[PATCH v2 0/2] Fix for race in relocation and avoid start and wait for unrelated IO

[PATCH v2 0/2] Fix for race in relocation and avoid start and wait for unrelated IO

[fdmanana]

From: Filipe Manana <fdmanana@suse.com>

The following patches fix a hard to hit race and unecessary flushing of
delalloc regions and waiting for unrelated IO (IO against extents outside
of the block group being relocated).

The race is between relocation and direct IO writes that lead to the
relocation process miss extents and not move them, and later makes us hit
a BUG_ON() if we attempt to read the extents.

Patch 1 just avoids waiting for unrelated ordered extents to complete
while patch 2 fixes the race and avoids flushing delalloc.

Thanks.

V2: Removed the first patch in the v1 series, it was meant for a false
    positive.

Filipe Manana (2):
  Btrfs: don't wait for unrelated IO to finish before relocation
  Btrfs: don't do unnecessary delalloc flushes when relocating

 fs/btrfs/ctree.h        | 14 ++++++++++
 fs/btrfs/dev-replace.c  |  4 +--
 fs/btrfs/extent-tree.c  | 69 ++++++++++++++++++++++++++++++++++++++++++++-----
 fs/btrfs/inode.c        |  8 ++++++
 fs/btrfs/ioctl.c        |  2 +-
 fs/btrfs/ordered-data.c | 26 ++++++++++++++-----
 fs/btrfs/ordered-data.h |  6 +++--
 fs/btrfs/relocation.c   | 10 +++----
 fs/btrfs/super.c        |  2 +-
 fs/btrfs/transaction.c  |  2 +-
 10 files changed, 117 insertions(+), 26 deletions(-)

-- 
2.7.0.rc3

[PATCH v2 1/2] Btrfs: don't wait for unrelated IO to finish before relocation

[fdmanana]

From: Filipe Manana <fdmanana@suse.com>

Before the relocation process of a block group starts, it sets the block
group to readonly mode, then flushes all delalloc writes and then finally
it waits for all ordered extents to complete. This last step includes
waiting for ordered extents destinated at extents allocated in other block
groups, making us waste unecessary time.

So improve this by waiting only for ordered extents that fall into the
block group's range.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
---
 fs/btrfs/dev-replace.c  |  4 ++--
 fs/btrfs/extent-tree.c  | 11 +++++++----
 fs/btrfs/ioctl.c        |  2 +-
 fs/btrfs/ordered-data.c | 26 +++++++++++++++++++-------
 fs/btrfs/ordered-data.h |  6 ++++--
 fs/btrfs/relocation.c   |  4 +++-
 fs/btrfs/super.c        |  2 +-
 fs/btrfs/transaction.c  |  2 +-
 8 files changed, 38 insertions(+), 19 deletions(-)

diff --git a/fs/btrfs/dev-replace.c b/fs/btrfs/dev-replace.c
index a1d6652..a0c1016 100644
--- a/fs/btrfs/dev-replace.c
+++ b/fs/btrfs/dev-replace.c
@@ -401,7 +401,7 @@ int btrfs_dev_replace_start(struct btrfs_root *root,
 	if (ret)
 		btrfs_err(root->fs_info, "kobj add dev failed %d\n", ret);
 
-	btrfs_wait_ordered_roots(root->fs_info, -1);
+	btrfs_wait_ordered_roots(root->fs_info, -1, 0, (u64)-1);
 
 	/* force writing the updated state information to disk */
 	trans = btrfs_start_transaction(root, 0);
@@ -493,7 +493,7 @@ static int btrfs_dev_replace_finishing(struct btrfs_fs_info *fs_info,
 		mutex_unlock(&dev_replace->lock_finishing_cancel_unmount);
 		return ret;
 	}
-	btrfs_wait_ordered_roots(root->fs_info, -1);
+	btrfs_wait_ordered_roots(root->fs_info, -1, 0, (u64)-1);
 
 	trans = btrfs_start_transaction(root, 0);
 	if (IS_ERR(trans)) {
diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 53e1297..0544f70 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -4141,7 +4141,7 @@ commit_trans:
 
 			if (need_commit > 0) {
 				btrfs_start_delalloc_roots(fs_info, 0, -1);
-				btrfs_wait_ordered_roots(fs_info, -1);
+				btrfs_wait_ordered_roots(fs_info, -1, 0, (u64)-1);
 			}
 
 			trans = btrfs_join_transaction(root);
@@ -4583,7 +4583,8 @@ static void btrfs_writeback_inodes_sb_nr(struct btrfs_root *root,
 		 */
 		btrfs_start_delalloc_roots(root->fs_info, 0, nr_items);
 		if (!current->journal_info)
-			btrfs_wait_ordered_roots(root->fs_info, nr_items);
+			btrfs_wait_ordered_roots(root->fs_info, nr_items,
+						 0, (u64)-1);
 	}
 }
 
@@ -4632,7 +4633,8 @@ static void shrink_delalloc(struct btrfs_root *root, u64 to_reclaim, u64 orig,
 		if (trans)
 			return;
 		if (wait_ordered)
-			btrfs_wait_ordered_roots(root->fs_info, items);
+			btrfs_wait_ordered_roots(root->fs_info, items,
+						 0, (u64)-1);
 		return;
 	}
 
@@ -4671,7 +4673,8 @@ skip_async:
 
 		loops++;
 		if (wait_ordered && !trans) {
-			btrfs_wait_ordered_roots(root->fs_info, items);
+			btrfs_wait_ordered_roots(root->fs_info, items,
+						 0, (u64)-1);
 		} else {
 			time_left = schedule_timeout_killable(1);
 			if (time_left)
diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 053e677..4d71273 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -681,7 +681,7 @@ static int create_snapshot(struct btrfs_root *root, struct inode *dir,
 	if (ret)
 		goto dec_and_free;
 
-	btrfs_wait_ordered_extents(root, -1);
+	btrfs_wait_ordered_extents(root, -1, 0, (u64)-1);
 
 	btrfs_init_block_rsv(&pending_snapshot->block_rsv,
 			     BTRFS_BLOCK_RSV_TEMP);
diff --git a/fs/btrfs/ordered-data.c b/fs/btrfs/ordered-data.c
index 0de7da5..5591704 100644
--- a/fs/btrfs/ordered-data.c
+++ b/fs/btrfs/ordered-data.c
@@ -661,14 +661,15 @@ static void btrfs_run_ordered_extent_work(struct btrfs_work *work)
  * wait for all the ordered extents in a root.  This is done when balancing
  * space between drives.
  */
-int btrfs_wait_ordered_extents(struct btrfs_root *root, int nr)
+int btrfs_wait_ordered_extents(struct btrfs_root *root, int nr,
+			       const u64 range_start, const u64 range_len)
 {
-	struct list_head splice, works;
+	LIST_HEAD(splice);
+	LIST_HEAD(skipped);
+	LIST_HEAD(works);
 	struct btrfs_ordered_extent *ordered, *next;
 	int count = 0;
-
-	INIT_LIST_HEAD(&splice);
-	INIT_LIST_HEAD(&works);
+	const u64 range_end = range_start + range_len;
 
 	mutex_lock(&root->ordered_extent_mutex);
 	spin_lock(&root->ordered_extent_lock);
@@ -676,6 +677,14 @@ int btrfs_wait_ordered_extents(struct btrfs_root *root, int nr)
 	while (!list_empty(&splice) && nr) {
 		ordered = list_first_entry(&splice, struct btrfs_ordered_extent,
 					   root_extent_list);
+
+		if (range_end <= ordered->start ||
+		    ordered->start + ordered->disk_len <= range_start) {
+			list_move_tail(&ordered->root_extent_list, &skipped);
+			cond_resched_lock(&root->ordered_extent_lock);
+			continue;
+		}
+
 		list_move_tail(&ordered->root_extent_list,
 			       &root->ordered_extents);
 		atomic_inc(&ordered->refs);
@@ -694,6 +703,7 @@ int btrfs_wait_ordered_extents(struct btrfs_root *root, int nr)
 			nr--;
 		count++;
 	}
+	list_splice_tail(&skipped, &root->ordered_extents);
 	list_splice_tail(&splice, &root->ordered_extents);
 	spin_unlock(&root->ordered_extent_lock);
 
@@ -708,7 +718,8 @@ int btrfs_wait_ordered_extents(struct btrfs_root *root, int nr)
 	return count;
 }
 
-void btrfs_wait_ordered_roots(struct btrfs_fs_info *fs_info, int nr)
+void btrfs_wait_ordered_roots(struct btrfs_fs_info *fs_info, int nr,
+			      const u64 range_start, const u64 range_len)
 {
 	struct btrfs_root *root;
 	struct list_head splice;
@@ -728,7 +739,8 @@ void btrfs_wait_ordered_roots(struct btrfs_fs_info *fs_info, int nr)
 			       &fs_info->ordered_roots);
 		spin_unlock(&fs_info->ordered_root_lock);
 
-		done = btrfs_wait_ordered_extents(root, nr);
+		done = btrfs_wait_ordered_extents(root, nr,
+						  range_start, range_len);
 		btrfs_put_fs_root(root);
 
 		spin_lock(&fs_info->ordered_root_lock);
diff --git a/fs/btrfs/ordered-data.h b/fs/btrfs/ordered-data.h
index 23c9605..8ef1262 100644
--- a/fs/btrfs/ordered-data.h
+++ b/fs/btrfs/ordered-data.h
@@ -197,8 +197,10 @@ int btrfs_ordered_update_i_size(struct inode *inode, u64 offset,
 				struct btrfs_ordered_extent *ordered);
 int btrfs_find_ordered_sum(struct inode *inode, u64 offset, u64 disk_bytenr,
 			   u32 *sum, int len);
-int btrfs_wait_ordered_extents(struct btrfs_root *root, int nr);
-void btrfs_wait_ordered_roots(struct btrfs_fs_info *fs_info, int nr);
+int btrfs_wait_ordered_extents(struct btrfs_root *root, int nr,
+			       const u64 range_start, const u64 range_len);
+void btrfs_wait_ordered_roots(struct btrfs_fs_info *fs_info, int nr,
+			      const u64 range_start, const u64 range_len);
 void btrfs_get_logged_extents(struct inode *inode,
 			      struct list_head *logged_list,
 			      const loff_t start,
diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
index 2bd0011..9c35ea2 100644
--- a/fs/btrfs/relocation.c
+++ b/fs/btrfs/relocation.c
@@ -4258,7 +4258,9 @@ int btrfs_relocate_block_group(struct btrfs_root *extent_root, u64 group_start)
 		err = ret;
 		goto out;
 	}
-	btrfs_wait_ordered_roots(fs_info, -1);
+	btrfs_wait_ordered_roots(fs_info, -1,
+				 rc->block_group->key.objectid,
+				 rc->block_group->key.offset);
 
 	while (1) {
 		mutex_lock(&fs_info->cleaner_mutex);
diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 00b8f37..89d1347 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -1160,7 +1160,7 @@ int btrfs_sync_fs(struct super_block *sb, int wait)
 		return 0;
 	}
 
-	btrfs_wait_ordered_roots(fs_info, -1);
+	btrfs_wait_ordered_roots(fs_info, -1, 0, (u64)-1);
 
 	trans = btrfs_attach_transaction_barrier(root);
 	if (IS_ERR(trans)) {
diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index 43885e5..f0bb54a 100644
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -1821,7 +1821,7 @@ static inline int btrfs_start_delalloc_flush(struct btrfs_fs_info *fs_info)
 static inline void btrfs_wait_delalloc_flush(struct btrfs_fs_info *fs_info)
 {
 	if (btrfs_test_opt(fs_info->tree_root, FLUSHONCOMMIT))
-		btrfs_wait_ordered_roots(fs_info, -1);
+		btrfs_wait_ordered_roots(fs_info, -1, 0, (u64)-1);
 }
 
 static inline void
-- 
2.7.0.rc3

Re: [PATCH v2 1/2] Btrfs: don't wait for unrelated IO to finish before relocation

[Josef Bacik]

On 04/26/2016 11:39 AM, fdmanana@kernel.org wrote:
> From: Filipe Manana <fdmanana@suse.com>
>
> Before the relocation process of a block group starts, it sets the block
> group to readonly mode, then flushes all delalloc writes and then finally
> it waits for all ordered extents to complete. This last step includes
> waiting for ordered extents destinated at extents allocated in other block
> groups, making us waste unecessary time.
>
> So improve this by waiting only for ordered extents that fall into the
> block group's range.
>
> Signed-off-by: Filipe Manana <fdmanana@suse.com>

Reviewed-by: Josef Bacik <jbacik@fb.com>

Thanks,

Josef

Re: [PATCH v2 1/2] Btrfs: don't wait for unrelated IO to finish before relocation

[Liu Bo]

On Tue, Apr 26, 2016 at 04:39:38PM +0100, fdmanana@kernel.org wrote:
> From: Filipe Manana <fdmanana@suse.com>
> 
> Before the relocation process of a block group starts, it sets the block
> group to readonly mode, then flushes all delalloc writes and then finally
> it waits for all ordered extents to complete. This last step includes
> waiting for ordered extents destinated at extents allocated in other block
> groups, making us waste unecessary time.
> 
> So improve this by waiting only for ordered extents that fall into the
> block group's range.

Reviewed-by: Liu Bo <bo.li.liu@oracle.com>

Thanks,

-liubo
> 
> Signed-off-by: Filipe Manana <fdmanana@suse.com>
> ---
>  fs/btrfs/dev-replace.c  |  4 ++--
>  fs/btrfs/extent-tree.c  | 11 +++++++----
>  fs/btrfs/ioctl.c        |  2 +-
>  fs/btrfs/ordered-data.c | 26 +++++++++++++++++++-------
>  fs/btrfs/ordered-data.h |  6 ++++--
>  fs/btrfs/relocation.c   |  4 +++-
>  fs/btrfs/super.c        |  2 +-
>  fs/btrfs/transaction.c  |  2 +-
>  8 files changed, 38 insertions(+), 19 deletions(-)
> 
> diff --git a/fs/btrfs/dev-replace.c b/fs/btrfs/dev-replace.c
> index a1d6652..a0c1016 100644
> --- a/fs/btrfs/dev-replace.c
> +++ b/fs/btrfs/dev-replace.c
> @@ -401,7 +401,7 @@ int btrfs_dev_replace_start(struct btrfs_root *root,
>  	if (ret)
>  		btrfs_err(root->fs_info, "kobj add dev failed %d\n", ret);
>  
> -	btrfs_wait_ordered_roots(root->fs_info, -1);
> +	btrfs_wait_ordered_roots(root->fs_info, -1, 0, (u64)-1);
>  
>  	/* force writing the updated state information to disk */
>  	trans = btrfs_start_transaction(root, 0);
> @@ -493,7 +493,7 @@ static int btrfs_dev_replace_finishing(struct btrfs_fs_info *fs_info,
>  		mutex_unlock(&dev_replace->lock_finishing_cancel_unmount);
>  		return ret;
>  	}
> -	btrfs_wait_ordered_roots(root->fs_info, -1);
> +	btrfs_wait_ordered_roots(root->fs_info, -1, 0, (u64)-1);
>  
>  	trans = btrfs_start_transaction(root, 0);
>  	if (IS_ERR(trans)) {
> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> index 53e1297..0544f70 100644
> --- a/fs/btrfs/extent-tree.c
> +++ b/fs/btrfs/extent-tree.c
> @@ -4141,7 +4141,7 @@ commit_trans:
>  
>  			if (need_commit > 0) {
>  				btrfs_start_delalloc_roots(fs_info, 0, -1);
> -				btrfs_wait_ordered_roots(fs_info, -1);
> +				btrfs_wait_ordered_roots(fs_info, -1, 0, (u64)-1);
>  			}
>  
>  			trans = btrfs_join_transaction(root);
> @@ -4583,7 +4583,8 @@ static void btrfs_writeback_inodes_sb_nr(struct btrfs_root *root,
>  		 */
>  		btrfs_start_delalloc_roots(root->fs_info, 0, nr_items);
>  		if (!current->journal_info)
> -			btrfs_wait_ordered_roots(root->fs_info, nr_items);
> +			btrfs_wait_ordered_roots(root->fs_info, nr_items,
> +						 0, (u64)-1);
>  	}
>  }
>  
> @@ -4632,7 +4633,8 @@ static void shrink_delalloc(struct btrfs_root *root, u64 to_reclaim, u64 orig,
>  		if (trans)
>  			return;
>  		if (wait_ordered)
> -			btrfs_wait_ordered_roots(root->fs_info, items);
> +			btrfs_wait_ordered_roots(root->fs_info, items,
> +						 0, (u64)-1);
>  		return;
>  	}
>  
> @@ -4671,7 +4673,8 @@ skip_async:
>  
>  		loops++;
>  		if (wait_ordered && !trans) {
> -			btrfs_wait_ordered_roots(root->fs_info, items);
> +			btrfs_wait_ordered_roots(root->fs_info, items,
> +						 0, (u64)-1);
>  		} else {
>  			time_left = schedule_timeout_killable(1);
>  			if (time_left)
> diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
> index 053e677..4d71273 100644
> --- a/fs/btrfs/ioctl.c
> +++ b/fs/btrfs/ioctl.c
> @@ -681,7 +681,7 @@ static int create_snapshot(struct btrfs_root *root, struct inode *dir,
>  	if (ret)
>  		goto dec_and_free;
>  
> -	btrfs_wait_ordered_extents(root, -1);
> +	btrfs_wait_ordered_extents(root, -1, 0, (u64)-1);
>  
>  	btrfs_init_block_rsv(&pending_snapshot->block_rsv,
>  			     BTRFS_BLOCK_RSV_TEMP);
> diff --git a/fs/btrfs/ordered-data.c b/fs/btrfs/ordered-data.c
> index 0de7da5..5591704 100644
> --- a/fs/btrfs/ordered-data.c
> +++ b/fs/btrfs/ordered-data.c
> @@ -661,14 +661,15 @@ static void btrfs_run_ordered_extent_work(struct btrfs_work *work)
>   * wait for all the ordered extents in a root.  This is done when balancing
>   * space between drives.
>   */
> -int btrfs_wait_ordered_extents(struct btrfs_root *root, int nr)
> +int btrfs_wait_ordered_extents(struct btrfs_root *root, int nr,
> +			       const u64 range_start, const u64 range_len)
>  {
> -	struct list_head splice, works;
> +	LIST_HEAD(splice);
> +	LIST_HEAD(skipped);
> +	LIST_HEAD(works);
>  	struct btrfs_ordered_extent *ordered, *next;
>  	int count = 0;
> -
> -	INIT_LIST_HEAD(&splice);
> -	INIT_LIST_HEAD(&works);
> +	const u64 range_end = range_start + range_len;
>  
>  	mutex_lock(&root->ordered_extent_mutex);
>  	spin_lock(&root->ordered_extent_lock);
> @@ -676,6 +677,14 @@ int btrfs_wait_ordered_extents(struct btrfs_root *root, int nr)
>  	while (!list_empty(&splice) && nr) {
>  		ordered = list_first_entry(&splice, struct btrfs_ordered_extent,
>  					   root_extent_list);
> +
> +		if (range_end <= ordered->start ||
> +		    ordered->start + ordered->disk_len <= range_start) {
> +			list_move_tail(&ordered->root_extent_list, &skipped);
> +			cond_resched_lock(&root->ordered_extent_lock);
> +			continue;
> +		}
> +
>  		list_move_tail(&ordered->root_extent_list,
>  			       &root->ordered_extents);
>  		atomic_inc(&ordered->refs);
> @@ -694,6 +703,7 @@ int btrfs_wait_ordered_extents(struct btrfs_root *root, int nr)
>  			nr--;
>  		count++;
>  	}
> +	list_splice_tail(&skipped, &root->ordered_extents);
>  	list_splice_tail(&splice, &root->ordered_extents);
>  	spin_unlock(&root->ordered_extent_lock);
>  
> @@ -708,7 +718,8 @@ int btrfs_wait_ordered_extents(struct btrfs_root *root, int nr)
>  	return count;
>  }
>  
> -void btrfs_wait_ordered_roots(struct btrfs_fs_info *fs_info, int nr)
> +void btrfs_wait_ordered_roots(struct btrfs_fs_info *fs_info, int nr,
> +			      const u64 range_start, const u64 range_len)
>  {
>  	struct btrfs_root *root;
>  	struct list_head splice;
> @@ -728,7 +739,8 @@ void btrfs_wait_ordered_roots(struct btrfs_fs_info *fs_info, int nr)
>  			       &fs_info->ordered_roots);
>  		spin_unlock(&fs_info->ordered_root_lock);
>  
> -		done = btrfs_wait_ordered_extents(root, nr);
> +		done = btrfs_wait_ordered_extents(root, nr,
> +						  range_start, range_len);
>  		btrfs_put_fs_root(root);
>  
>  		spin_lock(&fs_info->ordered_root_lock);
> diff --git a/fs/btrfs/ordered-data.h b/fs/btrfs/ordered-data.h
> index 23c9605..8ef1262 100644
> --- a/fs/btrfs/ordered-data.h
> +++ b/fs/btrfs/ordered-data.h
> @@ -197,8 +197,10 @@ int btrfs_ordered_update_i_size(struct inode *inode, u64 offset,
>  				struct btrfs_ordered_extent *ordered);
>  int btrfs_find_ordered_sum(struct inode *inode, u64 offset, u64 disk_bytenr,
>  			   u32 *sum, int len);
> -int btrfs_wait_ordered_extents(struct btrfs_root *root, int nr);
> -void btrfs_wait_ordered_roots(struct btrfs_fs_info *fs_info, int nr);
> +int btrfs_wait_ordered_extents(struct btrfs_root *root, int nr,
> +			       const u64 range_start, const u64 range_len);
> +void btrfs_wait_ordered_roots(struct btrfs_fs_info *fs_info, int nr,
> +			      const u64 range_start, const u64 range_len);
>  void btrfs_get_logged_extents(struct inode *inode,
>  			      struct list_head *logged_list,
>  			      const loff_t start,
> diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
> index 2bd0011..9c35ea2 100644
> --- a/fs/btrfs/relocation.c
> +++ b/fs/btrfs/relocation.c
> @@ -4258,7 +4258,9 @@ int btrfs_relocate_block_group(struct btrfs_root *extent_root, u64 group_start)
>  		err = ret;
>  		goto out;
>  	}
> -	btrfs_wait_ordered_roots(fs_info, -1);
> +	btrfs_wait_ordered_roots(fs_info, -1,
> +				 rc->block_group->key.objectid,
> +				 rc->block_group->key.offset);
>  
>  	while (1) {
>  		mutex_lock(&fs_info->cleaner_mutex);
> diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
> index 00b8f37..89d1347 100644
> --- a/fs/btrfs/super.c
> +++ b/fs/btrfs/super.c
> @@ -1160,7 +1160,7 @@ int btrfs_sync_fs(struct super_block *sb, int wait)
>  		return 0;
>  	}
>  
> -	btrfs_wait_ordered_roots(fs_info, -1);
> +	btrfs_wait_ordered_roots(fs_info, -1, 0, (u64)-1);
>  
>  	trans = btrfs_attach_transaction_barrier(root);
>  	if (IS_ERR(trans)) {
> diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
> index 43885e5..f0bb54a 100644
> --- a/fs/btrfs/transaction.c
> +++ b/fs/btrfs/transaction.c
> @@ -1821,7 +1821,7 @@ static inline int btrfs_start_delalloc_flush(struct btrfs_fs_info *fs_info)
>  static inline void btrfs_wait_delalloc_flush(struct btrfs_fs_info *fs_info)
>  {
>  	if (btrfs_test_opt(fs_info->tree_root, FLUSHONCOMMIT))
> -		btrfs_wait_ordered_roots(fs_info, -1);
> +		btrfs_wait_ordered_roots(fs_info, -1, 0, (u64)-1);
>  }
>  
>  static inline void
> -- 
> 2.7.0.rc3
> 
> --
> To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html

