Btrfs devel - Aug 2012 - [PATCH 1/2 RESEND] vfs: re-implement writeback_inodes_sb(_nr)_if_idle() and rename them

writeback_inodes_sb(_nr)_if_idle() is re-implemented by replacing down_read()
with down_read_trylock() because
- If ->s_umount is write locked, then the sb is not idle. That is
  writeback_inodes_sb(_nr)_if_idle() needn''t wait for the lock.
- writeback_inodes_sb(_nr)_if_idle() grabs s_umount lock when it want to start
  writeback, it may bring us deadlock problem when doing umount.
  (Ex. Btrfs has such a problem, see the following URL
       http://marc.info/?l=linux-btrfs&m=133540923510561&w=2)

The name of these two functions is cumbersome, so rename them to
try_to_writeback_inodes_sb(_nr).

This idea came from Christoph Hellwig.
Some code is from the patch of Kamal Mostafa.

Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
---
Many users who use btrfs met the deadlock problem caused by
writeback_inodes_sb(_nr)_if_idle(),
so I send this patch again and hope it will be received as soon as possible if
nobody objects.
---
 fs/btrfs/extent-tree.c    |    4 ++--
 fs/ext4/inode.c           |    2 +-
 fs/fs-writeback.c         |   45 ++++++++++++++++++++-------------------------
 include/linux/writeback.h |    6 +++---
 4 files changed, 26 insertions(+), 31 deletions(-)

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 4e1b153..175d802 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -3706,8 +3706,8 @@ static void shrink_delalloc(struct btrfs_root *root, u64
to_reclaim, u64 orig,
 	while (delalloc_bytes && loops < 3) {
 		max_reclaim = min(delalloc_bytes, to_reclaim);
 		nr_pages = max_reclaim >> PAGE_CACHE_SHIFT;
-		writeback_inodes_sb_nr_if_idle(root->fs_info->sb, nr_pages,
-					       WB_REASON_FS_FREE_SPACE);
+		try_to_writeback_inodes_sb_nr(root->fs_info->sb, nr_pages,
+					      WB_REASON_FS_FREE_SPACE);
 
 		spin_lock(&space_info->lock);
 		if (space_info->bytes_used + space_info->bytes_reserved +
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index dff171c..33ff5b5 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -2476,7 +2476,7 @@ static int ext4_nonda_switch(struct super_block *sb)
 	 * start pushing delalloc when 1/2 of free blocks are dirty.
 	 */
 	if (free_blocks < 2 * dirty_blocks)
-		writeback_inodes_sb_if_idle(sb, WB_REASON_FS_FREE_SPACE);
+		try_to_writeback_inodes_sb(sb, WB_REASON_FS_FREE_SPACE);
 
 	return 0;
 }
diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index be3efc4..20a3509 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -1316,7 +1316,6 @@ void writeback_inodes_sb_nr(struct super_block *sb,
 	bdi_queue_work(sb->s_bdi, &work);
 	wait_for_completion(&done);
 }
-EXPORT_SYMBOL(writeback_inodes_sb_nr);
 
 /**
  * writeback_inodes_sb	-	writeback dirty inodes from given super_block
@@ -1334,47 +1333,43 @@ void writeback_inodes_sb(struct super_block *sb, enum
wb_reason reason)
 EXPORT_SYMBOL(writeback_inodes_sb);
 
 /**
- * writeback_inodes_sb_if_idle	-	start writeback if none underway
+ * try_to_writeback_inodes_sb_nr - try to start writeback if none underway
  * @sb: the superblock
- * @reason: reason why some writeback work was initiated
+ * @nr: the number of pages to write
+ * @reason: the reason of writeback
  *
- * Invoke writeback_inodes_sb if no writeback is currently underway.
+ * Invoke writeback_inodes_sb_nr if no writeback is currently underway.
  * Returns 1 if writeback was started, 0 if not.
  */
-int writeback_inodes_sb_if_idle(struct super_block *sb, enum wb_reason reason)
+int try_to_writeback_inodes_sb_nr(struct super_block *sb,
+				  unsigned long nr,
+				  enum wb_reason reason)
 {
-	if (!writeback_in_progress(sb->s_bdi)) {
-		down_read(&sb->s_umount);
-		writeback_inodes_sb(sb, reason);
-		up_read(&sb->s_umount);
+	if (writeback_in_progress(sb->s_bdi))
 		return 1;
-	} else
+
+	if (!down_read_trylock(&sb->s_umount))
 		return 0;
+
+	writeback_inodes_sb_nr(sb, nr, reason);
+	up_read(&sb->s_umount);
+	return 1;
 }
-EXPORT_SYMBOL(writeback_inodes_sb_if_idle);
+EXPORT_SYMBOL(try_to_writeback_inodes_sb_nr);
 
