi_work *bdi_alloc_work(struct writeback_control *wbc)
 	if (work)
 		bdi_work_init(work, wbc);
 
-	return work;
+	bdi_queue_work(bdi, work);
 }
 
 void bdi_start_writeback(struct writeback_control *wbc)
@@ -205,11 +206,9 @@ void bdi_start_writeback(struct writeback_control *wbc)
 	 * bdi_queue_work() will wake up the thread and flush old data. This
 	 * should ensure some amount of progress in freeing memory.
 	 */
-	if (wbc->sync_mode != WB_SYNC_ALL) {
-		struct bdi_work *w = bdi_alloc_work(wbc);
-
-		bdi_queue_work(wbc->bdi, w);
-	} else {
+	if (wbc->sync_mode != WB_SYNC_ALL)
+		bdi_alloc_queue_work(wbc->bdi, wbc);
+	else {
 		struct bdi_work work;
 
 		bdi_work_init(&work, wbc);
@@ -840,67 +839,26 @@ int bdi_writeback_task(struct bdi_writeback *wb)
 }
 
 /*
- * Schedule writeback for all backing devices. Expensive! If this is a data
- * integrity operation, writeback will be complete when this returns. If
- * we are simply called for WB_SYNC_NONE, then writeback will merely be
- * scheduled to run.
+ * Schedule writeback for all backing devices. Can only be used for
+ * WB_SYNC_NONE writeback, WB_SYNC_ALL should use bdi_start_writeback()
+ * and pass in the superblock.
  */
 static void bdi_writeback_all(struct writeback_control *wbc)
 {
-	const bool must_wait = wbc->sync_mode == WB_SYNC_ALL;
 	struct backing_dev_info *bdi;
-	struct bdi_work *work;
-	LIST_HEAD(list);
 
-restart:
+	WARN_ON(wbc->sync_mode == WB_SYNC_ALL);
+
 	spin_lock(&bdi_lock);
 
 	list_for_each_entry(bdi, &bdi_list, bdi_list) {
-		struct bdi_work *work;
-
 		if (!bdi_has_dirty_io(bdi))
 			continue;
 
-		/*
-		 * If work allocation fails, do the writes inline. We drop
-		 * the lock and restart the list writeout. This should be OK,
-		 * since this happens rarely and because the writeout should
-		 * eventually make more free memory available.
-		 */
-		work = bdi_alloc_work(wbc);
-		if (!work) {
-			struct writeback_control __wbc;
-
-			/*
-			 * Not a data integrity writeout, just continue
-			 */
-			if (!must_wait)
-				continue;
-
-			spin_unlock(&bdi_lock);
-			__wbc = *wbc;
-			__wbc.bdi = bdi;
-			writeback_inodes_wbc(&__wbc);
-			goto restart;
-		}
-		if (must_wait)
-			list_add_tail(&work->wait_list, &list);
-
-		bdi_queue_work(bdi, work);
+		bdi_alloc_queue_work(bdi, wbc);
 	}
 
 	spin_unlock(&bdi_lock);
-
-	/*
-	 * If this is for WB_SYNC_ALL, wait for pending work to complete
-	 * before returning.
-	 */
-	while (!list_empty(&list)) {
-		work = list_entry(list.next, struct bdi_work, wait_list);
-		list_del(&work->wait_list);
-		bdi_wait_on_work_clear(work);
-		call_rcu(&work->rcu_head, bdi_work_free);
-	}
 }
 
 /*
@@ -1157,6 +1115,7 @@ long sync_inodes_sb(struct super_block *sb)
 {
 	struct writeback_control wbc = {
 		.sb		= sb,
+		.bdi		= sb->s_bdi,
 		.sync_mode	= WB_SYNC_ALL,
 		.range_start	= 0,
 		.range_end	= LLONG_MAX,
@@ -1164,7 +1123,7 @@ long sync_inodes_sb(struct super_block *sb)
 	long nr_to_write = LONG_MAX; /* doesn't actually matter */
 
 	wbc.nr_to_write = nr_to_write;
-	bdi_writeback_all(&wbc);
+	bdi_start_writeback(&wbc);
 	wait_sb_inodes(&wbc);
 	return nr_to_write - wbc.nr_to_write;
 }
-- 
1.6.4.1.207.g68ea

Re: [PATCH 3/4] writeback: only use bdi_writeback_all() for WB_SYNC_NONE writeout

[Christoph Hellwig]

> -static struct bdi_work *bdi_alloc_work(struct writeback_control *wbc)
> +static void bdi_alloc_queue_work(struct backing_dev_info *bdi,
> +				 struct writeback_control *wbc)
>  {
>  	struct bdi_work *work;
>  
> @@ -195,7 +196,7 @@ static struct bdi_work *bdi_alloc_work(struct writeback_control *wbc)
>  	if (work)
>  		bdi_work_init(work, wbc);
>  
> -	return work;
> +	bdi_queue_work(bdi, work);

This is now the only caller of bdi_queue_work that has a NULL work
argument.  I would recommend removing the !work half of bdi_queue_work
and just inline it into this function (or make it a separate helper).