[PATCH v2 2/2] Btrfs: don't do unnecessary delalloc flushes when relocating

[fdmanana]

From: Filipe Manana <fdmanana@suse.com>

Before we start the actual relocation process of a block group, we do
calls to flush delalloc of all inodes and then wait for ordered extents
to complete. However we do these flush calls just to make sure we don't
race with concurrent tasks that have actually already started to run
delalloc and have allocated an extent from the block group we want to
relocate, right before we set it to readonly mode, but have not yet
created the respective ordered extents. The flush calls make us wait
for such concurrent tasks because they end up calling
filemap_fdatawrite_range() (through btrfs_start_delalloc_roots() ->
__start_delalloc_inodes() -> btrfs_alloc_delalloc_work() ->
btrfs_run_delalloc_work()) which ends up serializing us with those tasks
due to attempts to lock the same pages (and the delalloc flush procedure
calls the allocator and creates the ordered extents before unlocking the
pages).

These flushing calls not only make us waste time (cpu, IO) but also reduce
the chances of writing larger extents (applications might be writing to
contiguous ranges and we flush before they finish dirtying the whole
ranges).

So make sure we don't flush delalloc and just wait for concurrent tasks
that have already started flushing delalloc and have allocated an extent
from the block group we are about to relocate.

This change also ends up fixing a race with direct IO writes that makes
relocation not wait for direct IO ordered extents. This race is
illustrated by the following diagram:

        CPU 1                                       CPU 2

 btrfs_relocate_block_group(bg X)

                                           starts direct IO write,
                                           target inode currently has no
                                           ordered extents ongoing nor
                                           dirty pages (delalloc regions),
                                           therefore the root for our inode
                                           is not in the list
                                           fs_info->ordered_roots

                                           btrfs_direct_IO()
                                             __blockdev_direct_IO()
                                               btrfs_get_blocks_direct()
                                                 btrfs_lock_extent_direct()
                                                   locks range in the io tree
                                                 btrfs_new_extent_direct()
                                                   btrfs_reserve_extent()
                                                     --> extent allocated
                                                         from bg X

   btrfs_inc_block_group_ro(bg X)

   btrfs_start_delalloc_roots()
     __start_delalloc_inodes()
       --> does nothing, no dealloc ranges
           in the inode's io tree so the
           inode's root is not in the list
           fs_info->delalloc_roots

   btrfs_wait_ordered_roots()
     --> does not find the inode's root in the
         list fs_info->ordered_roots

     --> ends up not waiting for the direct IO
         write started by the task at CPU 2

   relocate_block_group(rc->stage ==
     MOVE_DATA_EXTENTS)

     prepare_to_relocate()
       btrfs_commit_transaction()

     iterates the extent tree, using its
     commit root and moves extents into new
     locations

                                                   btrfs_add_ordered_extent_dio()
                                                     --> now a ordered extent is
                                                         created and added to the
                                                         list root->ordered_extents
                                                         and the root added to the
                                                         list fs_info->ordered_roots
                                                     --> this is too late and the
                                                         task at CPU 1 already
                                                         started the relocation

     btrfs_commit_transaction()

                                                   btrfs_finish_ordered_io()
                                                     btrfs_alloc_reserved_file_extent()
                                                       --> adds delayed data reference
                                                           for the extent allocated
                                                           from bg X

   relocate_block_group(rc->stage ==
     UPDATE_DATA_PTRS)

     prepare_to_relocate()
       btrfs_commit_transaction()
         --> delayed refs are run, so an extent
             item for the allocated extent from
             bg X is added to extent tree
         --> commit roots are switched, so the
             next scan in the extent tree will
             see the extent item

     sees the extent in the extent tree

When this happens the relocation produces the following warning when it
finishes:

[ 7260.832836] ------------[ cut here ]------------
[ 7260.834653] WARNING: CPU: 5 PID: 6765 at fs/btrfs/relocation.c:4318 btrfs_relocate_block_group+0x245/0x2a1 [btrfs]()
[ 7260.838268] Modules linked in: btrfs crc32c_generic xor ppdev raid6_pq psmouse sg acpi_cpufreq evdev i2c_piix4 tpm_tis serio_raw tpm i2c_core pcspkr parport_pc
[ 7260.850935] CPU: 5 PID: 6765 Comm: btrfs Not tainted 4.5.0-rc6-btrfs-next-28+ #1
[ 7260.852998] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS by qemu-project.org 04/01/2014
[ 7260.852998]  0000000000000000 ffff88020bf57bc0 ffffffff812648b3 0000000000000000
[ 7260.852998]  0000000000000009 ffff88020bf57bf8 ffffffff81051608 ffffffffa03c1b2d
[ 7260.852998]  ffff8800b2bbb800 0000000000000000 ffff8800b17bcc58 ffff8800399dd000
[ 7260.852998] Call Trace:
[ 7260.852998]  [<ffffffff812648b3>] dump_stack+0x67/0x90
[ 7260.852998]  [<ffffffff81051608>] warn_slowpath_common+0x99/0xb2
[ 7260.852998]  [<ffffffffa03c1b2d>] ? btrfs_relocate_block_group+0x245/0x2a1 [btrfs]
[ 7260.852998]  [<ffffffff810516d4>] warn_slowpath_null+0x1a/0x1c
[ 7260.852998]  [<ffffffffa03c1b2d>] btrfs_relocate_block_group+0x245/0x2a1 [btrfs]
[ 7260.852998]  [<ffffffffa039d9de>] btrfs_relocate_chunk.isra.29+0x66/0xdb [btrfs]
[ 7260.852998]  [<ffffffffa039f314>] btrfs_balance+0xde1/0xe4e [btrfs]
[ 7260.852998]  [<ffffffff8127d671>] ? debug_smp_processor_id+0x17/0x19
[ 7260.852998]  [<ffffffffa03a9583>] btrfs_ioctl_balance+0x255/0x2d3 [btrfs]
[ 7260.852998]  [<ffffffffa03ac96a>] btrfs_ioctl+0x11e0/0x1dff [btrfs]
[ 7260.852998]  [<ffffffff811451df>] ? handle_mm_fault+0x443/0xd63
[ 7260.852998]  [<ffffffff81491817>] ? _raw_spin_unlock+0x31/0x44
[ 7260.852998]  [<ffffffff8108b36a>] ? arch_local_irq_save+0x9/0xc
[ 7260.852998]  [<ffffffff811876ab>] vfs_ioctl+0x18/0x34
[ 7260.852998]  [<ffffffff81187cb2>] do_vfs_ioctl+0x550/0x5be
[ 7260.852998]  [<ffffffff81190c30>] ? __fget_light+0x4d/0x71
[ 7260.852998]  [<ffffffff81187d77>] SyS_ioctl+0x57/0x79
[ 7260.852998]  [<ffffffff81492017>] entry_SYSCALL_64_fastpath+0x12/0x6b
[ 7260.893268] ---[ end trace eb7803b24ebab8ad ]---

This is because at the end of the first stage, in relocate_block_group(),
we commit the current transaction, which makes delayed refs run, the
commit roots are switched and so the second stage will find the extent
item that the ordered extent added to the delayed refs. But this extent
was not moved (ordered extent completed after first stage finished), so
at the end of the relocation our block group item still has a positive
used bytes counter, triggering a warning at the end of
btrfs_relocate_block_group(). Later on when trying to read the extent
contents from disk we hit a BUG_ON() due to the inability to map a block
with a logical address that belongs to the block group we relocated and
is no longer valid, resulting in the following trace:

[ 7344.885290] BTRFS critical (device sdi): unable to find logical 12845056 len 4096
[ 7344.887518] ------------[ cut here ]------------
[ 7344.888431] kernel BUG at fs/btrfs/inode.c:1833!
[ 7344.888431] invalid opcode: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
[ 7344.888431] Modules linked in: btrfs crc32c_generic xor ppdev raid6_pq psmouse sg acpi_cpufreq evdev i2c_piix4 tpm_tis serio_raw tpm i2c_core pcspkr parport_pc
[ 7344.888431] CPU: 0 PID: 6831 Comm: od Tainted: G        W       4.5.0-rc6-btrfs-next-28+ #1
[ 7344.888431] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS by qemu-project.org 04/01/2014
[ 7344.888431] task: ffff880215818600 ti: ffff880204684000 task.ti: ffff880204684000
[ 7344.888431] RIP: 0010:[<ffffffffa037c88c>]  [<ffffffffa037c88c>] btrfs_merge_bio_hook+0x54/0x6b [btrfs]
[ 7344.888431] RSP: 0018:ffff8802046878f0  EFLAGS: 00010282
[ 7344.888431] RAX: 00000000ffffffea RBX: 0000000000001000 RCX: 0000000000000001
[ 7344.888431] RDX: ffff88023ec0f950 RSI: ffffffff8183b638 RDI: 00000000ffffffff
[ 7344.888431] RBP: ffff880204687908 R08: 0000000000000001 R09: 0000000000000000
[ 7344.888431] R10: ffff880204687770 R11: ffffffff82f2d52d R12: 0000000000001000
[ 7344.888431] R13: ffff88021afbfee8 R14: 0000000000006208 R15: ffff88006cd199b0
[ 7344.888431] FS:  00007f1f9e1d6700(0000) GS:ffff88023ec00000(0000) knlGS:0000000000000000
[ 7344.888431] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 7344.888431] CR2: 00007f1f9dc8cb60 CR3: 000000023e3b6000 CR4: 00000000000006f0
[ 7344.888431] Stack:
[ 7344.888431]  0000000000001000 0000000000001000 ffff880204687b98 ffff880204687950
[ 7344.888431]  ffffffffa0395c8f ffffea0004d64d48 0000000000000000 0000000000001000
[ 7344.888431]  ffffea0004d64d48 0000000000001000 0000000000000000 0000000000000000
[ 7344.888431] Call Trace:
[ 7344.888431]  [<ffffffffa0395c8f>] submit_extent_page+0xf5/0x16f [btrfs]
[ 7344.888431]  [<ffffffffa03970ac>] __do_readpage+0x4a0/0x4f1 [btrfs]
[ 7344.888431]  [<ffffffffa039680d>] ? btrfs_create_repair_bio+0xcb/0xcb [btrfs]
[ 7344.888431]  [<ffffffffa037eeb4>] ? btrfs_writepage_start_hook+0xbc/0xbc [btrfs]
[ 7344.888431]  [<ffffffff8108df55>] ? trace_hardirqs_on+0xd/0xf
[ 7344.888431]  [<ffffffffa039728c>] __do_contiguous_readpages.constprop.26+0xc2/0xe4 [btrfs]
[ 7344.888431]  [<ffffffffa037eeb4>] ? btrfs_writepage_start_hook+0xbc/0xbc [btrfs]
[ 7344.888431]  [<ffffffffa039739b>] __extent_readpages.constprop.25+0xed/0x100 [btrfs]
[ 7344.888431]  [<ffffffff81129d24>] ? lru_cache_add+0xe/0x10
[ 7344.888431]  [<ffffffffa0397ea8>] extent_readpages+0x160/0x1aa [btrfs]
[ 7344.888431]  [<ffffffffa037eeb4>] ? btrfs_writepage_start_hook+0xbc/0xbc [btrfs]
[ 7344.888431]  [<ffffffff8115daad>] ? alloc_pages_current+0xa9/0xcd
[ 7344.888431]  [<ffffffffa037cdc9>] btrfs_readpages+0x1f/0x21 [btrfs]
[ 7344.888431]  [<ffffffff81128316>] __do_page_cache_readahead+0x168/0x1fc
[ 7344.888431]  [<ffffffff811285a0>] ondemand_readahead+0x1f6/0x207
[ 7344.888431]  [<ffffffff811285a0>] ? ondemand_readahead+0x1f6/0x207
[ 7344.888431]  [<ffffffff8111cf34>] ? pagecache_get_page+0x2b/0x154
[ 7344.888431]  [<ffffffff8112870e>] page_cache_sync_readahead+0x3d/0x3f
[ 7344.888431]  [<ffffffff8111dbf7>] generic_file_read_iter+0x197/0x4e1
[ 7344.888431]  [<ffffffff8117773a>] __vfs_read+0x79/0x9d
[ 7344.888431]  [<ffffffff81178050>] vfs_read+0x8f/0xd2
[ 7344.888431]  [<ffffffff81178a38>] SyS_read+0x50/0x7e
[ 7344.888431]  [<ffffffff81492017>] entry_SYSCALL_64_fastpath+0x12/0x6b
[ 7344.888431] Code: 8d 4d e8 45 31 c9 45 31 c0 48 8b 00 48 c1 e2 09 48 8b 80 80 fc ff ff 4c 89 65 e8 48 8b b8 f0 01 00 00 e8 1d 42 02 00 85 c0 79 02 <0f> 0b 4c 0
[ 7344.888431] RIP  [<ffffffffa037c88c>] btrfs_merge_bio_hook+0x54/0x6b [btrfs]
[ 7344.888431]  RSP <ffff8802046878f0>
[ 7344.970544] ---[ end trace eb7803b24ebab8ae ]---

Signed-off-by: Filipe Manana <fdmanana@suse.com>
---
 fs/btrfs/ctree.h       | 14 ++++++++++++
 fs/btrfs/extent-tree.c | 58 ++++++++++++++++++++++++++++++++++++++++++++++++--
 fs/btrfs/inode.c       |  8 +++++++
 fs/btrfs/relocation.c  |  6 +-----
 4 files changed, 79 insertions(+), 7 deletions(-)

diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 84a6a5b..90e70e2 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -1408,6 +1408,17 @@ struct btrfs_block_group_cache {
 
 	struct btrfs_io_ctl io_ctl;
 
+	/*
+	 * Incremented when doing extent allocations and holding a read lock
+	 * on the space_info's groups_sem semaphore.
+	 * Decremented when an ordered extent that represents an IO against this
+	 * block group's range is created (after it's added to its inode's
+	 * root's list of ordered extents) or immediately after the allocation
+	 * if it's a metadata extent or fallocate extent (for these cases we
+	 * don't create ordered extents).
+	 */
+	atomic_t reservations;
+
 	/* Lock for free space tree operations. */
 	struct mutex free_space_lock;
 
@@ -3499,6 +3510,9 @@ int btrfs_should_throttle_delayed_refs(struct btrfs_trans_handle *trans,
 				       struct btrfs_root *root);
 int btrfs_check_space_for_delayed_refs(struct btrfs_trans_handle *trans,
 				       struct btrfs_root *root);
+void btrfs_dec_block_group_reservations(struct btrfs_fs_info *fs_info,
+					 const u64 start);
+void btrfs_wait_block_group_reservations(struct btrfs_block_group_cache *bg);
 void btrfs_put_block_group(struct btrfs_block_group_cache *cache);
 int btrfs_run_delayed_refs(struct btrfs_trans_handle *trans,
 			   struct btrfs_root *root, unsigned long count);
diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 0544f70..35e3ea9 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -6175,6 +6175,57 @@ int btrfs_exclude_logged_extents(struct btrfs_root *log,
 	return 0;
 }
 
+static void
+btrfs_inc_block_group_reservations(struct btrfs_block_group_cache *bg)
+{
+	atomic_inc(&bg->reservations);
+}
+
+void btrfs_dec_block_group_reservations(struct btrfs_fs_info *fs_info,
+					const u64 start)
+{
+	struct btrfs_block_group_cache *bg;
+
+	bg = btrfs_lookup_block_group(fs_info, start);
+	BUG_ON(!bg);
+	if (atomic_dec_and_test(&bg->reservations))
+		wake_up_atomic_t(&bg->reservations);
+	btrfs_put_block_group(bg);
+}
+
+static int btrfs_wait_bg_reservations_atomic_t(atomic_t *a)
+{
+	schedule();
+	return 0;
+}
+
+void btrfs_wait_block_group_reservations(struct btrfs_block_group_cache *bg)
+{
+	struct btrfs_space_info *space_info = bg->space_info;
+
+	ASSERT(bg->ro);
+
+	if (!(bg->flags & BTRFS_BLOCK_GROUP_DATA))
+		return;
+
+	/*
+	 * Our block group is read only but before we set it to read only,
+	 * some task might have had allocated an extent from it already, but it
+	 * has not yet created a respective ordered extent (and added it to a
+	 * root's list of ordered extents).
+	 * Therefore wait for any task currently allocating extents, since the
+	 * block group's reservations counter is incremented while a read lock
+	 * on the groups' semaphore is held and decremented after releasing
+	 * the read access on that semaphore and creating the ordered extent.
+	 */
+	down_write(&space_info->groups_sem);
+	up_write(&space_info->groups_sem);
+
+	wait_on_atomic_t(&bg->reservations,
+			 btrfs_wait_bg_reservations_atomic_t,
+			 TASK_UNINTERRUPTIBLE);
+}
+
 /**
  * btrfs_update_reserved_bytes - update the block_group and space info counters
  * @cache:	The cache we are manipulating
@@ -7434,6 +7485,7 @@ checks:
 			btrfs_add_free_space(block_group, offset, num_bytes);
 			goto loop;
 		}
+		btrfs_inc_block_group_reservations(block_group);
 