 /**
- * writeback_inodes_sb_nr_if_idle	-	start writeback if none underway
+ * try_to_writeback_inodes_sb - try to start writeback if none underway
  * @sb: the superblock
- * @nr: the number of pages to write
  * @reason: reason why some writeback work was initiated
  *
- * Invoke writeback_inodes_sb if no writeback is currently underway.
+ * Implement by try_to_writeback_inodes_sb_nr()
  * Returns 1 if writeback was started, 0 if not.
  */
-int writeback_inodes_sb_nr_if_idle(struct super_block *sb,
-				   unsigned long nr,
-				   enum wb_reason reason)
+int try_to_writeback_inodes_sb(struct super_block *sb, enum wb_reason reason)
 {
-	if (!writeback_in_progress(sb->s_bdi)) {
-		down_read(&sb->s_umount);
-		writeback_inodes_sb_nr(sb, nr, reason);
-		up_read(&sb->s_umount);
-		return 1;
-	} else
-		return 0;
+	return try_to_writeback_inodes_sb_nr(sb, get_nr_dirty_pages(), reason);
 }
-EXPORT_SYMBOL(writeback_inodes_sb_nr_if_idle);
+EXPORT_SYMBOL(try_to_writeback_inodes_sb);
 
 /**
  * sync_inodes_sb	-	sync sb inode pages
diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index 50c3e8f..ecdd0e0 100644
--- a/include/linux/writeback.h
+++ b/include/linux/writeback.h
@@ -87,9 +87,9 @@ int inode_wait(void *);
 void writeback_inodes_sb(struct super_block *, enum wb_reason reason);
 void writeback_inodes_sb_nr(struct super_block *, unsigned long nr,
 							enum wb_reason reason);
-int writeback_inodes_sb_if_idle(struct super_block *, enum wb_reason reason);
-int writeback_inodes_sb_nr_if_idle(struct super_block *, unsigned long nr,
-							enum wb_reason reason);
+int try_to_writeback_inodes_sb(struct super_block *, enum wb_reason reason);
+int try_to_writeback_inodes_sb_nr(struct super_block *, unsigned long nr,
+				  enum wb_reason reason);
 void sync_inodes_sb(struct super_block *);
 long writeback_inodes_wb(struct bdi_writeback *wb, long nr_pages,
 				enum wb_reason reason);
-- 
1.6.5.2
--
To unsubscribe from this list: send the line "unsubscribe linux-ext4"
in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html

On Wed, Aug 29, 2012 at 02:17:12PM +0800, Miao Xie
wrote:
> writeback_inodes_sb(_nr)_if_idle() is re-implemented by replacing
down_read()
> with down_read_trylock() because
> - If ->s_umount is write locked, then the sb is not idle. That is
>   writeback_inodes_sb(_nr)_if_idle() needn''t wait for the lock.
> - writeback_inodes_sb(_nr)_if_idle() grabs s_umount lock when it want to
start
>   writeback, it may bring us deadlock problem when doing umount.
>   (Ex. Btrfs has such a problem, see the following URL
>        http://marc.info/?l=linux-btrfs&m=133540923510561&w=2)
> 
> The name of these two functions is cumbersome, so rename them to
> try_to_writeback_inodes_sb(_nr).
> 
> This idea came from Christoph Hellwig.
> Some code is from the patch of Kamal Mostafa.
> 
> Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Tested-by: David Sterba <dsterba@suse.cz>
> ---
> Many users who use btrfs met the deadlock problem caused by
writeback_inodes_sb(_nr)_if_idle(),
> so I send this patch again and hope it will be received as soon as possible
if nobody objects.
Yes please, this makes testing of the ''autodefrag'' mount
option hard as
the deadlock occurs frequently upon umount (when there''s a background
snapshot cleanup in progress).

From the user''s POV, the autodefrag improves performance, it''s
not yet
on by default until it proves to be stable, the deadlocks were standing
in the way for long.

thanks,
david

On Wed, Sep 05, 2012 at 05:54:04PM +0200, David Sterba
wrote:
> > Many users who use btrfs met the deadlock problem caused by
writeback_inodes_sb(_nr)_if_idle(),
> > so I send this patch again and hope it will be received as soon as
possible if nobody objects.
> 
> Yes please, this makes testing of the ''autodefrag'' mount
option hard as
> the deadlock occurs frequently upon umount (when there''s a
background
> snapshot cleanup in progress).
We have another report of the deadlock and it''s without
''autodefrag''
mount, triggered by snapshot cleaning under low space conditions.

I have added the patch to SLES kernel some time ago and haven''t
observed
the deadlock since.

Please queue the patch for 3.7, we cannot fix it locally within btrfs.

Thanks,
david