>  /*
> + * Schedule writeback for all backing devices. Can only be used for
> + * WB_SYNC_NONE writeback, WB_SYNC_ALL should use bdi_start_writeback()
> + * and pass in the superblock.
>   */
>  static void bdi_writeback_all(struct writeback_control *wbc)
>  {
>  	struct backing_dev_info *bdi;
>  
> +	WARN_ON(wbc->sync_mode == WB_SYNC_ALL);
> +
>  	spin_lock(&bdi_lock);
>  
>  	list_for_each_entry(bdi, &bdi_list, bdi_list) {
>  		if (!bdi_has_dirty_io(bdi))
>  			continue;
>  
> +		bdi_alloc_queue_work(bdi, wbc);
>  	}


Much nicer than before.  Could be even further simplified to:

		if (bdi_has_dirty_io(bdi))
			bdi_alloc_queue_work(bdi, wbc);

> @@ -1157,6 +1115,7 @@ long sync_inodes_sb(struct super_block *sb)
>  {
>  	struct writeback_control wbc = {
>  		.sb		= sb,
> +		.bdi		= sb->s_bdi,
>  		.sync_mode	= WB_SYNC_ALL,
>  		.range_start	= 0,
>  		.range_end	= LLONG_MAX,
> @@ -1164,7 +1123,7 @@ long sync_inodes_sb(struct super_block *sb)
>  	long nr_to_write = LONG_MAX; /* doesn't actually matter */
>  
>  	wbc.nr_to_write = nr_to_write;
> -	bdi_writeback_all(&wbc);
> +	bdi_start_writeback(&wbc);

So here we have a WB_SYNC_ALL caller of bdi_writeback_all and the
only other caller is WB_SYNC_NONE.  Given that after patch two those
are entirely different codepathes in bdi_start_writeback I would just
split bdi_start_writeback into two separate functions.

Re: [PATCH 3/4] writeback: only use bdi_writeback_all() for WB_SYNC_NONE writeout

[Jens Axboe]

On Fri, Sep 11 2009, Christoph Hellwig wrote:
> > -static struct bdi_work *bdi_alloc_work(struct writeback_control *wbc)
> > +static void bdi_alloc_queue_work(struct backing_dev_info *bdi,
> > +				 struct writeback_control *wbc)
> >  {
> >  	struct bdi_work *work;
> >  
> > @@ -195,7 +196,7 @@ static struct bdi_work *bdi_alloc_work(struct writeback_control *wbc)
> >  	if (work)
> >  		bdi_work_init(work, wbc);
> >  
> > -	return work;
> > +	bdi_queue_work(bdi, work);
> 
> This is now the only caller of bdi_queue_work that has a NULL work
> argument.  I would recommend removing the !work half of bdi_queue_work
> and just inline it into this function (or make it a separate helper).

Yep agreed.

> >  /*
> > + * Schedule writeback for all backing devices. Can only be used for
> > + * WB_SYNC_NONE writeback, WB_SYNC_ALL should use bdi_start_writeback()
> > + * and pass in the superblock.
> >   */
> >  static void bdi_writeback_all(struct writeback_control *wbc)
> >  {
> >  	struct backing_dev_info *bdi;
> >  
> > +	WARN_ON(wbc->sync_mode == WB_SYNC_ALL);
> > +
> >  	spin_lock(&bdi_lock);
> >  
> >  	list_for_each_entry(bdi, &bdi_list, bdi_list) {
> >  		if (!bdi_has_dirty_io(bdi))
> >  			continue;
> >  
> > +		bdi_alloc_queue_work(bdi, wbc);
> >  	}
> 
> 
> Much nicer than before.  Could be even further simplified to:
> 
> 		if (bdi_has_dirty_io(bdi))
> 			bdi_alloc_queue_work(bdi, wbc);

Sure, matter of style.