 		/* we are all good, lets return */
 		ins->objectid = search_start;
@@ -7615,8 +7667,10 @@ again:
 	WARN_ON(num_bytes < root->sectorsize);
 	ret = find_free_extent(root, num_bytes, empty_size, hint_byte, ins,
 			       flags, delalloc);
-
-	if (ret == -ENOSPC) {
+	if (!ret && !is_data) {
+		btrfs_dec_block_group_reservations(root->fs_info,
+						   ins->objectid);
+	} else if (ret == -ENOSPC) {
 		if (!final_tried && ins->offset) {
 			num_bytes = min(num_bytes >> 1, ins->offset);
 			num_bytes = round_down(num_bytes, root->sectorsize);
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 41a5688..0085899 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -824,6 +824,7 @@ retry:
 						async_extent->ram_size - 1, 0);
 			goto out_free_reserve;
 		}
+		btrfs_dec_block_group_reservations(root->fs_info, ins.objectid);
 
 		/*
 		 * clear dirty, set writeback and unlock the pages.
@@ -861,6 +862,7 @@ retry:
 	}
 	return;
 out_free_reserve:
+	btrfs_dec_block_group_reservations(root->fs_info, ins.objectid);
 	btrfs_free_reserved_extent(root, ins.objectid, ins.offset, 1);
 out_free:
 	extent_clear_unlock_delalloc(inode, async_extent->start,
@@ -1038,6 +1040,8 @@ static noinline int cow_file_range(struct inode *inode,
 				goto out_drop_extent_cache;
 		}
 
+		btrfs_dec_block_group_reservations(root->fs_info, ins.objectid);
+
 		if (disk_num_bytes < cur_alloc_size)
 			break;
 
@@ -1066,6 +1070,7 @@ out:
 out_drop_extent_cache:
 	btrfs_drop_extent_cache(inode, start, start + ram_size - 1, 0);
 out_reserve:
+	btrfs_dec_block_group_reservations(root->fs_info, ins.objectid);
 	btrfs_free_reserved_extent(root, ins.objectid, ins.offset, 1);
 out_unlock:
 	extent_clear_unlock_delalloc(inode, start, end, locked_page,
@@ -7162,6 +7167,8 @@ static struct extent_map *btrfs_new_extent_direct(struct inode *inode,
 		return ERR_PTR(ret);
 	}
 
+	btrfs_dec_block_group_reservations(root->fs_info, ins.objectid);
+
 	em = create_pinned_em(inode, start, ins.offset, start, ins.objectid,
 			      ins.offset, ins.offset, ins.offset, 0);
 	if (IS_ERR(em)) {
@@ -9942,6 +9949,7 @@ static int __btrfs_prealloc_file_range(struct inode *inode, int mode,
 				btrfs_end_transaction(trans, root);
 			break;
 		}
+		btrfs_dec_block_group_reservations(root->fs_info, ins.objectid);
 
 		last_alloc = ins.offset;
 		ret = insert_reserved_file_extent(trans, inode,
diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
index 9c35ea2..2aed15c 100644
--- a/fs/btrfs/relocation.c
+++ b/fs/btrfs/relocation.c
@@ -4253,11 +4253,7 @@ int btrfs_relocate_block_group(struct btrfs_root *extent_root, u64 group_start)
 	btrfs_info(extent_root->fs_info, "relocating block group %llu flags %llu",
 	       rc->block_group->key.objectid, rc->block_group->flags);
 
-	ret = btrfs_start_delalloc_roots(fs_info, 0, -1);
-	if (ret < 0) {
-		err = ret;
-		goto out;
-	}
+	btrfs_wait_block_group_reservations(rc->block_group);
 	btrfs_wait_ordered_roots(fs_info, -1,
 				 rc->block_group->key.objectid,
 				 rc->block_group->key.offset);
-- 
2.7.0.rc3

Re: [PATCH v2 2/2] Btrfs: don't do unnecessary delalloc flushes when relocating

[Josef Bacik]

On 04/26/2016 11:39 AM, fdmanana@kernel.org wrote:
> From: Filipe Manana <fdmanana@suse.com>
>
> Before we start the actual relocation process of a block group, we do
> calls to flush delalloc of all inodes and then wait for ordered extents
> to complete. However we do these flush calls just to make sure we don't
> race with concurrent tasks that have actually already started to run
> delalloc and have allocated an extent from the block group we want to
> relocate, right before we set it to readonly mode, but have not yet
> created the respective ordered extents. The flush calls make us wait
> for such concurrent tasks because they end up calling
> filemap_fdatawrite_range() (through btrfs_start_delalloc_roots() ->
> __start_delalloc_inodes() -> btrfs_alloc_delalloc_work() ->
> btrfs_run_delalloc_work()) which ends up serializing us with those tasks
> due to attempts to lock the same pages (and the delalloc flush procedure
> calls the allocator and creates the ordered extents before unlocking the
> pages).
>
> These flushing calls not only make us waste time (cpu, IO) but also reduce
> the chances of writing larger extents (applications might be writing to
> contiguous ranges and we flush before they finish dirtying the whole
> ranges).
>
> So make sure we don't flush delalloc and just wait for concurrent tasks
> that have already started flushing delalloc and have allocated an extent
> from the block group we are about to relocate.
>
> This change also ends up fixing a race with direct IO writes that makes
> relocation not wait for direct IO ordered extents. This race is
> illustrated by the following diagram:
>
>         CPU 1                                       CPU 2
>
>  btrfs_relocate_block_group(bg X)
>
>                                            starts direct IO write,
>                                            target inode currently has no
>                                            ordered extents ongoing nor
>                                            dirty pages (delalloc regions),
>                                            therefore the root for our inode
>                                            is not in the list
>                                            fs_info->ordered_roots
>
>                                            btrfs_direct_IO()
>                                              __blockdev_direct_IO()
>                                                btrfs_get_blocks_direct()
>                                                  btrfs_lock_extent_direct()
>                                                    locks range in the io tree
>                                                  btrfs_new_extent_direct()
>                                                    btrfs_reserve_extent()
>                                                      --> extent allocated
>                                                          from bg X
>
>    btrfs_inc_block_group_ro(bg X)
>
>    btrfs_start_delalloc_roots()
>      __start_delalloc_inodes()
>        --> does nothing, no dealloc ranges
>            in the inode's io tree so the
>            inode's root is not in the list
>            fs_info->delalloc_roots
>
>    btrfs_wait_ordered_roots()
>      --> does not find the inode's root in the
>          list fs_info->ordered_roots
>
>      --> ends up not waiting for the direct IO
>          write started by the task at CPU 2
>
>    relocate_block_group(rc->stage ==
>      MOVE_DATA_EXTENTS)
>
>      prepare_to_relocate()
>        btrfs_commit_transaction()
>
>      iterates the extent tree, using its
>      commit root and moves extents into new
>      locations
>
>                                                    btrfs_add_ordered_extent_dio()
>                                                      --> now a ordered extent is
>                                                          created and added to the
>                                                          list root->ordered_extents
>                                                          and the root added to the
>                                                          list fs_info->ordered_roots
>                                                      --> this is too late and the
>                                                          task at CPU 1 already
>                                                          started the relocation
>
>      btrfs_commit_transaction()
>
>                                                    btrfs_finish_ordered_io()
>                                                      btrfs_alloc_reserved_file_extent()
>                                                        --> adds delayed data reference
>                                                            for the extent allocated
>                                                            from bg X
>
>    relocate_block_group(rc->stage ==
>      UPDATE_DATA_PTRS)
>
>      prepare_to_relocate()
>        btrfs_commit_transaction()
>          --> delayed refs are run, so an extent
>              item for the allocated extent from
>              bg X is added to extent tree
>          --> commit roots are switched, so the
>              next scan in the extent tree will
>              see the extent item
>
>      sees the extent in the extent tree
>
> When this happens the relocation produces the following warning when it
> finishes:
>
> [ 7260.832836] ------------[ cut here ]------------
> [ 7260.834653] WARNING: CPU: 5 PID: 6765 at fs/btrfs/relocation.c:4318 btrfs_relocate_block_group+0x245/0x2a1 [btrfs]()
> [ 7260.838268] Modules linked in: btrfs crc32c_generic xor ppdev raid6_pq psmouse sg acpi_cpufreq evdev i2c_piix4 tpm_tis serio_raw tpm i2c_core pcspkr parport_pc
> [ 7260.850935] CPU: 5 PID: 6765 Comm: btrfs Not tainted 4.5.0-rc6-btrfs-next-28+ #1
> [ 7260.852998] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS by qemu-project.org 04/01/2014
> [ 7260.852998]  0000000000000000 ffff88020bf57bc0 ffffffff812648b3 0000000000000000
> [ 7260.852998]  0000000000000009 ffff88020bf57bf8 ffffffff81051608 ffffffffa03c1b2d
> [ 7260.852998]  ffff8800b2bbb800 0000000000000000 ffff8800b17bcc58 ffff8800399dd000
> [ 7260.852998] Call Trace:
> [ 7260.852998]  [<ffffffff812648b3>] dump_stack+0x67/0x90
> [ 7260.852998]  [<ffffffff81051608>] warn_slowpath_common+0x99/0xb2
> [ 7260.852998]  [<ffffffffa03c1b2d>] ? btrfs_relocate_block_group+0x245/0x2a1 [btrfs]
> [ 7260.852998]  [<ffffffff810516d4>] warn_slowpath_null+0x1a/0x1c
> [ 7260.852998]  [<ffffffffa03c1b2d>] btrfs_relocate_block_group+0x245/0x2a1 [btrfs]
> [ 7260.852998]  [<ffffffffa039d9de>] btrfs_relocate_chunk.isra.29+0x66/0xdb [btrfs]
> [ 7260.852998]  [<ffffffffa039f314>] btrfs_balance+0xde1/0xe4e [btrfs]
> [ 7260.852998]  [<ffffffff8127d671>] ? debug_smp_processor_id+0x17/0x19
> [ 7260.852998]  [<ffffffffa03a9583>] btrfs_ioctl_balance+0x255/0x2d3 [btrfs]
> [ 7260.852998]  [<ffffffffa03ac96a>] btrfs_ioctl+0x11e0/0x1dff [btrfs]
> [ 7260.852998]  [<ffffffff811451df>] ? handle_mm_fault+0x443/0xd63
> [ 7260.852998]  [<ffffffff81491817>] ? _raw_spin_unlock+0x31/0x44
> [ 7260.852998]  [<ffffffff8108b36a>] ? arch_local_irq_save+0x9/0xc
> [ 7260.852998]  [<ffffffff811876ab>] vfs_ioctl+0x18/0x34
> [ 7260.852998]  [<ffffffff81187cb2>] do_vfs_ioctl+0x550/0x5be
> [ 7260.852998]  [<ffffffff81190c30>] ? __fget_light+0x4d/0x71
> [ 7260.852998]  [<ffffffff81187d77>] SyS_ioctl+0x57/0x79
> [ 7260.852998]  [<ffffffff81492017>] entry_SYSCALL_64_fastpath+0x12/0x6b
> [ 7260.893268] ---[ end trace eb7803b24ebab8ad ]---
>
> This is because at the end of the first stage, in relocate_block_group(),
> we commit the current transaction, which makes delayed refs run, the
> commit roots are switched and so the second stage will find the extent
> item that the ordered extent added to the delayed refs. But this extent
> was not moved (ordered extent completed after first stage finished), so
> at the end of the relocation our block group item still has a positive
> used bytes counter, triggering a warning at the end of
> btrfs_relocate_block_group(). Later on when trying to read the extent
> contents from disk we hit a BUG_ON() due to the inability to map a block
> with a logical address that belongs to the block group we relocated and
> is no longer valid, resulting in the following trace:
>
> [ 7344.885290] BTRFS critical (device sdi): unable to find logical 12845056 len 4096
> [ 7344.887518] ------------[ cut here ]------------
> [ 7344.888431] kernel BUG at fs/btrfs/inode.c:1833!
> [ 7344.888431] invalid opcode: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
> [ 7344.888431] Modules linked in: btrfs crc32c_generic xor ppdev raid6_pq psmouse sg acpi_cpufreq evdev i2c_piix4 tpm_tis serio_raw tpm i2c_core pcspkr parport_pc
> [ 7344.888431] CPU: 0 PID: 6831 Comm: od Tainted: G        W       4.5.0-rc6-btrfs-next-28+ #1
> [ 7344.888431] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS by qemu-project.org 04/01/2014
> [ 7344.888431] task: ffff880215818600 ti: ffff880204684000 task.ti: ffff880204684000
> [ 7344.888431] RIP: 0010:[<ffffffffa037c88c>]  [<ffffffffa037c88c>] btrfs_merge_bio_hook+0x54/0x6b [btrfs]
> [ 7344.888431] RSP: 0018:ffff8802046878f0  EFLAGS: 00010282
> [ 7344.888431] RAX: 00000000ffffffea RBX: 0000000000001000 RCX: 0000000000000001
> [ 7344.888431] RDX: ffff88023ec0f950 RSI: ffffffff8183b638 RDI: 00000000ffffffff
> [ 7344.888431] RBP: ffff880204687908 R08: 0000000000000001 R09: 0000000000000000
> [ 7344.888431] R10: ffff880204687770 R11: ffffffff82f2d52d R12: 0000000000001000
> [ 7344.888431] R13: ffff88021afbfee8 R14: 0000000000006208 R15: ffff88006cd199b0
> [ 7344.888431] FS:  00007f1f9e1d6700(0000) GS:ffff88023ec00000(0000) knlGS:0000000000000000
> [ 7344.888431] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [ 7344.888431] CR2: 00007f1f9dc8cb60 CR3: 000000023e3b6000 CR4: 00000000000006f0
> [ 7344.888431] Stack:
> [ 7344.888431]  0000000000001000 0000000000001000 ffff880204687b98 ffff880204687950
> [ 7344.888431]  ffffffffa0395c8f ffffea0004d64d48 0000000000000000 0000000000001000
> [ 7344.888431]  ffffea0004d64d48 0000000000001000 0000000000000000 0000000000000000
> [ 7344.888431] Call Trace:
> [ 7344.888431]  [<ffffffffa0395c8f>] submit_extent_page+0xf5/0x16f [btrfs]
> [ 7344.888431]  [<ffffffffa03970ac>] __do_readpage+0x4a0/0x4f1 [btrfs]
> [ 7344.888431]  [<ffffffffa039680d>] ? btrfs_create_repair_bio+0xcb/0xcb [btrfs]
> [ 7344.888431]  [<ffffffffa037eeb4>] ? btrfs_writepage_start_hook+0xbc/0xbc [btrfs]
> [ 7344.888431]  [<ffffffff8108df55>] ? trace_hardirqs_on+0xd/0xf
> [ 7344.888431]  [<ffffffffa039728c>] __do_contiguous_readpages.constprop.26+0xc2/0xe4 [btrfs]
> [ 7344.888431]  [<ffffffffa037eeb4>] ? btrfs_writepage_start_hook+0xbc/0xbc [btrfs]
> [ 7344.888431]  [<ffffffffa039739b>] __extent_readpages.constprop.25+0xed/0x100 [btrfs]
> [ 7344.888431]  [<ffffffff81129d24>] ? lru_cache_add+0xe/0x10
> [ 7344.888431]  [<ffffffffa0397ea8>] extent_readpages+0x160/0x1aa [btrfs]
> [ 7344.888431]  [<ffffffffa037eeb4>] ? btrfs_writepage_start_hook+0xbc/0xbc [btrfs]
> [ 7344.888431]  [<ffffffff8115daad>] ? alloc_pages_current+0xa9/0xcd
> [ 7344.888431]  [<ffffffffa037cdc9>] btrfs_readpages+0x1f/0x21 [btrfs]
> [ 7344.888431]  [<ffffffff81128316>] __do_page_cache_readahead+0x168/0x1fc
> [ 7344.888431]  [<ffffffff811285a0>] ondemand_readahead+0x1f6/0x207
> [ 7344.888431]  [<ffffffff811285a0>] ? ondemand_readahead+0x1f6/0x207
> [ 7344.888431]  [<ffffffff8111cf34>] ? pagecache_get_page+0x2b/0x154
> [ 7344.888431]  [<ffffffff8112870e>] page_cache_sync_readahead+0x3d/0x3f
> [ 7344.888431]  [<ffffffff8111dbf7>] generic_file_read_iter+0x197/0x4e1
> [ 7344.888431]  [<ffffffff8117773a>] __vfs_read+0x79/0x9d
> [ 7344.888431]  [<ffffffff81178050>] vfs_read+0x8f/0xd2
> [ 7344.888431]  [<ffffffff81178a38>] SyS_read+0x50/0x7e
> [ 7344.888431]  [<ffffffff81492017>] entry_SYSCALL_64_fastpath+0x12/0x6b
> [ 7344.888431] Code: 8d 4d e8 45 31 c9 45 31 c0 48 8b 00 48 c1 e2 09 48 8b 80 80 fc ff ff 4c 89 65 e8 48 8b b8 f0 01 00 00 e8 1d 42 02 00 85 c0 79 02 <0f> 0b 4c 0
> [ 7344.888431] RIP  [<ffffffffa037c88c>] btrfs_merge_bio_hook+0x54/0x6b [btrfs]
> [ 7344.888431]  RSP <ffff8802046878f0>
> [ 7344.970544] ---[ end trace eb7803b24ebab8ae ]---
>
> Signed-off-by: Filipe Manana <fdmanana@suse.com>
> ---
>  fs/btrfs/ctree.h       | 14 ++++++++++++
>  fs/btrfs/extent-tree.c | 58 ++++++++++++++++++++++++++++++++++++++++++++++++--
>  fs/btrfs/inode.c       |  8 +++++++
>  fs/btrfs/relocation.c  |  6 +-----
>  4 files changed, 79 insertions(+), 7 deletions(-)
>
> diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
> index 84a6a5b..90e70e2 100644
> --- a/fs/btrfs/ctree.h
> +++ b/fs/btrfs/ctree.h
> @@ -1408,6 +1408,17 @@ struct btrfs_block_group_cache {
>
>  	struct btrfs_io_ctl io_ctl;
>
> +	/*
> +	 * Incremented when doing extent allocations and holding a read lock
> +	 * on the space_info's groups_sem semaphore.
> +	 * Decremented when an ordered extent that represents an IO against this
> +	 * block group's range is created (after it's added to its inode's
> +	 * root's list of ordered extents) or immediately after the allocation
> +	 * if it's a metadata extent or fallocate extent (for these cases we
> +	 * don't create ordered extents).
> +	 */
> +	atomic_t reservations;
> +
>  	/* Lock for free space tree operations. */
>  	struct mutex free_space_lock;
>
> @@ -3499,6 +3510,9 @@ int btrfs_should_throttle_delayed_refs(struct btrfs_trans_handle *trans,
>  				       struct btrfs_root *root);
>  int btrfs_check_space_for_delayed_refs(struct btrfs_trans_handle *trans,
>  				       struct btrfs_root *root);
> +void btrfs_dec_block_group_reservations(struct btrfs_fs_info *fs_info,
> +					 const u64 start);
> +void btrfs_wait_block_group_reservations(struct btrfs_block_group_cache *bg);
>  void btrfs_put_block_group(struct btrfs_block_group_cache *cache);
>  int btrfs_run_delayed_refs(struct btrfs_trans_handle *trans,
>  			   struct btrfs_root *root, unsigned long count);
> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> index 0544f70..35e3ea9 100644
> --- a/fs/btrfs/extent-tree.c
> +++ b/fs/btrfs/extent-tree.c
> @@ -6175,6 +6175,57 @@ int btrfs_exclude_logged_extents(struct btrfs_root *log,
>  	return 0;
>  }
>
> +static void
> +btrfs_inc_block_group_reservations(struct btrfs_block_group_cache *bg)
> +{
> +	atomic_inc(&bg->reservations);
> +}
> +
> +void btrfs_dec_block_group_reservations(struct btrfs_fs_info *fs_info,
> +					const u64 start)
> +{
> +	struct btrfs_block_group_cache *bg;
> +
> +	bg = btrfs_lookup_block_group(fs_info, start);
> +	BUG_ON(!bg);

ASSERT(bg) instead please.

> +	if (atomic_dec_and_test(&bg->reservations))
> +		wake_up_atomic_t(&bg->reservations);
> +	btrfs_put_block_group(bg);
> +}
> +
> +static int btrfs_wait_bg_reservations_atomic_t(atomic_t *a)
> +{
> +	schedule();
> +	return 0;
> +}
> +
> +void btrfs_wait_block_group_reservations(struct btrfs_block_group_cache *bg)
> +{
> +	struct btrfs_space_info *space_info = bg->space_info;
> +
> +	ASSERT(bg->ro);
> +
> +	if (!(bg->flags & BTRFS_BLOCK_GROUP_DATA))
> +		return;
> +
> +	/*
> +	 * Our block group is read only but before we set it to read only,
> +	 * some task might have had allocated an extent from it already, but it
> +	 * has not yet created a respective ordered extent (and added it to a
> +	 * root's list of ordered extents).
> +	 * Therefore wait for any task currently allocating extents, since the
> +	 * block group's reservations counter is incremented while a read lock
> +	 * on the groups' semaphore is held and decremented after releasing
> +	 * the read access on that semaphore and creating the ordered extent.
> +	 */
> +	down_write(&space_info->groups_sem);
> +	up_write(&space_info->groups_sem);
> +
> +	wait_on_atomic_t(&bg->reservations,
> +			 btrfs_wait_bg_reservations_atomic_t,
> +			 TASK_UNINTERRUPTIBLE);
> +}
> +
>  /**
>   * btrfs_update_reserved_bytes - update the block_group and space info counters
>   * @cache:	The cache we are manipulating
> @@ -7434,6 +7485,7 @@ checks:
>  			btrfs_add_free_space(block_group, offset, num_bytes);
>  			goto loop;
>  		}
> +		btrfs_inc_block_group_reservations(block_group);
>
>  		/* we are all good, lets return */
>  		ins->objectid = search_start;
> @@ -7615,8 +7667,10 @@ again:
>  	WARN_ON(num_bytes < root->sectorsize);
>  	ret = find_free_extent(root, num_bytes, empty_size, hint_byte, ins,
>  			       flags, delalloc);
> -
> -	if (ret == -ENOSPC) {
> +	if (!ret && !is_data) {
> +		btrfs_dec_block_group_reservations(root->fs_info,
> +						   ins->objectid);
> +	} else if (ret == -ENOSPC) {
>  		if (!final_tried && ins->offset) {
>  			num_bytes = min(num_bytes >> 1, ins->offset);
>  			num_bytes = round_down(num_bytes, root->sectorsize);
> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
> index 41a5688..0085899 100644
> --- a/fs/btrfs/inode.c
> +++ b/fs/btrfs/inode.c
> @@ -824,6 +824,7 @@ retry:
>  						async_extent->ram_size - 1, 0);
>  			goto out_free_reserve;
>  		}
> +		btrfs_dec_block_group_reservations(root->fs_info, ins.objectid);
>
>  		/*
>  		 * clear dirty, set writeback and unlock the pages.
> @@ -861,6 +862,7 @@ retry:
>  	}
>  	return;
>  out_free_reserve:
> +	btrfs_dec_block_group_reservations(root->fs_info, ins.objectid);
>  	btrfs_free_reserved_extent(root, ins.objectid, ins.offset, 1);
>  out_free:
>  	extent_clear_unlock_delalloc(inode, async_extent->start,
> @@ -1038,6 +1040,8 @@ static noinline int cow_file_range(struct inode *inode,
>  				goto out_drop_extent_cache;
>  		}
>
> +		btrfs_dec_block_group_reservations(root->fs_info, ins.objectid);
> +
>  		if (disk_num_bytes < cur_alloc_size)
>  			break;
>
> @@ -1066,6 +1070,7 @@ out:
>  out_drop_extent_cache:
>  	btrfs_drop_extent_cache(inode, start, start + ram_size - 1, 0);
>  out_reserve:
> +	btrfs_dec_block_group_reservations(root->fs_info, ins.objectid);
>  	btrfs_free_reserved_extent(root, ins.objectid, ins.offset, 1);
>  out_unlock:
>  	extent_clear_unlock_delalloc(inode, start, end, locked_page,
> @@ -7162,6 +7167,8 @@ static struct extent_map *btrfs_new_extent_direct(struct inode *inode,
>  		return ERR_PTR(ret);
>  	}
>
> +	btrfs_dec_block_group_reservations(root->fs_info, ins.objectid);
> +
>  	em = create_pinned_em(inode, start, ins.offset, start, ins.objectid,
>  			      ins.offset, ins.offset, ins.offset, 0);
>  	if (IS_ERR(em)) {
> @@ -9942,6 +9949,7 @@ static int __btrfs_prealloc_file_range(struct inode *inode, int mode,
>  				btrfs_end_transaction(trans, root);
>  			break;
>  		}
> +		btrfs_dec_block_group_reservations(root->fs_info, ins.objectid);
>
>  		last_alloc = ins.offset;
>  		ret = insert_reserved_file_extent(trans, inode,

Instead of doing all this, why not just add the dec to the end of 
__btrfs_add_ordered_extents?  It seems like it would be much cleaner. 
Thanks,

Josef

Re: [PATCH v2 2/2] Btrfs: don't do unnecessary delalloc flushes when relocating

[Filipe Manana]

On Tue, Apr 26, 2016 at 5:02 PM, Josef Bacik <jbacik@fb.com> wrote:
> On 04/26/2016 11:39 AM, fdmanana@kernel.org wrote:
>>
>> From: Filipe Manana <fdmanana@suse.com>
>>
>> Before we start the actual relocation process of a block group, we do
>> calls to flush delalloc of all inodes and then wait for ordered extents
>> to complete. However we do these flush calls just to make sure we don't
>> race with concurrent tasks that have actually already started to run
>> delalloc and have allocated an extent from the block group we want to
>> relocate, right before we set it to readonly mode, but have not yet
>> created the respective ordered extents. The flush calls make us wait
>> for such concurrent tasks because they end up calling
>> filemap_fdatawrite_range() (through btrfs_start_delalloc_roots() ->
>> __start_delalloc_inodes() -> btrfs_alloc_delalloc_work() ->
>> btrfs_run_delalloc_work()) which ends up serializing us with those tasks
>> due to attempts to lock the same pages (and the delalloc flush procedure
>> calls the allocator and creates the ordered extents before unlocking the
>> pages).
>>
>> These flushing calls not only make us waste time (cpu, IO) but also reduce
>> the chances of writing larger extents (applications might be writing to
>> contiguous ranges and we flush before they finish dirtying the whole
>> ranges).
>>
>> So make sure we don't flush delalloc and just wait for concurrent tasks
>> that have already started flushing delalloc and have allocated an extent
>> from the block group we are about to relocate.
>>
>> This change also ends up fixing a race with direct IO writes that makes
>> relocation not wait for direct IO ordered extents. This race is
>> illustrated by the following diagram:
>>
>>         CPU 1                                       CPU 2
>>
>>  btrfs_relocate_block_group(bg X)
>>
>>                                            starts direct IO write,
>>                                            target inode currently has no
>>                                            ordered extents ongoing nor
>>                                            dirty pages (delalloc regions),
>>                                            therefore the root for our
>> inode
>>                                            is not in the list
>>                                            fs_info->ordered_roots
>>
>>                                            btrfs_direct_IO()
>>                                              __blockdev_direct_IO()
>>                                                btrfs_get_blocks_direct()
>>
>> btrfs_lock_extent_direct()
>>                                                    locks range in the io
>> tree
>>                                                  btrfs_new_extent_direct()
>>                                                    btrfs_reserve_extent()
>>                                                      --> extent allocated
>>                                                          from bg X
>>
>>    btrfs_inc_block_group_ro(bg X)
>>
>>    btrfs_start_delalloc_roots()
>>      __start_delalloc_inodes()
>>        --> does nothing, no dealloc ranges
>>            in the inode's io tree so the
>>            inode's root is not in the list
>>            fs_info->delalloc_roots
>>
>>    btrfs_wait_ordered_roots()
>>      --> does not find the inode's root in the
>>          list fs_info->ordered_roots
>>
>>      --> ends up not waiting for the direct IO
>>          write started by the task at CPU 2
>>
>>    relocate_block_group(rc->stage ==
>>      MOVE_DATA_EXTENTS)
>>
>>      prepare_to_relocate()
>>        btrfs_commit_transaction()
>>
>>      iterates the extent tree, using its
>>      commit root and moves extents into new
>>      locations
>>
>>
>> btrfs_add_ordered_extent_dio()
>>                                                      --> now a ordered
>> extent is
>>                                                          created and added
>> to the
>>                                                          list
>> root->ordered_extents
>>                                                          and the root
>> added to the
>>                                                          list
>> fs_info->ordered_roots
>>                                                      --> this is too late
>> and the
>>                                                          task at CPU 1
>> already
>>                                                          started the
>> relocation
>>
>>      btrfs_commit_transaction()
>>
>>
>> btrfs_finish_ordered_io()
>>
>> btrfs_alloc_reserved_file_extent()
>>                                                        --> adds delayed
>> data reference
>>                                                            for the extent
>> allocated
>>                                                            from bg X
>>
>>    relocate_block_group(rc->stage ==
>>      UPDATE_DATA_PTRS)
>>
>>      prepare_to_relocate()
>>        btrfs_commit_transaction()
>>          --> delayed refs are run, so an extent
>>              item for the allocated extent from
>>              bg X is added to extent tree
>>          --> commit roots are switched, so the
>>              next scan in the extent tree will
>>              see the extent item
>>
>>      sees the extent in the extent tree
>>
>> When this happens the relocation produces the following warning when it
>> finishes:
>>
>> [ 7260.832836] ------------[ cut here ]------------
>> [ 7260.834653] WARNING: CPU: 5 PID: 6765 at fs/btrfs/relocation.c:4318
>> btrfs_relocate_block_group+0x245/0x2a1 [btrfs]()
>> [ 7260.838268] Modules linked in: btrfs crc32c_generic xor ppdev raid6_pq
>> psmouse sg acpi_cpufreq evdev i2c_piix4 tpm_tis serio_raw tpm i2c_core
>> pcspkr parport_pc
>> [ 7260.850935] CPU: 5 PID: 6765 Comm: btrfs Not tainted
>> 4.5.0-rc6-btrfs-next-28+ #1
>> [ 7260.852998] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
>> by qemu-project.org 04/01/2014
>> [ 7260.852998]  0000000000000000 ffff88020bf57bc0 ffffffff812648b3
>> 0000000000000000
>> [ 7260.852998]  0000000000000009 ffff88020bf57bf8 ffffffff81051608
>> ffffffffa03c1b2d
>> [ 7260.852998]  ffff8800b2bbb800 0000000000000000 ffff8800b17bcc58
>> ffff8800399dd000
>> [ 7260.852998] Call Trace:
>> [ 7260.852998]  [<ffffffff812648b3>] dump_stack+0x67/0x90
>> [ 7260.852998]  [<ffffffff81051608>] warn_slowpath_common+0x99/0xb2
>> [ 7260.852998]  [<ffffffffa03c1b2d>] ?
>> btrfs_relocate_block_group+0x245/0x2a1 [btrfs]
>> [ 7260.852998]  [<ffffffff810516d4>] warn_slowpath_null+0x1a/0x1c
>> [ 7260.852998]  [<ffffffffa03c1b2d>]
>> btrfs_relocate_block_group+0x245/0x2a1 [btrfs]
>> [ 7260.852998]  [<ffffffffa039d9de>]
>> btrfs_relocate_chunk.isra.29+0x66/0xdb [btrfs]
>> [ 7260.852998]  [<ffffffffa039f314>] btrfs_balance+0xde1/0xe4e [btrfs]
>> [ 7260.852998]  [<ffffffff8127d671>] ? debug_smp_processor_id+0x17/0x19
>> [ 7260.852998]  [<ffffffffa03a9583>] btrfs_ioctl_balance+0x255/0x2d3
>> [btrfs]
>> [ 7260.852998]  [<ffffffffa03ac96a>] btrfs_ioctl+0x11e0/0x1dff [btrfs]
>> [ 7260.852998]  [<ffffffff811451df>] ? handle_mm_fault+0x443/0xd63
>> [ 7260.852998]  [<ffffffff81491817>] ? _raw_spin_unlock+0x31/0x44
>> [ 7260.852998]  [<ffffffff8108b36a>] ? arch_local_irq_save+0x9/0xc
>> [ 7260.852998]  [<ffffffff811876ab>] vfs_ioctl+0x18/0x34
>> [ 7260.852998]  [<ffffffff81187cb2>] do_vfs_ioctl+0x550/0x5be
>> [ 7260.852998]  [<ffffffff81190c30>] ? __fget_light+0x4d/0x71
>> [ 7260.852998]  [<ffffffff81187d77>] SyS_ioctl+0x57/0x79
>> [ 7260.852998]  [<ffffffff81492017>] entry_SYSCALL_64_fastpath+0x12/0x6b
>> [ 7260.893268] ---[ end trace eb7803b24ebab8ad ]---
>>
>> This is because at the end of the first stage, in relocate_block_group(),
>> we commit the current transaction, which makes delayed refs run, the
>> commit roots are switched and so the second stage will find the extent
>> item that the ordered extent added to the delayed refs. But this extent
>> was not moved (ordered extent completed after first stage finished), so
>> at the end of the relocation our block group item still has a positive
>> used bytes counter, triggering a warning at the end of
>> btrfs_relocate_block_group(). Later on when trying to read the extent
>> contents from disk we hit a BUG_ON() due to the inability to map a block
>> with a logical address that belongs to the block group we relocated and
>> is no longer valid, resulting in the following trace:
>>
>> [ 7344.885290] BTRFS critical (device sdi): unable to find logical
>> 12845056 len 4096
>> [ 7344.887518] ------------[ cut here ]------------
>> [ 7344.888431] kernel BUG at fs/btrfs/inode.c:1833!
>> [ 7344.888431] invalid opcode: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
>> [ 7344.888431] Modules linked in: btrfs crc32c_generic xor ppdev raid6_pq
>> psmouse sg acpi_cpufreq evdev i2c_piix4 tpm_tis serio_raw tpm i2c_core
>> pcspkr parport_pc
>> [ 7344.888431] CPU: 0 PID: 6831 Comm: od Tainted: G        W
>> 4.5.0-rc6-btrfs-next-28+ #1
>> [ 7344.888431] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
>> by qemu-project.org 04/01/2014
>> [ 7344.888431] task: ffff880215818600 ti: ffff880204684000 task.ti:
>> ffff880204684000
>> [ 7344.888431] RIP: 0010:[<ffffffffa037c88c>]  [<ffffffffa037c88c>]
>> btrfs_merge_bio_hook+0x54/0x6b [btrfs]
>> [ 7344.888431] RSP: 0018:ffff8802046878f0  EFLAGS: 00010282
>> [ 7344.888431] RAX: 00000000ffffffea RBX: 0000000000001000 RCX:
>> 0000000000000001
>> [ 7344.888431] RDX: ffff88023ec0f950 RSI: ffffffff8183b638 RDI:
>> 00000000ffffffff
>> [ 7344.888431] RBP: ffff880204687908 R08: 0000000000000001 R09:
>> 0000000000000000
>> [ 7344.888431] R10: ffff880204687770 R11: ffffffff82f2d52d R12:
>> 0000000000001000
>> [ 7344.888431] R13: ffff88021afbfee8 R14: 0000000000006208 R15:
>> ffff88006cd199b0
>> [ 7344.888431] FS:  00007f1f9e1d6700(0000) GS:ffff88023ec00000(0000)
>> knlGS:0000000000000000
>> [ 7344.888431] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> [ 7344.888431] CR2: 00007f1f9dc8cb60 CR3: 000000023e3b6000 CR4:
>> 00000000000006f0
>> [ 7344.888431] Stack:
>> [ 7344.888431]  0000000000001000 0000000000001000 ffff880204687b98
>> ffff880204687950
>> [ 7344.888431]  ffffffffa0395c8f ffffea0004d64d48 0000000000000000
>> 0000000000001000
>> [ 7344.888431]  ffffea0004d64d48 0000000000001000 0000000000000000
>> 0000000000000000
>> [ 7344.888431] Call Trace:
>> [ 7344.888431]  [<ffffffffa0395c8f>] submit_extent_page+0xf5/0x16f [btrfs]
>> [ 7344.888431]  [<ffffffffa03970ac>] __do_readpage+0x4a0/0x4f1 [btrfs]
>> [ 7344.888431]  [<ffffffffa039680d>] ? btrfs_create_repair_bio+0xcb/0xcb
>> [btrfs]
>> [ 7344.888431]  [<ffffffffa037eeb4>] ?
>> btrfs_writepage_start_hook+0xbc/0xbc [btrfs]
>> [ 7344.888431]  [<ffffffff8108df55>] ? trace_hardirqs_on+0xd/0xf
>> [ 7344.888431]  [<ffffffffa039728c>]
>> __do_contiguous_readpages.constprop.26+0xc2/0xe4 [btrfs]
>> [ 7344.888431]  [<ffffffffa037eeb4>] ?
>> btrfs_writepage_start_hook+0xbc/0xbc [btrfs]
>> [ 7344.888431]  [<ffffffffa039739b>]
>> __extent_readpages.constprop.25+0xed/0x100 [btrfs]
>> [ 7344.888431]  [<ffffffff81129d24>] ? lru_cache_add+0xe/0x10
>> [ 7344.888431]  [<ffffffffa0397ea8>] extent_readpages+0x160/0x1aa [btrfs]
>> [ 7344.888431]  [<ffffffffa037eeb4>] ?
>> btrfs_writepage_start_hook+0xbc/0xbc [btrfs]
>> [ 7344.888431]  [<ffffffff8115daad>] ? alloc_pages_current+0xa9/0xcd
>> [ 7344.888431]  [<ffffffffa037cdc9>] btrfs_readpages+0x1f/0x21 [btrfs]
>> [ 7344.888431]  [<ffffffff81128316>] __do_page_cache_readahead+0x168/0x1fc
>> [ 7344.888431]  [<ffffffff811285a0>] ondemand_readahead+0x1f6/0x207
>> [ 7344.888431]  [<ffffffff811285a0>] ? ondemand_readahead+0x1f6/0x207
>> [ 7344.888431]  [<ffffffff8111cf34>] ? pagecache_get_page+0x2b/0x154
>> [ 7344.888431]  [<ffffffff8112870e>] page_cache_sync_readahead+0x3d/0x3f
>> [ 7344.888431]  [<ffffffff8111dbf7>] generic_file_read_iter+0x197/0x4e1
>> [ 7344.888431]  [<ffffffff8117773a>] __vfs_read+0x79/0x9d
>> [ 7344.888431]  [<ffffffff81178050>] vfs_read+0x8f/0xd2
>> [ 7344.888431]  [<ffffffff81178a38>] SyS_read+0x50/0x7e
>> [ 7344.888431]  [<ffffffff81492017>] entry_SYSCALL_64_fastpath+0x12/0x6b
>> [ 7344.888431] Code: 8d 4d e8 45 31 c9 45 31 c0 48 8b 00 48 c1 e2 09 48 8b
>> 80 80 fc ff ff 4c 89 65 e8 48 8b b8 f0 01 00 00 e8 1d 42 02 00 85 c0 79 02
>> <0f> 0b 4c 0
>> [ 7344.888431] RIP  [<ffffffffa037c88c>] btrfs_merge_bio_hook+0x54/0x6b
>> [btrfs]
>> [ 7344.888431]  RSP <ffff8802046878f0>
>> [ 7344.970544] ---[ end trace eb7803b24ebab8ae ]---
>>
>> Signed-off-by: Filipe Manana <fdmanana@suse.com>
>> ---
>>  fs/btrfs/ctree.h       | 14 ++++++++++++
>>  fs/btrfs/extent-tree.c | 58
>> ++++++++++++++++++++++++++++++++++++++++++++++++--
>>  fs/btrfs/inode.c       |  8 +++++++
>>  fs/btrfs/relocation.c  |  6 +-----
>>  4 files changed, 79 insertions(+), 7 deletions(-)
>>
>> diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
>> index 84a6a5b..90e70e2 100644
>> --- a/fs/btrfs/ctree.h
>> +++ b/fs/btrfs/ctree.h
>> @@ -1408,6 +1408,17 @@ struct btrfs_block_group_cache {
>>
>>         struct btrfs_io_ctl io_ctl;
>>
>> +       /*
>> +        * Incremented when doing extent allocations and holding a read
>> lock
>> +        * on the space_info's groups_sem semaphore.
>> +        * Decremented when an ordered extent that represents an IO
>> against this
>> +        * block group's range is created (after it's added to its inode's
>> +        * root's list of ordered extents) or immediately after the
>> allocation
>> +        * if it's a metadata extent or fallocate extent (for these cases
>> we
>> +        * don't create ordered extents).
>> +        */
>> +       atomic_t reservations;
>> +
>>         /* Lock for free space tree operations. */
>>         struct mutex free_space_lock;
>>
>> @@ -3499,6 +3510,9 @@ int btrfs_should_throttle_delayed_refs(struct
>> btrfs_trans_handle *trans,
>>                                        struct btrfs_root *root);
>>  int btrfs_check_space_for_delayed_refs(struct btrfs_trans_handle *trans,
>>                                        struct btrfs_root *root);
>> +void btrfs_dec_block_group_reservations(struct btrfs_fs_info *fs_info,
>> +                                        const u64 start);
>> +void btrfs_wait_block_group_reservations(struct btrfs_block_group_cache
>> *bg);
>>  void btrfs_put_block_group(struct btrfs_block_group_cache *cache);
>>  int btrfs_run_delayed_refs(struct btrfs_trans_handle *trans,
>>                            struct btrfs_root *root, unsigned long count);
>> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
>> index 0544f70..35e3ea9 100644
>> --- a/fs/btrfs/extent-tree.c
>> +++ b/fs/btrfs/extent-tree.c
>> @@ -6175,6 +6175,57 @@ int btrfs_exclude_logged_extents(struct btrfs_root
>> *log,
>>         return 0;
>>  }
>>
>> +static void
>> +btrfs_inc_block_group_reservations(struct btrfs_block_group_cache *bg)
>> +{
>> +       atomic_inc(&bg->reservations);
>> +}
>> +
>> +void btrfs_dec_block_group_reservations(struct btrfs_fs_info *fs_info,
>> +                                       const u64 start)
>> +{
>> +       struct btrfs_block_group_cache *bg;
>> +
>> +       bg = btrfs_lookup_block_group(fs_info, start);
>> +       BUG_ON(!bg);
>
>
> ASSERT(bg) instead please.
>
>
>> +       if (atomic_dec_and_test(&bg->reservations))
>> +               wake_up_atomic_t(&bg->reservations);
>> +       btrfs_put_block_group(bg);
>> +}
>> +
>> +static int btrfs_wait_bg_reservations_atomic_t(atomic_t *a)
>> +{
>> +       schedule();
>> +       return 0;
>> +}
>> +
>> +void btrfs_wait_block_group_reservations(struct btrfs_block_group_cache
>> *bg)
>> +{
>> +       struct btrfs_space_info *space_info = bg->space_info;
>> +
>> +       ASSERT(bg->ro);
>> +
>> +       if (!(bg->flags & BTRFS_BLOCK_GROUP_DATA))
>> +               return;
>> +
>> +       /*
>> +        * Our block group is read only but before we set it to read only,
>> +        * some task might have had allocated an extent from it already,
>> but it
>> +        * has not yet created a respective ordered extent (and added it
>> to a
>> +        * root's list of ordered extents).
>> +        * Therefore wait for any task currently allocating extents, since
>> the
>> +        * block group's reservations counter is incremented while a read
>> lock
>> +        * on the groups' semaphore is held and decremented after
>> releasing
>> +        * the read access on that semaphore and creating the ordered
>> extent.
>> +        */
>> +       down_write(&space_info->groups_sem);
>> +       up_write(&space_info->groups_sem);
>> +
>> +       wait_on_atomic_t(&bg->reservations,
>> +                        btrfs_wait_bg_reservations_atomic_t,
>> +                        TASK_UNINTERRUPTIBLE);
>> +}
>> +
>>  /**
>>   * btrfs_update_reserved_bytes - update the block_group and space info
>> counters
>>   * @cache:     The cache we are manipulating
>> @@ -7434,6 +7485,7 @@ checks:
>>                         btrfs_add_free_space(block_group, offset,
>> num_bytes);
>>                         goto loop;
>>                 }
>> +               btrfs_inc_block_group_reservations(block_group);
>>
>>                 /* we are all good, lets return */
>>                 ins->objectid = search_start;
>> @@ -7615,8 +7667,10 @@ again:
>>         WARN_ON(num_bytes < root->sectorsize);
>>         ret = find_free_extent(root, num_bytes, empty_size, hint_byte,
>> ins,
>>                                flags, delalloc);
>> -
>> -       if (ret == -ENOSPC) {
>> +       if (!ret && !is_data) {
>> +               btrfs_dec_block_group_reservations(root->fs_info,
>> +                                                  ins->objectid);
>> +       } else if (ret == -ENOSPC) {
>>                 if (!final_tried && ins->offset) {
>>                         num_bytes = min(num_bytes >> 1, ins->offset);
>>                         num_bytes = round_down(num_bytes,
>> root->sectorsize);
>> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
>> index 41a5688..0085899 100644
>> --- a/fs/btrfs/inode.c
>> +++ b/fs/btrfs/inode.c
>> @@ -824,6 +824,7 @@ retry:
>>                                                 async_extent->ram_size -
>> 1, 0);
>>                         goto out_free_reserve;
>>                 }
>> +               btrfs_dec_block_group_reservations(root->fs_info,
>> ins.objectid);
>>
>>                 /*
>>                  * clear dirty, set writeback and unlock the pages.
>> @@ -861,6 +862,7 @@ retry:
>>         }
>>         return;
>>  out_free_reserve:
>> +       btrfs_dec_block_group_reservations(root->fs_info, ins.objectid);
>>         btrfs_free_reserved_extent(root, ins.objectid, ins.offset, 1);
>>  out_free:
>>         extent_clear_unlock_delalloc(inode, async_extent->start,
>> @@ -1038,6 +1040,8 @@ static noinline int cow_file_range(struct inode
>> *inode,
>>                                 goto out_drop_extent_cache;
>>                 }
>>
>> +               btrfs_dec_block_group_reservations(root->fs_info,
>> ins.objectid);
>> +
>>                 if (disk_num_bytes < cur_alloc_size)
>>                         break;
>>
>> @@ -1066,6 +1070,7 @@ out:
>>  out_drop_extent_cache:
>>         btrfs_drop_extent_cache(inode, start, start + ram_size - 1, 0);
>>  out_reserve:
>> +       btrfs_dec_block_group_reservations(root->fs_info, ins.objectid);
>>         btrfs_free_reserved_extent(root, ins.objectid, ins.offset, 1);
>>  out_unlock:
>>         extent_clear_unlock_delalloc(inode, start, end, locked_page,
>> @@ -7162,6 +7167,8 @@ static struct extent_map
>> *btrfs_new_extent_direct(struct inode *inode,
>>                 return ERR_PTR(ret);
>>         }
>>
>> +       btrfs_dec_block_group_reservations(root->fs_info, ins.objectid);
>> +
>>         em = create_pinned_em(inode, start, ins.offset, start,
>> ins.objectid,
>>                               ins.offset, ins.offset, ins.offset, 0);
>>         if (IS_ERR(em)) {
>> @@ -9942,6 +9949,7 @@ static int __btrfs_prealloc_file_range(struct inode
>> *inode, int mode,
>>                                 btrfs_end_transaction(trans, root);
>>                         break;
>>                 }
>> +               btrfs_dec_block_group_reservations(root->fs_info,
>> ins.objectid);
>>
>>                 last_alloc = ins.offset;
>>                 ret = insert_reserved_file_extent(trans, inode,
>
>
> Instead of doing all this, why not just add the dec to the end of
> __btrfs_add_ordered_extents?  It seems like it would be much cleaner.