> > @@ -1157,6 +1115,7 @@ long sync_inodes_sb(struct super_block *sb)
> >  {
> >  	struct writeback_control wbc = {
> >  		.sb		= sb,
> > +		.bdi		= sb->s_bdi,
> >  		.sync_mode	= WB_SYNC_ALL,
> >  		.range_start	= 0,
> >  		.range_end	= LLONG_MAX,
> > @@ -1164,7 +1123,7 @@ long sync_inodes_sb(struct super_block *sb)
> >  	long nr_to_write = LONG_MAX; /* doesn't actually matter */
> >  
> >  	wbc.nr_to_write = nr_to_write;
> > -	bdi_writeback_all(&wbc);
> > +	bdi_start_writeback(&wbc);
> 
> So here we have a WB_SYNC_ALL caller of bdi_writeback_all and the
> only other caller is WB_SYNC_NONE.  Given that after patch two those
> are entirely different codepathes in bdi_start_writeback I would just
> split bdi_start_writeback into two separate functions.

Sure, it'll draw a sharper line between WB_SYNC_NONE and WB_SYNC_ALL.
I'll work on this and the bdi cap dirty flag removal as a follow up.

-- 
Jens Axboe

Re: [PATCH 3/4] writeback: only use bdi_writeback_all() for WB_SYNC_NONE writeout

[Jens Axboe]

On Fri, Sep 11 2009, Christoph Hellwig wrote:
> > -static struct bdi_work *bdi_alloc_work(struct writeback_control *wbc)
> > +static void bdi_alloc_queue_work(struct backing_dev_info *bdi,
> > +				 struct writeback_control *wbc)
> >  {
> >  	struct bdi_work *work;
> >  
> > @@ -195,7 +196,7 @@ static struct bdi_work *bdi_alloc_work(struct writeback_control *wbc)
> >  	if (work)
> >  		bdi_work_init(work, wbc);
> >  
> > -	return work;
> > +	bdi_queue_work(bdi, work);
> 
> This is now the only caller of bdi_queue_work that has a NULL work
> argument.  I would recommend removing the !work half of bdi_queue_work
> and just inline it into this function (or make it a separate helper).

http://git.kernel.dk/?p=linux-2.6-block.git;a=commit;h=cd932613c5b86a5bb2eefee726749acb0ed1a5b8

 
> > @@ -1157,6 +1115,7 @@ long sync_inodes_sb(struct super_block *sb)
> >  {
> >  	struct writeback_control wbc = {
> >  		.sb		= sb,
> > +		.bdi		= sb->s_bdi,
> >  		.sync_mode	= WB_SYNC_ALL,
> >  		.range_start	= 0,
> >  		.range_end	= LLONG_MAX,
> > @@ -1164,7 +1123,7 @@ long sync_inodes_sb(struct super_block *sb)
> >  	long nr_to_write = LONG_MAX; /* doesn't actually matter */
> >  
> >  	wbc.nr_to_write = nr_to_write;
> > -	bdi_writeback_all(&wbc);
> > +	bdi_start_writeback(&wbc);
> 
> So here we have a WB_SYNC_ALL caller of bdi_writeback_all and the
> only other caller is WB_SYNC_NONE.  Given that after patch two those
> are entirely different codepathes in bdi_start_writeback I would just
> split bdi_start_writeback into two separate functions.

http://git.kernel.dk/?p=linux-2.6-block.git;a=commit;h=479364efda68ae0d954f325fa52a3bbe94916783

-- 
Jens Axboe

[PATCH 4/4] writeback: use RCU to protect bdi_list

[Jens Axboe]

Now that bdi_writeback_all() no longer handles integrity writeback,
it doesn't have to block anymore. This means that we can switch
bdi_list reader side protection to RCU.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
---
 fs/fs-writeback.c           |    4 +-
 include/linux/backing-dev.h |    1 +
 mm/backing-dev.c            |   76 +++++++++++++++++++++++++++++++------------
 mm/page-writeback.c         |    8 ++--
 4 files changed, 62 insertions(+), 27 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 5d4bd1c..d7592c7 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -849,7 +849,7 @@ static void bdi_writeback_all(struct writeback_control *wbc)
 
 	WARN_ON(wbc->sync_mode == WB_SYNC_ALL);
 
-	spin_lock(&bdi_lock);
+	rcu_read_lock();
 
 	list_for_each_entry(bdi, &bdi_list, bdi_list) {
 		if (!bdi_has_dirty_io(bdi))
@@ -858,7 +858,7 @@ static void bdi_writeback_all(struct writeback_control *wbc)
 		bdi_alloc_queue_work(bdi, wbc);
 	}
 