Indeed, and it was something I tried first.
The reason I didn't end up doing it like that is that we still need to
do it in error paths (before we call the add_ordered_extent variants),
so I ended up preferring to leave the dec operation always to the
responsibility of any extent allocation caller (so they do it for both
error and success paths).
What do you think?

> Thanks,
>
> Josef
>

Re: [PATCH v2 2/2] Btrfs: don't do unnecessary delalloc flushes when relocating

[Josef Bacik]

On 04/26/2016 12:09 PM, Filipe Manana wrote:
> On Tue, Apr 26, 2016 at 5:02 PM, Josef Bacik <jbacik@fb.com> wrote:
>> On 04/26/2016 11:39 AM, fdmanana@kernel.org wrote:
>>>
>>> From: Filipe Manana <fdmanana@suse.com>
>>>
>>> Before we start the actual relocation process of a block group, we do
>>> calls to flush delalloc of all inodes and then wait for ordered extents
>>> to complete. However we do these flush calls just to make sure we don't
>>> race with concurrent tasks that have actually already started to run
>>> delalloc and have allocated an extent from the block group we want to
>>> relocate, right before we set it to readonly mode, but have not yet
>>> created the respective ordered extents. The flush calls make us wait
>>> for such concurrent tasks because they end up calling
>>> filemap_fdatawrite_range() (through btrfs_start_delalloc_roots() ->
>>> __start_delalloc_inodes() -> btrfs_alloc_delalloc_work() ->
>>> btrfs_run_delalloc_work()) which ends up serializing us with those tasks
>>> due to attempts to lock the same pages (and the delalloc flush procedure
>>> calls the allocator and creates the ordered extents before unlocking the
>>> pages).
>>>
>>> These flushing calls not only make us waste time (cpu, IO) but also reduce
>>> the chances of writing larger extents (applications might be writing to
>>> contiguous ranges and we flush before they finish dirtying the whole
>>> ranges).
>>>
>>> So make sure we don't flush delalloc and just wait for concurrent tasks
>>> that have already started flushing delalloc and have allocated an extent
>>> from the block group we are about to relocate.
>>>
>>> This change also ends up fixing a race with direct IO writes that makes
>>> relocation not wait for direct IO ordered extents. This race is
>>> illustrated by the following diagram:
>>>
>>>         CPU 1                                       CPU 2
>>>
>>>  btrfs_relocate_block_group(bg X)
>>>
>>>                                            starts direct IO write,
>>>                                            target inode currently has no
>>>                                            ordered extents ongoing nor
>>>                                            dirty pages (delalloc regions),
>>>                                            therefore the root for our
>>> inode
>>>                                            is not in the list
>>>                                            fs_info->ordered_roots
>>>
>>>                                            btrfs_direct_IO()
>>>                                              __blockdev_direct_IO()
>>>                                                btrfs_get_blocks_direct()
>>>
>>> btrfs_lock_extent_direct()
>>>                                                    locks range in the io
>>> tree
>>>                                                  btrfs_new_extent_direct()
>>>                                                    btrfs_reserve_extent()
>>>                                                      --> extent allocated
>>>                                                          from bg X
>>>
>>>    btrfs_inc_block_group_ro(bg X)
>>>
>>>    btrfs_start_delalloc_roots()
>>>      __start_delalloc_inodes()
>>>        --> does nothing, no dealloc ranges
>>>            in the inode's io tree so the
>>>            inode's root is not in the list
>>>            fs_info->delalloc_roots
>>>
>>>    btrfs_wait_ordered_roots()
>>>      --> does not find the inode's root in the
>>>          list fs_info->ordered_roots
>>>
>>>      --> ends up not waiting for the direct IO
>>>          write started by the task at CPU 2
>>>
>>>    relocate_block_group(rc->stage ==
>>>      MOVE_DATA_EXTENTS)
>>>
>>>      prepare_to_relocate()
>>>        btrfs_commit_transaction()
>>>
>>>      iterates the extent tree, using its
>>>      commit root and moves extents into new
>>>      locations
>>>
>>>
>>> btrfs_add_ordered_extent_dio()
>>>                                                      --> now a ordered
>>> extent is
>>>                                                          created and added
>>> to the
>>>                                                          list
>>> root->ordered_extents
>>>                                                          and the root
>>> added to the
>>>                                                          list
>>> fs_info->ordered_roots
>>>                                                      --> this is too late
>>> and the
>>>                                                          task at CPU 1
>>> already
>>>                                                          started the
>>> relocation
>>>
>>>      btrfs_commit_transaction()
>>>
>>>
>>> btrfs_finish_ordered_io()
>>>
>>> btrfs_alloc_reserved_file_extent()
>>>                                                        --> adds delayed
>>> data reference
>>>                                                            for the extent
>>> allocated
>>>                                                            from bg X
>>>
>>>    relocate_block_group(rc->stage ==
>>>      UPDATE_DATA_PTRS)
>>>
>>>      prepare_to_relocate()
>>>        btrfs_commit_transaction()
>>>          --> delayed refs are run, so an extent
>>>              item for the allocated extent from
>>>              bg X is added to extent tree
>>>          --> commit roots are switched, so the
>>>              next scan in the extent tree will
>>>              see the extent item
>>>
>>>      sees the extent in the extent tree
>>>
>>> When this happens the relocation produces the following warning when it
>>> finishes:
>>>
>>> [ 7260.832836] ------------[ cut here ]------------
>>> [ 7260.834653] WARNING: CPU: 5 PID: 6765 at fs/btrfs/relocation.c:4318
>>> btrfs_relocate_block_group+0x245/0x2a1 [btrfs]()
>>> [ 7260.838268] Modules linked in: btrfs crc32c_generic xor ppdev raid6_pq
>>> psmouse sg acpi_cpufreq evdev i2c_piix4 tpm_tis serio_raw tpm i2c_core
>>> pcspkr parport_pc
>>> [ 7260.850935] CPU: 5 PID: 6765 Comm: btrfs Not tainted
>>> 4.5.0-rc6-btrfs-next-28+ #1
>>> [ 7260.852998] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
>>> by qemu-project.org 04/01/2014
>>> [ 7260.852998]  0000000000000000 ffff88020bf57bc0 ffffffff812648b3
>>> 0000000000000000
>>> [ 7260.852998]  0000000000000009 ffff88020bf57bf8 ffffffff81051608
>>> ffffffffa03c1b2d
>>> [ 7260.852998]  ffff8800b2bbb800 0000000000000000 ffff8800b17bcc58
>>> ffff8800399dd000
>>> [ 7260.852998] Call Trace:
>>> [ 7260.852998]  [<ffffffff812648b3>] dump_stack+0x67/0x90
>>> [ 7260.852998]  [<ffffffff81051608>] warn_slowpath_common+0x99/0xb2
>>> [ 7260.852998]  [<ffffffffa03c1b2d>] ?
>>> btrfs_relocate_block_group+0x245/0x2a1 [btrfs]
>>> [ 7260.852998]  [<ffffffff810516d4>] warn_slowpath_null+0x1a/0x1c
>>> [ 7260.852998]  [<ffffffffa03c1b2d>]
>>> btrfs_relocate_block_group+0x245/0x2a1 [btrfs]
>>> [ 7260.852998]  [<ffffffffa039d9de>]
>>> btrfs_relocate_chunk.isra.29+0x66/0xdb [btrfs]
>>> [ 7260.852998]  [<ffffffffa039f314>] btrfs_balance+0xde1/0xe4e [btrfs]
>>> [ 7260.852998]  [<ffffffff8127d671>] ? debug_smp_processor_id+0x17/0x19
>>> [ 7260.852998]  [<ffffffffa03a9583>] btrfs_ioctl_balance+0x255/0x2d3
>>> [btrfs]
>>> [ 7260.852998]  [<ffffffffa03ac96a>] btrfs_ioctl+0x11e0/0x1dff [btrfs]
>>> [ 7260.852998]  [<ffffffff811451df>] ? handle_mm_fault+0x443/0xd63
>>> [ 7260.852998]  [<ffffffff81491817>] ? _raw_spin_unlock+0x31/0x44
>>> [ 7260.852998]  [<ffffffff8108b36a>] ? arch_local_irq_save+0x9/0xc
>>> [ 7260.852998]  [<ffffffff811876ab>] vfs_ioctl+0x18/0x34
>>> [ 7260.852998]  [<ffffffff81187cb2>] do_vfs_ioctl+0x550/0x5be
>>> [ 7260.852998]  [<ffffffff81190c30>] ? __fget_light+0x4d/0x71
>>> [ 7260.852998]  [<ffffffff81187d77>] SyS_ioctl+0x57/0x79
>>> [ 7260.852998]  [<ffffffff81492017>] entry_SYSCALL_64_fastpath+0x12/0x6b
>>> [ 7260.893268] ---[ end trace eb7803b24ebab8ad ]---
>>>
>>> This is because at the end of the first stage, in relocate_block_group(),
>>> we commit the current transaction, which makes delayed refs run, the
>>> commit roots are switched and so the second stage will find the extent
>>> item that the ordered extent added to the delayed refs. But this extent
>>> was not moved (ordered extent completed after first stage finished), so
>>> at the end of the relocation our block group item still has a positive
>>> used bytes counter, triggering a warning at the end of
>>> btrfs_relocate_block_group(). Later on when trying to read the extent
>>> contents from disk we hit a BUG_ON() due to the inability to map a block
>>> with a logical address that belongs to the block group we relocated and
>>> is no longer valid, resulting in the following trace:
>>>
>>> [ 7344.885290] BTRFS critical (device sdi): unable to find logical
>>> 12845056 len 4096
>>> [ 7344.887518] ------------[ cut here ]------------
>>> [ 7344.888431] kernel BUG at fs/btrfs/inode.c:1833!
>>> [ 7344.888431] invalid opcode: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
>>> [ 7344.888431] Modules linked in: btrfs crc32c_generic xor ppdev raid6_pq
>>> psmouse sg acpi_cpufreq evdev i2c_piix4 tpm_tis serio_raw tpm i2c_core
>>> pcspkr parport_pc
>>> [ 7344.888431] CPU: 0 PID: 6831 Comm: od Tainted: G        W
>>> 4.5.0-rc6-btrfs-next-28+ #1
>>> [ 7344.888431] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
>>> by qemu-project.org 04/01/2014
>>> [ 7344.888431] task: ffff880215818600 ti: ffff880204684000 task.ti:
>>> ffff880204684000
>>> [ 7344.888431] RIP: 0010:[<ffffffffa037c88c>]  [<ffffffffa037c88c>]
>>> btrfs_merge_bio_hook+0x54/0x6b [btrfs]
>>> [ 7344.888431] RSP: 0018:ffff8802046878f0  EFLAGS: 00010282
>>> [ 7344.888431] RAX: 00000000ffffffea RBX: 0000000000001000 RCX:
>>> 0000000000000001
>>> [ 7344.888431] RDX: ffff88023ec0f950 RSI: ffffffff8183b638 RDI:
>>> 00000000ffffffff
>>> [ 7344.888431] RBP: ffff880204687908 R08: 0000000000000001 R09:
>>> 0000000000000000
>>> [ 7344.888431] R10: ffff880204687770 R11: ffffffff82f2d52d R12:
>>> 0000000000001000
>>> [ 7344.888431] R13: ffff88021afbfee8 R14: 0000000000006208 R15:
>>> ffff88006cd199b0
>>> [ 7344.888431] FS:  00007f1f9e1d6700(0000) GS:ffff88023ec00000(0000)
>>> knlGS:0000000000000000
>>> [ 7344.888431] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>> [ 7344.888431] CR2: 00007f1f9dc8cb60 CR3: 000000023e3b6000 CR4:
>>> 00000000000006f0
>>> [ 7344.888431] Stack:
>>> [ 7344.888431]  0000000000001000 0000000000001000 ffff880204687b98
>>> ffff880204687950
>>> [ 7344.888431]  ffffffffa0395c8f ffffea0004d64d48 0000000000000000
>>> 0000000000001000
>>> [ 7344.888431]  ffffea0004d64d48 0000000000001000 0000000000000000
>>> 0000000000000000
>>> [ 7344.888431] Call Trace:
>>> [ 7344.888431]  [<ffffffffa0395c8f>] submit_extent_page+0xf5/0x16f [btrfs]
>>> [ 7344.888431]  [<ffffffffa03970ac>] __do_readpage+0x4a0/0x4f1 [btrfs]
>>> [ 7344.888431]  [<ffffffffa039680d>] ? btrfs_create_repair_bio+0xcb/0xcb
>>> [btrfs]
>>> [ 7344.888431]  [<ffffffffa037eeb4>] ?
>>> btrfs_writepage_start_hook+0xbc/0xbc [btrfs]
>>> [ 7344.888431]  [<ffffffff8108df55>] ? trace_hardirqs_on+0xd/0xf
>>> [ 7344.888431]  [<ffffffffa039728c>]
>>> __do_contiguous_readpages.constprop.26+0xc2/0xe4 [btrfs]
>>> [ 7344.888431]  [<ffffffffa037eeb4>] ?
>>> btrfs_writepage_start_hook+0xbc/0xbc [btrfs]
>>> [ 7344.888431]  [<ffffffffa039739b>]
>>> __extent_readpages.constprop.25+0xed/0x100 [btrfs]
>>> [ 7344.888431]  [<ffffffff81129d24>] ? lru_cache_add+0xe/0x10
>>> [ 7344.888431]  [<ffffffffa0397ea8>] extent_readpages+0x160/0x1aa [btrfs]
>>> [ 7344.888431]  [<ffffffffa037eeb4>] ?
>>> btrfs_writepage_start_hook+0xbc/0xbc [btrfs]
>>> [ 7344.888431]  [<ffffffff8115daad>] ? alloc_pages_current+0xa9/0xcd
>>> [ 7344.888431]  [<ffffffffa037cdc9>] btrfs_readpages+0x1f/0x21 [btrfs]
>>> [ 7344.888431]  [<ffffffff81128316>] __do_page_cache_readahead+0x168/0x1fc
>>> [ 7344.888431]  [<ffffffff811285a0>] ondemand_readahead+0x1f6/0x207
>>> [ 7344.888431]  [<ffffffff811285a0>] ? ondemand_readahead+0x1f6/0x207
>>> [ 7344.888431]  [<ffffffff8111cf34>] ? pagecache_get_page+0x2b/0x154
>>> [ 7344.888431]  [<ffffffff8112870e>] page_cache_sync_readahead+0x3d/0x3f
>>> [ 7344.888431]  [<ffffffff8111dbf7>] generic_file_read_iter+0x197/0x4e1
>>> [ 7344.888431]  [<ffffffff8117773a>] __vfs_read+0x79/0x9d
>>> [ 7344.888431]  [<ffffffff81178050>] vfs_read+0x8f/0xd2
>>> [ 7344.888431]  [<ffffffff81178a38>] SyS_read+0x50/0x7e
>>> [ 7344.888431]  [<ffffffff81492017>] entry_SYSCALL_64_fastpath+0x12/0x6b
>>> [ 7344.888431] Code: 8d 4d e8 45 31 c9 45 31 c0 48 8b 00 48 c1 e2 09 48 8b
>>> 80 80 fc ff ff 4c 89 65 e8 48 8b b8 f0 01 00 00 e8 1d 42 02 00 85 c0 79 02
>>> <0f> 0b 4c 0
>>> [ 7344.888431] RIP  [<ffffffffa037c88c>] btrfs_merge_bio_hook+0x54/0x6b
>>> [btrfs]
>>> [ 7344.888431]  RSP <ffff8802046878f0>
>>> [ 7344.970544] ---[ end trace eb7803b24ebab8ae ]---
>>>
>>> Signed-off-by: Filipe Manana <fdmanana@suse.com>
>>> ---
>>>  fs/btrfs/ctree.h       | 14 ++++++++++++
>>>  fs/btrfs/extent-tree.c | 58
>>> ++++++++++++++++++++++++++++++++++++++++++++++++--
>>>  fs/btrfs/inode.c       |  8 +++++++
>>>  fs/btrfs/relocation.c  |  6 +-----
>>>  4 files changed, 79 insertions(+), 7 deletions(-)
>>>
>>> diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
>>> index 84a6a5b..90e70e2 100644
>>> --- a/fs/btrfs/ctree.h
>>> +++ b/fs/btrfs/ctree.h
>>> @@ -1408,6 +1408,17 @@ struct btrfs_block_group_cache {
>>>
>>>         struct btrfs_io_ctl io_ctl;
>>>
>>> +       /*
>>> +        * Incremented when doing extent allocations and holding a read
>>> lock
>>> +        * on the space_info's groups_sem semaphore.
>>> +        * Decremented when an ordered extent that represents an IO
>>> against this
>>> +        * block group's range is created (after it's added to its inode's
>>> +        * root's list of ordered extents) or immediately after the
>>> allocation
>>> +        * if it's a metadata extent or fallocate extent (for these cases
>>> we
>>> +        * don't create ordered extents).
>>> +        */
>>> +       atomic_t reservations;
>>> +
>>>         /* Lock for free space tree operations. */
>>>         struct mutex free_space_lock;
>>>
>>> @@ -3499,6 +3510,9 @@ int btrfs_should_throttle_delayed_refs(struct
>>> btrfs_trans_handle *trans,
>>>                                        struct btrfs_root *root);
>>>  int btrfs_check_space_for_delayed_refs(struct btrfs_trans_handle *trans,
>>>                                        struct btrfs_root *root);
>>> +void btrfs_dec_block_group_reservations(struct btrfs_fs_info *fs_info,
>>> +                                        const u64 start);
>>> +void btrfs_wait_block_group_reservations(struct btrfs_block_group_cache
>>> *bg);
>>>  void btrfs_put_block_group(struct btrfs_block_group_cache *cache);
>>>  int btrfs_run_delayed_refs(struct btrfs_trans_handle *trans,
>>>                            struct btrfs_root *root, unsigned long count);
>>> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
>>> index 0544f70..35e3ea9 100644
>>> --- a/fs/btrfs/extent-tree.c
>>> +++ b/fs/btrfs/extent-tree.c
>>> @@ -6175,6 +6175,57 @@ int btrfs_exclude_logged_extents(struct btrfs_root
>>> *log,
>>>         return 0;
>>>  }
>>>
>>> +static void
>>> +btrfs_inc_block_group_reservations(struct btrfs_block_group_cache *bg)
>>> +{
>>> +       atomic_inc(&bg->reservations);
>>> +}
>>> +
>>> +void btrfs_dec_block_group_reservations(struct btrfs_fs_info *fs_info,
>>> +                                       const u64 start)
>>> +{
>>> +       struct btrfs_block_group_cache *bg;
>>> +
>>> +       bg = btrfs_lookup_block_group(fs_info, start);
>>> +       BUG_ON(!bg);
>>
>>
>> ASSERT(bg) instead please.
>>
>>
>>> +       if (atomic_dec_and_test(&bg->reservations))
>>> +               wake_up_atomic_t(&bg->reservations);
>>> +       btrfs_put_block_group(bg);
>>> +}
>>> +
>>> +static int btrfs_wait_bg_reservations_atomic_t(atomic_t *a)
>>> +{
>>> +       schedule();
>>> +       return 0;
>>> +}
>>> +
>>> +void btrfs_wait_block_group_reservations(struct btrfs_block_group_cache
>>> *bg)
>>> +{
>>> +       struct btrfs_space_info *space_info = bg->space_info;
>>> +
>>> +       ASSERT(bg->ro);
>>> +
>>> +       if (!(bg->flags & BTRFS_BLOCK_GROUP_DATA))
>>> +               return;
>>> +
>>> +       /*
>>> +        * Our block group is read only but before we set it to read only,
>>> +        * some task might have had allocated an extent from it already,
>>> but it
>>> +        * has not yet created a respective ordered extent (and added it
>>> to a
>>> +        * root's list of ordered extents).
>>> +        * Therefore wait for any task currently allocating extents, since
>>> the
>>> +        * block group's reservations counter is incremented while a read
>>> lock
>>> +        * on the groups' semaphore is held and decremented after
>>> releasing
>>> +        * the read access on that semaphore and creating the ordered
>>> extent.
>>> +        */
>>> +       down_write(&space_info->groups_sem);
>>> +       up_write(&space_info->groups_sem);
>>> +
>>> +       wait_on_atomic_t(&bg->reservations,
>>> +                        btrfs_wait_bg_reservations_atomic_t,
>>> +                        TASK_UNINTERRUPTIBLE);
>>> +}
>>> +
>>>  /**
>>>   * btrfs_update_reserved_bytes - update the block_group and space info
>>> counters
>>>   * @cache:     The cache we are manipulating
>>> @@ -7434,6 +7485,7 @@ checks:
>>>                         btrfs_add_free_space(block_group, offset,
>>> num_bytes);
>>>                         goto loop;
>>>                 }
>>> +               btrfs_inc_block_group_reservations(block_group);
>>>
>>>                 /* we are all good, lets return */
>>>                 ins->objectid = search_start;
>>> @@ -7615,8 +7667,10 @@ again:
>>>         WARN_ON(num_bytes < root->sectorsize);
>>>         ret = find_free_extent(root, num_bytes, empty_size, hint_byte,
>>> ins,
>>>                                flags, delalloc);
>>> -
>>> -       if (ret == -ENOSPC) {
>>> +       if (!ret && !is_data) {
>>> +               btrfs_dec_block_group_reservations(root->fs_info,
>>> +                                                  ins->objectid);
>>> +       } else if (ret == -ENOSPC) {
>>>                 if (!final_tried && ins->offset) {
>>>                         num_bytes = min(num_bytes >> 1, ins->offset);
>>>                         num_bytes = round_down(num_bytes,
>>> root->sectorsize);
>>> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
>>> index 41a5688..0085899 100644
>>> --- a/fs/btrfs/inode.c
>>> +++ b/fs/btrfs/inode.c
>>> @@ -824,6 +824,7 @@ retry:
>>>                                                 async_extent->ram_size -
>>> 1, 0);
>>>                         goto out_free_reserve;
>>>                 }
>>> +               btrfs_dec_block_group_reservations(root->fs_info,
>>> ins.objectid);
>>>
>>>                 /*
>>>                  * clear dirty, set writeback and unlock the pages.
>>> @@ -861,6 +862,7 @@ retry:
>>>         }
>>>         return;
>>>  out_free_reserve:
>>> +       btrfs_dec_block_group_reservations(root->fs_info, ins.objectid);
>>>         btrfs_free_reserved_extent(root, ins.objectid, ins.offset, 1);
>>>  out_free:
>>>         extent_clear_unlock_delalloc(inode, async_extent->start,
>>> @@ -1038,6 +1040,8 @@ static noinline int cow_file_range(struct inode
>>> *inode,
>>>                                 goto out_drop_extent_cache;
>>>                 }
>>>
>>> +               btrfs_dec_block_group_reservations(root->fs_info,
>>> ins.objectid);
>>> +
>>>                 if (disk_num_bytes < cur_alloc_size)
>>>                         break;
>>>
>>> @@ -1066,6 +1070,7 @@ out:
>>>  out_drop_extent_cache:
>>>         btrfs_drop_extent_cache(inode, start, start + ram_size - 1, 0);
>>>  out_reserve:
>>> +       btrfs_dec_block_group_reservations(root->fs_info, ins.objectid);
>>>         btrfs_free_reserved_extent(root, ins.objectid, ins.offset, 1);
>>>  out_unlock:
>>>         extent_clear_unlock_delalloc(inode, start, end, locked_page,
>>> @@ -7162,6 +7167,8 @@ static struct extent_map
>>> *btrfs_new_extent_direct(struct inode *inode,
>>>                 return ERR_PTR(ret);
>>>         }
>>>
>>> +       btrfs_dec_block_group_reservations(root->fs_info, ins.objectid);
>>> +
>>>         em = create_pinned_em(inode, start, ins.offset, start,
>>> ins.objectid,
>>>                               ins.offset, ins.offset, ins.offset, 0);
>>>         if (IS_ERR(em)) {
>>> @@ -9942,6 +9949,7 @@ static int __btrfs_prealloc_file_range(struct inode
>>> *inode, int mode,
>>>                                 btrfs_end_transaction(trans, root);
>>>                         break;
>>>                 }
>>> +               btrfs_dec_block_group_reservations(root->fs_info,
>>> ins.objectid);
>>>
>>>                 last_alloc = ins.offset;
>>>                 ret = insert_reserved_file_extent(trans, inode,
>>
>>
>> Instead of doing all this, why not just add the dec to the end of
>> __btrfs_add_ordered_extents?  It seems like it would be much cleaner.
>
> Indeed, and it was something I tried first.
> The reason I didn't end up doing it like that is that we still need to
> do it in error paths (before we call the add_ordered_extent variants),
> so I ended up preferring to leave the dec operation always to the
> responsibility of any extent allocation caller (so they do it for both
> error and success paths).
> What do you think?
>

We talked about this on irc, just updating here so nobody is confused. 
You are right, this is probably the best thing for now.  Thanks,

Josef

[PATCH v3 2/2] Btrfs: don't do unnecessary delalloc flushes when relocating

[fdmanana]

From: Filipe Manana <fdmanana@suse.com>

Before we start the actual relocation process of a block group, we do
calls to flush delalloc of all inodes and then wait for ordered extents
to complete. However we do these flush calls just to make sure we don't
race with concurrent tasks that have actually already started to run
delalloc and have allocated an extent from the block group we want to
relocate, right before we set it to readonly mode, but have not yet
created the respective ordered extents. The flush calls make us wait
for such concurrent tasks because they end up calling
filemap_fdatawrite_range() (through btrfs_start_delalloc_roots() ->
__start_delalloc_inodes() -> btrfs_alloc_delalloc_work() ->
btrfs_run_delalloc_work()) which ends up serializing us with those tasks
due to attempts to lock the same pages (and the delalloc flush procedure
calls the allocator and creates the ordered extents before unlocking the
pages).

These flushing calls not only make us waste time (cpu, IO) but also reduce
the chances of writing larger extents (applications might be writing to
contiguous ranges and we flush before they finish dirtying the whole
ranges).

So make sure we don't flush delalloc and just wait for concurrent tasks
that have already started flushing delalloc and have allocated an extent
from the block group we are about to relocate.

This change also ends up fixing a race with direct IO writes that makes
relocation not wait for direct IO ordered extents. This race is
illustrated by the following diagram:

        CPU 1                                       CPU 2

 btrfs_relocate_block_group(bg X)

                                           starts direct IO write,
                                           target inode currently has no
                                           ordered extents ongoing nor
                                           dirty pages (delalloc regions),
                                           therefore the root for our inode
                                           is not in the list
                                           fs_info->ordered_roots

                                           btrfs_direct_IO()
                                             __blockdev_direct_IO()
                                               btrfs_get_blocks_direct()
                                                 btrfs_lock_extent_direct()
                                                   locks range in the io tree
                                                 btrfs_new_extent_direct()
                                                   btrfs_reserve_extent()
                                                     --> extent allocated
                                                         from bg X

   btrfs_inc_block_group_ro(bg X)

   btrfs_start_delalloc_roots()
     __start_delalloc_inodes()
       --> does nothing, no dealloc ranges
           in the inode's io tree so the
           inode's root is not in the list
           fs_info->delalloc_roots

   btrfs_wait_ordered_roots()
     --> does not find the inode's root in the
         list fs_info->ordered_roots

     --> ends up not waiting for the direct IO
         write started by the task at CPU 2

   relocate_block_group(rc->stage ==
     MOVE_DATA_EXTENTS)

     prepare_to_relocate()
       btrfs_commit_transaction()

     iterates the extent tree, using its
     commit root and moves extents into new
     locations

                                                   btrfs_add_ordered_extent_dio()
                                                     --> now a ordered extent is
                                                         created and added to the
                                                         list root->ordered_extents
                                                         and the root added to the
                                                         list fs_info->ordered_roots
                                                     --> this is too late and the
                                                         task at CPU 1 already
                                                         started the relocation

     btrfs_commit_transaction()

                                                   btrfs_finish_ordered_io()
                                                     btrfs_alloc_reserved_file_extent()
                                                       --> adds delayed data reference
                                                           for the extent allocated
                                                           from bg X

   relocate_block_group(rc->stage ==
     UPDATE_DATA_PTRS)

     prepare_to_relocate()
       btrfs_commit_transaction()
         --> delayed refs are run, so an extent
             item for the allocated extent from
             bg X is added to extent tree
         --> commit roots are switched, so the
             next scan in the extent tree will
             see the extent item

     sees the extent in the extent tree

When this happens the relocation produces the following warning when it
finishes:

[ 7260.832836] ------------[ cut here ]------------
[ 7260.834653] WARNING: CPU: 5 PID: 6765 at fs/btrfs/relocation.c:4318 btrfs_relocate_block_group+0x245/0x2a1 [btrfs]()
[ 7260.838268] Modules linked in: btrfs crc32c_generic xor ppdev raid6_pq psmouse sg acpi_cpufreq evdev i2c_piix4 tpm_tis serio_raw tpm i2c_core pcspkr parport_pc
[ 7260.850935] CPU: 5 PID: 6765 Comm: btrfs Not tainted 4.5.0-rc6-btrfs-next-28+ #1
[ 7260.852998] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS by qemu-project.org 04/01/2014
[ 7260.852998]  0000000000000000 ffff88020bf57bc0 ffffffff812648b3 0000000000000000
[ 7260.852998]  0000000000000009 ffff88020bf57bf8 ffffffff81051608 ffffffffa03c1b2d
[ 7260.852998]  ffff8800b2bbb800 0000000000000000 ffff8800b17bcc58 ffff8800399dd000
[ 7260.852998] Call Trace:
[ 7260.852998]  [<ffffffff812648b3>] dump_stack+0x67/0x90
[ 7260.852998]  [<ffffffff81051608>] warn_slowpath_common+0x99/0xb2
[ 7260.852998]  [<ffffffffa03c1b2d>] ? btrfs_relocate_block_group+0x245/0x2a1 [btrfs]
[ 7260.852998]  [<ffffffff810516d4>] warn_slowpath_null+0x1a/0x1c
[ 7260.852998]  [<ffffffffa03c1b2d>] btrfs_relocate_block_group+0x245/0x2a1 [btrfs]
[ 7260.852998]  [<ffffffffa039d9de>] btrfs_relocate_chunk.isra.29+0x66/0xdb [btrfs]
[ 7260.852998]  [<ffffffffa039f314>] btrfs_balance+0xde1/0xe4e [btrfs]
[ 7260.852998]  [<ffffffff8127d671>] ? debug_smp_processor_id+0x17/0x19
[ 7260.852998]  [<ffffffffa03a9583>] btrfs_ioctl_balance+0x255/0x2d3 [btrfs]
[ 7260.852998]  [<ffffffffa03ac96a>] btrfs_ioctl+0x11e0/0x1dff [btrfs]
[ 7260.852998]  [<ffffffff811451df>] ? handle_mm_fault+0x443/0xd63
[ 7260.852998]  [<ffffffff81491817>] ? _raw_spin_unlock+0x31/0x44
[ 7260.852998]  [<ffffffff8108b36a>] ? arch_local_irq_save+0x9/0xc
[ 7260.852998]  [<ffffffff811876ab>] vfs_ioctl+0x18/0x34
[ 7260.852998]  [<ffffffff81187cb2>] do_vfs_ioctl+0x550/0x5be
[ 7260.852998]  [<ffffffff81190c30>] ? __fget_light+0x4d/0x71
[ 7260.852998]  [<ffffffff81187d77>] SyS_ioctl+0x57/0x79
[ 7260.852998]  [<ffffffff81492017>] entry_SYSCALL_64_fastpath+0x12/0x6b
[ 7260.893268] ---[ end trace eb7803b24ebab8ad ]---

This is because at the end of the first stage, in relocate_block_group(),
we commit the current transaction, which makes delayed refs run, the
commit roots are switched and so the second stage will find the extent
item that the ordered extent added to the delayed refs. But this extent
was not moved (ordered extent completed after first stage finished), so
at the end of the relocation our block group item still has a positive
used bytes counter, triggering a warning at the end of
btrfs_relocate_block_group(). Later on when trying to read the extent
contents from disk we hit a BUG_ON() due to the inability to map a block
with a logical address that belongs to the block group we relocated and
is no longer valid, resulting in the following trace:

[ 7344.885290] BTRFS critical (device sdi): unable to find logical 12845056 len 4096
[ 7344.887518] ------------[ cut here ]------------
[ 7344.888431] kernel BUG at fs/btrfs/inode.c:1833!
[ 7344.888431] invalid opcode: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
[ 7344.888431] Modules linked in: btrfs crc32c_generic xor ppdev raid6_pq psmouse sg acpi_cpufreq evdev i2c_piix4 tpm_tis serio_raw tpm i2c_core pcspkr parport_pc
[ 7344.888431] CPU: 0 PID: 6831 Comm: od Tainted: G        W       4.5.0-rc6-btrfs-next-28+ #1
[ 7344.888431] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS by qemu-project.org 04/01/2014
[ 7344.888431] task: ffff880215818600 ti: ffff880204684000 task.ti: ffff880204684000
[ 7344.888431] RIP: 0010:[<ffffffffa037c88c>]  [<ffffffffa037c88c>] btrfs_merge_bio_hook+0x54/0x6b [btrfs]
[ 7344.888431] RSP: 0018:ffff8802046878f0  EFLAGS: 00010282
[ 7344.888431] RAX: 00000000ffffffea RBX: 0000000000001000 RCX: 0000000000000001
[ 7344.888431] RDX: ffff88023ec0f950 RSI: ffffffff8183b638 RDI: 00000000ffffffff
[ 7344.888431] RBP: ffff880204687908 R08: 0000000000000001 R09: 0000000000000000
[ 7344.888431] R10: ffff880204687770 R11: ffffffff82f2d52d R12: 0000000000001000
[ 7344.888431] R13: ffff88021afbfee8 R14: 0000000000006208 R15: ffff88006cd199b0
[ 7344.888431] FS:  00007f1f9e1d6700(0000) GS:ffff88023ec00000(0000) knlGS:0000000000000000
[ 7344.888431] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 7344.888431] CR2: 00007f1f9dc8cb60 CR3: 000000023e3b6000 CR4: 00000000000006f0
[ 7344.888431] Stack:
[ 7344.888431]  0000000000001000 0000000000001000 ffff880204687b98 ffff880204687950
[ 7344.888431]  ffffffffa0395c8f ffffea0004d64d48 0000000000000000 0000000000001000
[ 7344.888431]  ffffea0004d64d48 0000000000001000 0000000000000000 0000000000000000
[ 7344.888431] Call Trace:
[ 7344.888431]  [<ffffffffa0395c8f>] submit_extent_page+0xf5/0x16f [btrfs]
[ 7344.888431]  [<ffffffffa03970ac>] __do_readpage+0x4a0/0x4f1 [btrfs]
[ 7344.888431]  [<ffffffffa039680d>] ? btrfs_create_repair_bio+0xcb/0xcb [btrfs]
[ 7344.888431]  [<ffffffffa037eeb4>] ? btrfs_writepage_start_hook+0xbc/0xbc [btrfs]
[ 7344.888431]  [<ffffffff8108df55>] ? trace_hardirqs_on+0xd/0xf
[ 7344.888431]  [<ffffffffa039728c>] __do_contiguous_readpages.constprop.26+0xc2/0xe4 [btrfs]
[ 7344.888431]  [<ffffffffa037eeb4>] ? btrfs_writepage_start_hook+0xbc/0xbc [btrfs]
[ 7344.888431]  [<ffffffffa039739b>] __extent_readpages.constprop.25+0xed/0x100 [btrfs]
[ 7344.888431]  [<ffffffff81129d24>] ? lru_cache_add+0xe/0x10
[ 7344.888431]  [<ffffffffa0397ea8>] extent_readpages+0x160/0x1aa [btrfs]
[ 7344.888431]  [<ffffffffa037eeb4>] ? btrfs_writepage_start_hook+0xbc/0xbc [btrfs]
[ 7344.888431]  [<ffffffff8115daad>] ? alloc_pages_current+0xa9/0xcd
[ 7344.888431]  [<ffffffffa037cdc9>] btrfs_readpages+0x1f/0x21 [btrfs]
[ 7344.888431]  [<ffffffff81128316>] __do_page_cache_readahead+0x168/0x1fc
[ 7344.888431]  [<ffffffff811285a0>] ondemand_readahead+0x1f6/0x207
[ 7344.888431]  [<ffffffff811285a0>] ? ondemand_readahead+0x1f6/0x207
[ 7344.888431]  [<ffffffff8111cf34>] ? pagecache_get_page+0x2b/0x154
[ 7344.888431]  [<ffffffff8112870e>] page_cache_sync_readahead+0x3d/0x3f
[ 7344.888431]  [<ffffffff8111dbf7>] generic_file_read_iter+0x197/0x4e1
[ 7344.888431]  [<ffffffff8117773a>] __vfs_read+0x79/0x9d
[ 7344.888431]  [<ffffffff81178050>] vfs_read+0x8f/0xd2
[ 7344.888431]  [<ffffffff81178a38>] SyS_read+0x50/0x7e
[ 7344.888431]  [<ffffffff81492017>] entry_SYSCALL_64_fastpath+0x12/0x6b
[ 7344.888431] Code: 8d 4d e8 45 31 c9 45 31 c0 48 8b 00 48 c1 e2 09 48 8b 80 80 fc ff ff 4c 89 65 e8 48 8b b8 f0 01 00 00 e8 1d 42 02 00 85 c0 79 02 <0f> 0b 4c 0
[ 7344.888431] RIP  [<ffffffffa037c88c>] btrfs_merge_bio_hook+0x54/0x6b [btrfs]
[ 7344.888431]  RSP <ffff8802046878f0>
[ 7344.970544] ---[ end trace eb7803b24ebab8ae ]---

Signed-off-by: Filipe Manana <fdmanana@suse.com>
---

v3: Replaced a BUG_ON(!bg) with ASSERT(bg).

 fs/btrfs/ctree.h       | 14 ++++++++++++
 fs/btrfs/extent-tree.c | 58 ++++++++++++++++++++++++++++++++++++++++++++++++--
 fs/btrfs/inode.c       |  8 +++++++
 fs/btrfs/relocation.c  |  6 +-----
 4 files changed, 79 insertions(+), 7 deletions(-)

diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 84a6a5b..90e70e2 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -1408,6 +1408,17 @@ struct btrfs_block_group_cache {
 
 	struct btrfs_io_ctl io_ctl;
 
+	/*
+	 * Incremented when doing extent allocations and holding a read lock
+	 * on the space_info's groups_sem semaphore.
+	 * Decremented when an ordered extent that represents an IO against this
+	 * block group's range is created (after it's added to its inode's
+	 * root's list of ordered extents) or immediately after the allocation
+	 * if it's a metadata extent or fallocate extent (for these cases we
+	 * don't create ordered extents).
+	 */
+	atomic_t reservations;
+
 	/* Lock for free space tree operations. */
 	struct mutex free_space_lock;
 
@@ -3499,6 +3510,9 @@ int btrfs_should_throttle_delayed_refs(struct btrfs_trans_handle *trans,
 				       struct btrfs_root *root);
 int btrfs_check_space_for_delayed_refs(struct btrfs_trans_handle *trans,
 				       struct btrfs_root *root);
+void btrfs_dec_block_group_reservations(struct btrfs_fs_info *fs_info,
+					 const u64 start);
+void btrfs_wait_block_group_reservations(struct btrfs_block_group_cache *bg);
 void btrfs_put_block_group(struct btrfs_block_group_cache *cache);
 int btrfs_run_delayed_refs(struct btrfs_trans_handle *trans,
 			   struct btrfs_root *root, unsigned long count);
diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 0544f70..1e009bc 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -6175,6 +6175,57 @@ int btrfs_exclude_logged_extents(struct btrfs_root *log,
 	return 0;
 }
 
+static void
+btrfs_inc_block_group_reservations(struct btrfs_block_group_cache *bg)
+{
+	atomic_inc(&bg->reservations);
+}
+
+void btrfs_dec_block_group_reservations(struct btrfs_fs_info *fs_info,
+					const u64 start)
+{
+	struct btrfs_block_group_cache *bg;
+
+	bg = btrfs_lookup_block_group(fs_info, start);
+	ASSERT(bg);
+	if (atomic_dec_and_test(&bg->reservations))
+		wake_up_atomic_t(&bg->reservations);
+	btrfs_put_block_group(bg);
+}
+
+static int btrfs_wait_bg_reservations_atomic_t(atomic_t *a)
+{
+	schedule();
+	return 0;
+}
+
+void btrfs_wait_block_group_reservations(struct btrfs_block_group_cache *bg)
+{
+	struct btrfs_space_info *space_info = bg->space_info;
+
+	ASSERT(bg->ro);
+
+	if (!(bg->flags & BTRFS_BLOCK_GROUP_DATA))
+		return;
+
+	/*
+	 * Our block group is read only but before we set it to read only,
+	 * some task might have had allocated an extent from it already, but it
+	 * has not yet created a respective ordered extent (and added it to a
+	 * root's list of ordered extents).
+	 * Therefore wait for any task currently allocating extents, since the
+	 * block group's reservations counter is incremented while a read lock
+	 * on the groups' semaphore is held and decremented after releasing
+	 * the read access on that semaphore and creating the ordered extent.
+	 */
+	down_write(&space_info->groups_sem);
+	up_write(&space_info->groups_sem);
+
+	wait_on_atomic_t(&bg->reservations,
+			 btrfs_wait_bg_reservations_atomic_t,
+			 TASK_UNINTERRUPTIBLE);
+}
+
 /**
  * btrfs_update_reserved_bytes - update the block_group and space info counters
  * @cache:	The cache we are manipulating
@@ -7434,6 +7485,7 @@ checks:
 			btrfs_add_free_space(block_group, offset, num_bytes);
 			goto loop;
 		}
+		btrfs_inc_block_group_reservations(block_group);
 
 		/* we are all good, lets return */
 		ins->objectid = search_start;
@@ -7615,8 +7667,10 @@ again:
 	WARN_ON(num_bytes < root->sectorsize);
 	ret = find_free_extent(root, num_bytes, empty_size, hint_byte, ins,
 			       flags, delalloc);
-
-	if (ret == -ENOSPC) {
+	if (!ret && !is_data) {
+		btrfs_dec_block_group_reservations(root->fs_info,
+						   ins->objectid);
+	} else if (ret == -ENOSPC) {
 		if (!final_tried && ins->offset) {
 			num_bytes = min(num_bytes >> 1, ins->offset);
 			num_bytes = round_down(num_bytes, root->sectorsize);
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 41a5688..0085899 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -824,6 +824,7 @@ retry:
 						async_extent->ram_size - 1, 0);
 			goto out_free_reserve;
 		}
+		btrfs_dec_block_group_reservations(root->fs_info, ins.objectid);
 
 		/*
 		 * clear dirty, set writeback and unlock the pages.
@@ -861,6 +862,7 @@ retry:
 	}
 	return;
 out_free_reserve:
+	btrfs_dec_block_group_reservations(root->fs_info, ins.objectid);
 	btrfs_free_reserved_extent(root, ins.objectid, ins.offset, 1);
 out_free:
 	extent_clear_unlock_delalloc(inode, async_extent->start,
@@ -1038,6 +1040,8 @@ static noinline int cow_file_range(struct inode *inode,
 				goto out_drop_extent_cache;
 		}
 
+		btrfs_dec_block_group_reservations(root->fs_info, ins.objectid);
+
 		if (disk_num_bytes < cur_alloc_size)
 			break;
 
@@ -1066,6 +1070,7 @@ out:
 out_drop_extent_cache:
 	btrfs_drop_extent_cache(inode, start, start + ram_size - 1, 0);
 out_reserve:
+	btrfs_dec_block_group_reservations(root->fs_info, ins.objectid);
 	btrfs_free_reserved_extent(root, ins.objectid, ins.offset, 1);
 out_unlock:
 	extent_clear_unlock_delalloc(inode, start, end, locked_page,
@@ -7162,6 +7167,8 @@ static struct extent_map *btrfs_new_extent_direct(struct inode *inode,
 		return ERR_PTR(ret);
 	}
 
+	btrfs_dec_block_group_reservations(root->fs_info, ins.objectid);
+
 	em = create_pinned_em(inode, start, ins.offset, start, ins.objectid,
 			      ins.offset, ins.offset, ins.offset, 0);
 	if (IS_ERR(em)) {
@@ -9942,6 +9949,7 @@ static int __btrfs_prealloc_file_range(struct inode *inode, int mode,
 				btrfs_end_transaction(trans, root);
 			break;
 		}
+		btrfs_dec_block_group_reservations(root->fs_info, ins.objectid);
 
 		last_alloc = ins.offset;
 		ret = insert_reserved_file_extent(trans, inode,
diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
index 9c35ea2..2aed15c 100644
--- a/fs/btrfs/relocation.c
+++ b/fs/btrfs/relocation.c
@@ -4253,11 +4253,7 @@ int btrfs_relocate_block_group(struct btrfs_root *extent_root, u64 group_start)
 	btrfs_info(extent_root->fs_info, "relocating block group %llu flags %llu",
 	       rc->block_group->key.objectid, rc->block_group->flags);
 
-	ret = btrfs_start_delalloc_roots(fs_info, 0, -1);
-	if (ret < 0) {
-		err = ret;
-		goto out;
-	}
+	btrfs_wait_block_group_reservations(rc->block_group);
 	btrfs_wait_ordered_roots(fs_info, -1,
 				 rc->block_group->key.objectid,
 				 rc->block_group->key.offset);
-- 
2.7.0.rc3

Re: [PATCH v3 2/2] Btrfs: don't do unnecessary delalloc flushes when relocating

[Josef Bacik]

On 04/26/2016 01:45 PM, fdmanana@kernel.org wrote:
> From: Filipe Manana <fdmanana@suse.com>
>
> Before we start the actual relocation process of a block group, we do
> calls to flush delalloc of all inodes and then wait for ordered extents
> to complete. However we do these flush calls just to make sure we don't
> race with concurrent tasks that have actually already started to run
> delalloc and have allocated an extent from the block group we want to
> relocate, right before we set it to readonly mode, but have not yet
> created the respective ordered extents. The flush calls make us wait
> for such concurrent tasks because they end up calling
> filemap_fdatawrite_range() (through btrfs_start_delalloc_roots() ->
> __start_delalloc_inodes() -> btrfs_alloc_delalloc_work() ->
> btrfs_run_delalloc_work()) which ends up serializing us with those tasks
> due to attempts to lock the same pages (and the delalloc flush procedure
> calls the allocator and creates the ordered extents before unlocking the
> pages).
>
> These flushing calls not only make us waste time (cpu, IO) but also reduce
> the chances of writing larger extents (applications might be writing to
> contiguous ranges and we flush before they finish dirtying the whole
> ranges).
>
> So make sure we don't flush delalloc and just wait for concurrent tasks
> that have already started flushing delalloc and have allocated an extent
> from the block group we are about to relocate.
>
> This change also ends up fixing a race with direct IO writes that makes
> relocation not wait for direct IO ordered extents. This race is
> illustrated by the following diagram:
>
>         CPU 1                                       CPU 2
>
>  btrfs_relocate_block_group(bg X)
>
>                                            starts direct IO write,
>                                            target inode currently has no
>                                            ordered extents ongoing nor
>                                            dirty pages (delalloc regions),
>                                            therefore the root for our inode
>                                            is not in the list
>                                            fs_info->ordered_roots
>
>                                            btrfs_direct_IO()
>                                              __blockdev_direct_IO()
>                                                btrfs_get_blocks_direct()
>                                                  btrfs_lock_extent_direct()
>                                                    locks range in the io tree
>                                                  btrfs_new_extent_direct()
>                                                    btrfs_reserve_extent()
>                                                      --> extent allocated
>                                                          from bg X
>
>    btrfs_inc_block_group_ro(bg X)
>
>    btrfs_start_delalloc_roots()
>      __start_delalloc_inodes()
>        --> does nothing, no dealloc ranges
>            in the inode's io tree so the
>            inode's root is not in the list
>            fs_info->delalloc_roots
>
>    btrfs_wait_ordered_roots()
>      --> does not find the inode's root in the
>          list fs_info->ordered_roots
>
>      --> ends up not waiting for the direct IO
>          write started by the task at CPU 2
>
>    relocate_block_group(rc->stage ==
>      MOVE_DATA_EXTENTS)
>
>      prepare_to_relocate()
>        btrfs_commit_transaction()
>
>      iterates the extent tree, using its
>      commit root and moves extents into new
>      locations
>
>                                                    btrfs_add_ordered_extent_dio()
>                                                      --> now a ordered extent is
>                                                          created and added to the
>                                                          list root->ordered_extents
>                                                          and the root added to the
>                                                          list fs_info->ordered_roots
>                                                      --> this is too late and the
>                                                          task at CPU 1 already
>                                                          started the relocation
>
>      btrfs_commit_transaction()
>
>                                                    btrfs_finish_ordered_io()
>                                                      btrfs_alloc_reserved_file_extent()
>                                                        --> adds delayed data reference
>                                                            for the extent allocated
>                                                            from bg X
>
>    relocate_block_group(rc->stage ==
>      UPDATE_DATA_PTRS)
>
>      prepare_to_relocate()
>        btrfs_commit_transaction()
>          --> delayed refs are run, so an extent
>              item for the allocated extent from
>              bg X is added to extent tree
>          --> commit roots are switched, so the
>              next scan in the extent tree will
>              see the extent item
>
>      sees the extent in the extent tree
>
> When this happens the relocation produces the following warning when it
> finishes:
>
> [ 7260.832836] ------------[ cut here ]------------
> [ 7260.834653] WARNING: CPU: 5 PID: 6765 at fs/btrfs/relocation.c:4318 btrfs_relocate_block_group+0x245/0x2a1 [btrfs]()
> [ 7260.838268] Modules linked in: btrfs crc32c_generic xor ppdev raid6_pq psmouse sg acpi_cpufreq evdev i2c_piix4 tpm_tis serio_raw tpm i2c_core pcspkr parport_pc
> [ 7260.850935] CPU: 5 PID: 6765 Comm: btrfs Not tainted 4.5.0-rc6-btrfs-next-28+ #1
> [ 7260.852998] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS by qemu-project.org 04/01/2014
> [ 7260.852998]  0000000000000000 ffff88020bf57bc0 ffffffff812648b3 0000000000000000
> [ 7260.852998]  0000000000000009 ffff88020bf57bf8 ffffffff81051608 ffffffffa03c1b2d
> [ 7260.852998]  ffff8800b2bbb800 0000000000000000 ffff8800b17bcc58 ffff8800399dd000
> [ 7260.852998] Call Trace:
> [ 7260.852998]  [<ffffffff812648b3>] dump_stack+0x67/0x90
> [ 7260.852998]  [<ffffffff81051608>] warn_slowpath_common+0x99/0xb2
> [ 7260.852998]  [<ffffffffa03c1b2d>] ? btrfs_relocate_block_group+0x245/0x2a1 [btrfs]
> [ 7260.852998]  [<ffffffff810516d4>] warn_slowpath_null+0x1a/0x1c
> [ 7260.852998]  [<ffffffffa03c1b2d>] btrfs_relocate_block_group+0x245/0x2a1 [btrfs]
> [ 7260.852998]  [<ffffffffa039d9de>] btrfs_relocate_chunk.isra.29+0x66/0xdb [btrfs]
> [ 7260.852998]  [<ffffffffa039f314>] btrfs_balance+0xde1/0xe4e [btrfs]
> [ 7260.852998]  [<ffffffff8127d671>] ? debug_smp_processor_id+0x17/0x19
> [ 7260.852998]  [<ffffffffa03a9583>] btrfs_ioctl_balance+0x255/0x2d3 [btrfs]
> [ 7260.852998]  [<ffffffffa03ac96a>] btrfs_ioctl+0x11e0/0x1dff [btrfs]
> [ 7260.852998]  [<ffffffff811451df>] ? handle_mm_fault+0x443/0xd63
> [ 7260.852998]  [<ffffffff81491817>] ? _raw_spin_unlock+0x31/0x44
> [ 7260.852998]  [<ffffffff8108b36a>] ? arch_local_irq_save+0x9/0xc
> [ 7260.852998]  [<ffffffff811876ab>] vfs_ioctl+0x18/0x34
> [ 7260.852998]  [<ffffffff81187cb2>] do_vfs_ioctl+0x550/0x5be
> [ 7260.852998]  [<ffffffff81190c30>] ? __fget_light+0x4d/0x71
> [ 7260.852998]  [<ffffffff81187d77>] SyS_ioctl+0x57/0x79
> [ 7260.852998]  [<ffffffff81492017>] entry_SYSCALL_64_fastpath+0x12/0x6b
> [ 7260.893268] ---[ end trace eb7803b24ebab8ad ]---
>
> This is because at the end of the first stage, in relocate_block_group(),
> we commit the current transaction, which makes delayed refs run, the
> commit roots are switched and so the second stage will find the extent
> item that the ordered extent added to the delayed refs. But this extent
> was not moved (ordered extent completed after first stage finished), so
> at the end of the relocation our block group item still has a positive
> used bytes counter, triggering a warning at the end of
> btrfs_relocate_block_group(). Later on when trying to read the extent
> contents from disk we hit a BUG_ON() due to the inability to map a block
> with a logical address that belongs to the block group we relocated and
> is no longer valid, resulting in the following trace:
>
> [ 7344.885290] BTRFS critical (device sdi): unable to find logical 12845056 len 4096
> [ 7344.887518] ------------[ cut here ]------------
> [ 7344.888431] kernel BUG at fs/btrfs/inode.c:1833!
> [ 7344.888431] invalid opcode: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
> [ 7344.888431] Modules linked in: btrfs crc32c_generic xor ppdev raid6_pq psmouse sg acpi_cpufreq evdev i2c_piix4 tpm_tis serio_raw tpm i2c_core pcspkr parport_pc
> [ 7344.888431] CPU: 0 PID: 6831 Comm: od Tainted: G        W       4.5.0-rc6-btrfs-next-28+ #1
> [ 7344.888431] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS by qemu-project.org 04/01/2014
> [ 7344.888431] task: ffff880215818600 ti: ffff880204684000 task.ti: ffff880204684000
> [ 7344.888431] RIP: 0010:[<ffffffffa037c88c>]  [<ffffffffa037c88c>] btrfs_merge_bio_hook+0x54/0x6b [btrfs]
> [ 7344.888431] RSP: 0018:ffff8802046878f0  EFLAGS: 00010282
> [ 7344.888431] RAX: 00000000ffffffea RBX: 0000000000001000 RCX: 0000000000000001
> [ 7344.888431] RDX: ffff88023ec0f950 RSI: ffffffff8183b638 RDI: 00000000ffffffff
> [ 7344.888431] RBP: ffff880204687908 R08: 0000000000000001 R09: 0000000000000000
> [ 7344.888431] R10: ffff880204687770 R11: ffffffff82f2d52d R12: 0000000000001000
> [ 7344.888431] R13: ffff88021afbfee8 R14: 0000000000006208 R15: ffff88006cd199b0
> [ 7344.888431] FS:  00007f1f9e1d6700(0000) GS:ffff88023ec00000(0000) knlGS:0000000000000000
> [ 7344.888431] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [ 7344.888431] CR2: 00007f1f9dc8cb60 CR3: 000000023e3b6000 CR4: 00000000000006f0
> [ 7344.888431] Stack:
> [ 7344.888431]  0000000000001000 0000000000001000 ffff880204687b98 ffff880204687950
> [ 7344.888431]  ffffffffa0395c8f ffffea0004d64d48 0000000000000000 0000000000001000
> [ 7344.888431]  ffffea0004d64d48 0000000000001000 0000000000000000 0000000000000000
> [ 7344.888431] Call Trace:
> [ 7344.888431]  [<ffffffffa0395c8f>] submit_extent_page+0xf5/0x16f [btrfs]
> [ 7344.888431]  [<ffffffffa03970ac>] __do_readpage+0x4a0/0x4f1 [btrfs]
> [ 7344.888431]  [<ffffffffa039680d>] ? btrfs_create_repair_bio+0xcb/0xcb [btrfs]
> [ 7344.888431]  [<ffffffffa037eeb4>] ? btrfs_writepage_start_hook+0xbc/0xbc [btrfs]
> [ 7344.888431]  [<ffffffff8108df55>] ? trace_hardirqs_on+0xd/0xf
> [ 7344.888431]  [<ffffffffa039728c>] __do_contiguous_readpages.constprop.26+0xc2/0xe4 [btrfs]
> [ 7344.888431]  [<ffffffffa037eeb4>] ? btrfs_writepage_start_hook+0xbc/0xbc [btrfs]
> [ 7344.888431]  [<ffffffffa039739b>] __extent_readpages.constprop.25+0xed/0x100 [btrfs]
> [ 7344.888431]  [<ffffffff81129d24>] ? lru_cache_add+0xe/0x10
> [ 7344.888431]  [<ffffffffa0397ea8>] extent_readpages+0x160/0x1aa [btrfs]
> [ 7344.888431]  [<ffffffffa037eeb4>] ? btrfs_writepage_start_hook+0xbc/0xbc [btrfs]
> [ 7344.888431]  [<ffffffff8115daad>] ? alloc_pages_current+0xa9/0xcd
> [ 7344.888431]  [<ffffffffa037cdc9>] btrfs_readpages+0x1f/0x21 [btrfs]
> [ 7344.888431]  [<ffffffff81128316>] __do_page_cache_readahead+0x168/0x1fc
> [ 7344.888431]  [<ffffffff811285a0>] ondemand_readahead+0x1f6/0x207
> [ 7344.888431]  [<ffffffff811285a0>] ? ondemand_readahead+0x1f6/0x207
> [ 7344.888431]  [<ffffffff8111cf34>] ? pagecache_get_page+0x2b/0x154
> [ 7344.888431]  [<ffffffff8112870e>] page_cache_sync_readahead+0x3d/0x3f
> [ 7344.888431]  [<ffffffff8111dbf7>] generic_file_read_iter+0x197/0x4e1
> [ 7344.888431]  [<ffffffff8117773a>] __vfs_read+0x79/0x9d
> [ 7344.888431]  [<ffffffff81178050>] vfs_read+0x8f/0xd2
> [ 7344.888431]  [<ffffffff81178a38>] SyS_read+0x50/0x7e
> [ 7344.888431]  [<ffffffff81492017>] entry_SYSCALL_64_fastpath+0x12/0x6b
> [ 7344.888431] Code: 8d 4d e8 45 31 c9 45 31 c0 48 8b 00 48 c1 e2 09 48 8b 80 80 fc ff ff 4c 89 65 e8 48 8b b8 f0 01 00 00 e8 1d 42 02 00 85 c0 79 02 <0f> 0b 4c 0
> [ 7344.888431] RIP  [<ffffffffa037c88c>] btrfs_merge_bio_hook+0x54/0x6b [btrfs]
> [ 7344.888431]  RSP <ffff8802046878f0>
> [ 7344.970544] ---[ end trace eb7803b24ebab8ae ]---
>
> Signed-off-by: Filipe Manana <fdmanana@suse.com>
> ---
>