-	spin_unlock(&bdi_lock);
+	rcu_read_unlock();
 }
 
 /*
diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
index f169bcb..859e797 100644
--- a/include/linux/backing-dev.h
+++ b/include/linux/backing-dev.h
@@ -59,6 +59,7 @@ struct bdi_writeback {
 
 struct backing_dev_info {
 	struct list_head bdi_list;
+	struct rcu_head rcu_head;
 	unsigned long ra_pages;	/* max readahead in PAGE_CACHE_SIZE units */
 	unsigned long state;	/* Always use atomic bitops on this */
 	unsigned int capabilities; /* Device capabilities */
diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index d3ca0da..fd93566 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -26,6 +26,12 @@ struct backing_dev_info default_backing_dev_info = {
 EXPORT_SYMBOL_GPL(default_backing_dev_info);
 
 static struct class *bdi_class;
+
+/*
+ * bdi_lock protects updates to bdi_list and bdi_pending_list, as well as
+ * reader side protection for bdi_pending_list. bdi_list has RCU reader side
+ * locking.
+ */
 DEFINE_SPINLOCK(bdi_lock);
 LIST_HEAD(bdi_list);
 LIST_HEAD(bdi_pending_list);
@@ -284,9 +290,9 @@ static int bdi_start_fn(void *ptr)
 	/*
 	 * Add us to the active bdi_list
 	 */
-	spin_lock(&bdi_lock);
-	list_add(&bdi->bdi_list, &bdi_list);
-	spin_unlock(&bdi_lock);
+	spin_lock_bh(&bdi_lock);
+	list_add_rcu(&bdi->bdi_list, &bdi_list);
+	spin_unlock_bh(&bdi_lock);
 
 	bdi_task_init(bdi, wb);
 
@@ -389,7 +395,7 @@ static int bdi_forker_task(void *ptr)
 		if (wb_has_dirty_io(me) || !list_empty(&me->bdi->work_list))
 			wb_do_writeback(me, 0);
 
-		spin_lock(&bdi_lock);
+		spin_lock_bh(&bdi_lock);
 
 		/*
 		 * Check if any existing bdi's have dirty data without
@@ -410,7 +416,7 @@ static int bdi_forker_task(void *ptr)
 		if (list_empty(&bdi_pending_list)) {
 			unsigned long wait;
 
-			spin_unlock(&bdi_lock);
+			spin_unlock_bh(&bdi_lock);
 			wait = msecs_to_jiffies(dirty_writeback_interval * 10);
 			schedule_timeout(wait);
 			try_to_freeze();
@@ -426,7 +432,7 @@ static int bdi_forker_task(void *ptr)
 		bdi = list_entry(bdi_pending_list.next, struct backing_dev_info,
 				 bdi_list);
 		list_del_init(&bdi->bdi_list);
-		spin_unlock(&bdi_lock);
+		spin_unlock_bh(&bdi_lock);
 
 		wb = &bdi->wb;
 		wb->task = kthread_run(bdi_start_fn, wb, "flush-%s",
@@ -445,9 +451,9 @@ static int bdi_forker_task(void *ptr)
 			 * a chance to flush other bdi's to free
 			 * memory.
 			 */
-			spin_lock(&bdi_lock);
+			spin_lock_bh(&bdi_lock);
 			list_add_tail(&bdi->bdi_list, &bdi_pending_list);
-			spin_unlock(&bdi_lock);
+			spin_unlock_bh(&bdi_lock);
 
 			bdi_flush_io(bdi);
 		}
@@ -456,6 +462,24 @@ static int bdi_forker_task(void *ptr)
 	return 0;
 }
 
+static void bdi_add_to_pending(struct rcu_head *head)
+{
+	struct backing_dev_info *bdi;
+
+	bdi = container_of(head, struct backing_dev_info, rcu_head);
+	INIT_LIST_HEAD(&bdi->bdi_list);
+
+	spin_lock(&bdi_lock);
+	list_add_tail(&bdi->bdi_list, &bdi_pending_list);
+	spin_unlock(&bdi_lock);
+
+	/*
+	 * We are now on the pending list, wake up bdi_forker_task()
+	 * to finish the job and add us back to the active bdi_list
+	 */
+	wake_up_process(default_backing_dev_info.wb.task);
+}
+
 /*
  * Add the default flusher task that gets created for any bdi
  * that has dirty data pending writeout
@@ -478,16 +502,29 @@ void static bdi_add_default_flusher_task(struct backing_dev_info *bdi)
 	 * waiting for previous additions to finish.
 	 */
 	if (!test_and_set_bit(BDI_pending, &bdi->state)) {
-		list_move_tail(&bdi->bdi_list, &bdi_pending_list);
+		list_del_rcu(&bdi->bdi_list);
 
 		/*
-		 * We are now on the pending list, wake up bdi_forker_task()
-		 * to finish the job and add us back to the active bdi_list
+		 * We must wait for the current RCU period to end before
+		 * moving to the pending list. So schedule that operation
+		 * from an RCU callback.
 		 */
-		wake_up_process(default_backing_dev_info.wb.task);
+		call_rcu(&bdi->rcu_head, bdi_add_to_pending);
 	}
 }
 
+/*
+ * Remove bdi from bdi_list, and ensure that it is no longer visible
+ */
+static void bdi_remove_from_list(struct backing_dev_info *bdi)
+{
+	spin_lock_bh(&bdi_lock);
+	list_del_rcu(&bdi->bdi_list);
+	spin_unlock_bh(&bdi_lock);
+
+	synchronize_rcu();
+}
+
 int bdi_register(struct backing_dev_info *bdi, struct device *parent,
 		const char *fmt, ...)
 {
@@ -506,9 +543,9 @@ int bdi_register(struct backing_dev_info *bdi, struct device *parent,
 		goto exit;
 	}
 
-	spin_lock(&bdi_lock);
-	list_add_tail(&bdi->bdi_list, &bdi_list);
-	spin_unlock(&bdi_lock);
+	spin_lock_bh(&bdi_lock);
+	list_add_tail_rcu(&bdi->bdi_list, &bdi_list);
+	spin_unlock_bh(&bdi_lock);
 
 	bdi->dev = dev;
 
@@ -526,9 +563,7 @@ int bdi_register(struct backing_dev_info *bdi, struct device *parent,
 			wb->task = NULL;
 			ret = -ENOMEM;
 
-			spin_lock(&bdi_lock);
-			list_del(&bdi->bdi_list);
-			spin_unlock(&bdi_lock);
+			bdi_remove_from_list(bdi);
 			goto exit;
 		}
 	}
@@ -565,9 +600,7 @@ static void bdi_wb_shutdown(struct backing_dev_info *bdi)
 	/*
 	 * Make sure nobody finds us on the bdi_list anymore
 	 */
-	spin_lock(&bdi_lock);
-	list_del(&bdi->bdi_list);
-	spin_unlock(&bdi_lock);
+	bdi_remove_from_list(bdi);
 
 	/*
 	 * Finally, kill the kernel threads. We don't need to be RCU
@@ -599,6 +632,7 @@ int bdi_init(struct backing_dev_info *bdi)
 	bdi->max_ratio = 100;
 	bdi->max_prop_frac = PROP_FRAC_BASE;
 	spin_lock_init(&bdi->wb_lock);
+	INIT_RCU_HEAD(&bdi->rcu_head);
 	INIT_LIST_HEAD(&bdi->bdi_list);
 	INIT_LIST_HEAD(&bdi->wb_list);
 	INIT_LIST_HEAD(&bdi->work_list);
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 25e7770..a5f0f76 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -315,7 +315,7 @@ int bdi_set_min_ratio(struct backing_dev_info *bdi, unsigned int min_ratio)
 {
 	int ret = 0;
 
-	spin_lock(&bdi_lock);
+	spin_lock_bh(&bdi_lock);
 	if (min_ratio > bdi->max_ratio) {
 		ret = -EINVAL;
 	} else {
@@ -327,7 +327,7 @@ int bdi_set_min_ratio(struct backing_dev_info *bdi, unsigned int min_ratio)
 			ret = -EINVAL;
 		}
 	}
-	spin_unlock(&bdi_lock);
+	spin_unlock_bh(&bdi_lock);
 
 	return ret;
 }
@@ -339,14 +339,14 @@ int bdi_set_max_ratio(struct backing_dev_info *bdi, unsigned max_ratio)
 	if (max_ratio > 100)
 		return -EINVAL;
 
-	spin_lock(&bdi_lock);
+	spin_lock_bh(&bdi_lock);
 	if (bdi->min_ratio > max_ratio) {
 		ret = -EINVAL;
 	} else {
 		bdi->max_ratio = max_ratio;
 		bdi->max_prop_frac = (PROP_FRAC_BASE * max_ratio) / 100;
 	}
-	spin_unlock(&bdi_lock);
+	spin_unlock_bh(&bdi_lock);
 
 	return ret;
 }
-- 
1.6.4.1.207.g68ea