Reviewed-by: Josef Bacik <jbacik@fb.com>

Thanks,

Josef

Re: [PATCH v3 2/2] Btrfs: don't do unnecessary delalloc flushes when relocating

[Liu Bo]

On Tue, Apr 26, 2016 at 06:45:24PM +0100, fdmanana@kernel.org wrote:
> From: Filipe Manana <fdmanana@suse.com>
> 
> Before we start the actual relocation process of a block group, we do
> calls to flush delalloc of all inodes and then wait for ordered extents
> to complete. However we do these flush calls just to make sure we don't
> race with concurrent tasks that have actually already started to run
> delalloc and have allocated an extent from the block group we want to
> relocate, right before we set it to readonly mode, but have not yet
> created the respective ordered extents. The flush calls make us wait
> for such concurrent tasks because they end up calling
> filemap_fdatawrite_range() (through btrfs_start_delalloc_roots() ->
> __start_delalloc_inodes() -> btrfs_alloc_delalloc_work() ->
> btrfs_run_delalloc_work()) which ends up serializing us with those tasks
> due to attempts to lock the same pages (and the delalloc flush procedure
> calls the allocator and creates the ordered extents before unlocking the
> pages).
> 
> These flushing calls not only make us waste time (cpu, IO) but also reduce
> the chances of writing larger extents (applications might be writing to
> contiguous ranges and we flush before they finish dirtying the whole
> ranges).
> 
> So make sure we don't flush delalloc and just wait for concurrent tasks
> that have already started flushing delalloc and have allocated an extent
> from the block group we are about to relocate.
> 
> This change also ends up fixing a race with direct IO writes that makes
> relocation not wait for direct IO ordered extents. This race is
> illustrated by the following diagram:
> 
>         CPU 1                                       CPU 2
> 
>  btrfs_relocate_block_group(bg X)
> 
>                                            starts direct IO write,
>                                            target inode currently has no
>                                            ordered extents ongoing nor
>                                            dirty pages (delalloc regions),
>                                            therefore the root for our inode
>                                            is not in the list
>                                            fs_info->ordered_roots
> 
>                                            btrfs_direct_IO()
>                                              __blockdev_direct_IO()
>                                                btrfs_get_blocks_direct()
>                                                  btrfs_lock_extent_direct()
>                                                    locks range in the io tree
>                                                  btrfs_new_extent_direct()
>                                                    btrfs_reserve_extent()
>                                                      --> extent allocated
>                                                          from bg X
> 
>    btrfs_inc_block_group_ro(bg X)
> 
>    btrfs_start_delalloc_roots()
>      __start_delalloc_inodes()
>        --> does nothing, no dealloc ranges
>            in the inode's io tree so the
>            inode's root is not in the list
>            fs_info->delalloc_roots
> 
>    btrfs_wait_ordered_roots()
>      --> does not find the inode's root in the
>          list fs_info->ordered_roots
> 
>      --> ends up not waiting for the direct IO
>          write started by the task at CPU 2
> 
>    relocate_block_group(rc->stage ==
>      MOVE_DATA_EXTENTS)
> 
>      prepare_to_relocate()
>        btrfs_commit_transaction()
> 
>      iterates the extent tree, using its
>      commit root and moves extents into new
>      locations
> 
>                                                    btrfs_add_ordered_extent_dio()
>                                                      --> now a ordered extent is
>                                                          created and added to the
>                                                          list root->ordered_extents
>                                                          and the root added to the
>                                                          list fs_info->ordered_roots
>                                                      --> this is too late and the
>                                                          task at CPU 1 already
>                                                          started the relocation
> 
>      btrfs_commit_transaction()
> 
>                                                    btrfs_finish_ordered_io()
>                                                      btrfs_alloc_reserved_file_extent()
>                                                        --> adds delayed data reference
>                                                            for the extent allocated
>                                                            from bg X
> 
>    relocate_block_group(rc->stage ==
>      UPDATE_DATA_PTRS)
> 
>      prepare_to_relocate()
>        btrfs_commit_transaction()
>          --> delayed refs are run, so an extent
>              item for the allocated extent from
>              bg X is added to extent tree
>          --> commit roots are switched, so the
>              next scan in the extent tree will
>              see the extent item
> 
>      sees the extent in the extent tree
> 
> When this happens the relocation produces the following warning when it
> finishes:
> 
> [ 7260.832836] ------------[ cut here ]------------
> [ 7260.834653] WARNING: CPU: 5 PID: 6765 at fs/btrfs/relocation.c:4318 btrfs_relocate_block_group+0x245/0x2a1 [btrfs]()
> [ 7260.838268] Modules linked in: btrfs crc32c_generic xor ppdev raid6_pq psmouse sg acpi_cpufreq evdev i2c_piix4 tpm_tis serio_raw tpm i2c_core pcspkr parport_pc
> [ 7260.850935] CPU: 5 PID: 6765 Comm: btrfs Not tainted 4.5.0-rc6-btrfs-next-28+ #1
> [ 7260.852998] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS by qemu-project.org 04/01/2014
> [ 7260.852998]  0000000000000000 ffff88020bf57bc0 ffffffff812648b3 0000000000000000
> [ 7260.852998]  0000000000000009 ffff88020bf57bf8 ffffffff81051608 ffffffffa03c1b2d
> [ 7260.852998]  ffff8800b2bbb800 0000000000000000 ffff8800b17bcc58 ffff8800399dd000
> [ 7260.852998] Call Trace:
> [ 7260.852998]  [<ffffffff812648b3>] dump_stack+0x67/0x90
> [ 7260.852998]  [<ffffffff81051608>] warn_slowpath_common+0x99/0xb2
> [ 7260.852998]  [<ffffffffa03c1b2d>] ? btrfs_relocate_block_group+0x245/0x2a1 [btrfs]
> [ 7260.852998]  [<ffffffff810516d4>] warn_slowpath_null+0x1a/0x1c
> [ 7260.852998]  [<ffffffffa03c1b2d>] btrfs_relocate_block_group+0x245/0x2a1 [btrfs]
> [ 7260.852998]  [<ffffffffa039d9de>] btrfs_relocate_chunk.isra.29+0x66/0xdb [btrfs]
> [ 7260.852998]  [<ffffffffa039f314>] btrfs_balance+0xde1/0xe4e [btrfs]
> [ 7260.852998]  [<ffffffff8127d671>] ? debug_smp_processor_id+0x17/0x19
> [ 7260.852998]  [<ffffffffa03a9583>] btrfs_ioctl_balance+0x255/0x2d3 [btrfs]
> [ 7260.852998]  [<ffffffffa03ac96a>] btrfs_ioctl+0x11e0/0x1dff [btrfs]
> [ 7260.852998]  [<ffffffff811451df>] ? handle_mm_fault+0x443/0xd63
> [ 7260.852998]  [<ffffffff81491817>] ? _raw_spin_unlock+0x31/0x44
> [ 7260.852998]  [<ffffffff8108b36a>] ? arch_local_irq_save+0x9/0xc
> [ 7260.852998]  [<ffffffff811876ab>] vfs_ioctl+0x18/0x34
> [ 7260.852998]  [<ffffffff81187cb2>] do_vfs_ioctl+0x550/0x5be
> [ 7260.852998]  [<ffffffff81190c30>] ? __fget_light+0x4d/0x71
> [ 7260.852998]  [<ffffffff81187d77>] SyS_ioctl+0x57/0x79
> [ 7260.852998]  [<ffffffff81492017>] entry_SYSCALL_64_fastpath+0x12/0x6b
> [ 7260.893268] ---[ end trace eb7803b24ebab8ad ]---
> 
> This is because at the end of the first stage, in relocate_block_group(),
> we commit the current transaction, which makes delayed refs run, the
> commit roots are switched and so the second stage will find the extent
> item that the ordered extent added to the delayed refs. But this extent
> was not moved (ordered extent completed after first stage finished), so
> at the end of the relocation our block group item still has a positive
> used bytes counter, triggering a warning at the end of
> btrfs_relocate_block_group(). Later on when trying to read the extent
> contents from disk we hit a BUG_ON() due to the inability to map a block
> with a logical address that belongs to the block group we relocated and
> is no longer valid, resulting in the following trace:
> 
> [ 7344.885290] BTRFS critical (device sdi): unable to find logical 12845056 len 4096
> [ 7344.887518] ------------[ cut here ]------------
> [ 7344.888431] kernel BUG at fs/btrfs/inode.c:1833!
> [ 7344.888431] invalid opcode: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
> [ 7344.888431] Modules linked in: btrfs crc32c_generic xor ppdev raid6_pq psmouse sg acpi_cpufreq evdev i2c_piix4 tpm_tis serio_raw tpm i2c_core pcspkr parport_pc
> [ 7344.888431] CPU: 0 PID: 6831 Comm: od Tainted: G        W       4.5.0-rc6-btrfs-next-28+ #1
> [ 7344.888431] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS by qemu-project.org 04/01/2014
> [ 7344.888431] task: ffff880215818600 ti: ffff880204684000 task.ti: ffff880204684000
> [ 7344.888431] RIP: 0010:[<ffffffffa037c88c>]  [<ffffffffa037c88c>] btrfs_merge_bio_hook+0x54/0x6b [btrfs]
> [ 7344.888431] RSP: 0018:ffff8802046878f0  EFLAGS: 00010282
> [ 7344.888431] RAX: 00000000ffffffea RBX: 0000000000001000 RCX: 0000000000000001
> [ 7344.888431] RDX: ffff88023ec0f950 RSI: ffffffff8183b638 RDI: 00000000ffffffff
> [ 7344.888431] RBP: ffff880204687908 R08: 0000000000000001 R09: 0000000000000000
> [ 7344.888431] R10: ffff880204687770 R11: ffffffff82f2d52d R12: 0000000000001000
> [ 7344.888431] R13: ffff88021afbfee8 R14: 0000000000006208 R15: ffff88006cd199b0
> [ 7344.888431] FS:  00007f1f9e1d6700(0000) GS:ffff88023ec00000(0000) knlGS:0000000000000000
> [ 7344.888431] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [ 7344.888431] CR2: 00007f1f9dc8cb60 CR3: 000000023e3b6000 CR4: 00000000000006f0
> [ 7344.888431] Stack:
> [ 7344.888431]  0000000000001000 0000000000001000 ffff880204687b98 ffff880204687950
> [ 7344.888431]  ffffffffa0395c8f ffffea0004d64d48 0000000000000000 0000000000001000
> [ 7344.888431]  ffffea0004d64d48 0000000000001000 0000000000000000 0000000000000000
> [ 7344.888431] Call Trace:
> [ 7344.888431]  [<ffffffffa0395c8f>] submit_extent_page+0xf5/0x16f [btrfs]
> [ 7344.888431]  [<ffffffffa03970ac>] __do_readpage+0x4a0/0x4f1 [btrfs]
> [ 7344.888431]  [<ffffffffa039680d>] ? btrfs_create_repair_bio+0xcb/0xcb [btrfs]
> [ 7344.888431]  [<ffffffffa037eeb4>] ? btrfs_writepage_start_hook+0xbc/0xbc [btrfs]
> [ 7344.888431]  [<ffffffff8108df55>] ? trace_hardirqs_on+0xd/0xf
> [ 7344.888431]  [<ffffffffa039728c>] __do_contiguous_readpages.constprop.26+0xc2/0xe4 [btrfs]
> [ 7344.888431]  [<ffffffffa037eeb4>] ? btrfs_writepage_start_hook+0xbc/0xbc [btrfs]
> [ 7344.888431]  [<ffffffffa039739b>] __extent_readpages.constprop.25+0xed/0x100 [btrfs]
> [ 7344.888431]  [<ffffffff81129d24>] ? lru_cache_add+0xe/0x10
> [ 7344.888431]  [<ffffffffa0397ea8>] extent_readpages+0x160/0x1aa [btrfs]
> [ 7344.888431]  [<ffffffffa037eeb4>] ? btrfs_writepage_start_hook+0xbc/0xbc [btrfs]
> [ 7344.888431]  [<ffffffff8115daad>] ? alloc_pages_current+0xa9/0xcd
> [ 7344.888431]  [<ffffffffa037cdc9>] btrfs_readpages+0x1f/0x21 [btrfs]
> [ 7344.888431]  [<ffffffff81128316>] __do_page_cache_readahead+0x168/0x1fc
> [ 7344.888431]  [<ffffffff811285a0>] ondemand_readahead+0x1f6/0x207
> [ 7344.888431]  [<ffffffff811285a0>] ? ondemand_readahead+0x1f6/0x207
> [ 7344.888431]  [<ffffffff8111cf34>] ? pagecache_get_page+0x2b/0x154
> [ 7344.888431]  [<ffffffff8112870e>] page_cache_sync_readahead+0x3d/0x3f
> [ 7344.888431]  [<ffffffff8111dbf7>] generic_file_read_iter+0x197/0x4e1
> [ 7344.888431]  [<ffffffff8117773a>] __vfs_read+0x79/0x9d
> [ 7344.888431]  [<ffffffff81178050>] vfs_read+0x8f/0xd2
> [ 7344.888431]  [<ffffffff81178a38>] SyS_read+0x50/0x7e
> [ 7344.888431]  [<ffffffff81492017>] entry_SYSCALL_64_fastpath+0x12/0x6b
> [ 7344.888431] Code: 8d 4d e8 45 31 c9 45 31 c0 48 8b 00 48 c1 e2 09 48 8b 80 80 fc ff ff 4c 89 65 e8 48 8b b8 f0 01 00 00 e8 1d 42 02 00 85 c0 79 02 <0f> 0b 4c 0
> [ 7344.888431] RIP  [<ffffffffa037c88c>] btrfs_merge_bio_hook+0x54/0x6b [btrfs]
> [ 7344.888431]  RSP <ffff8802046878f0>
> [ 7344.970544] ---[ end trace eb7803b24ebab8ae ]---
> 
> Signed-off-by: Filipe Manana <fdmanana@suse.com>
> ---
> 
> v3: Replaced a BUG_ON(!bg) with ASSERT(bg).
> 
>  fs/btrfs/ctree.h       | 14 ++++++++++++
>  fs/btrfs/extent-tree.c | 58 ++++++++++++++++++++++++++++++++++++++++++++++++--
>  fs/btrfs/inode.c       |  8 +++++++
>  fs/btrfs/relocation.c  |  6 +-----
>  4 files changed, 79 insertions(+), 7 deletions(-)
> 
> diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
> index 84a6a5b..90e70e2 100644
> --- a/fs/btrfs/ctree.h
> +++ b/fs/btrfs/ctree.h
> @@ -1408,6 +1408,17 @@ struct btrfs_block_group_cache {
>  
>  	struct btrfs_io_ctl io_ctl;
>  
> +	/*
> +	 * Incremented when doing extent allocations and holding a read lock
> +	 * on the space_info's groups_sem semaphore.
> +	 * Decremented when an ordered extent that represents an IO against this
> +	 * block group's range is created (after it's added to its inode's
> +	 * root's list of ordered extents) or immediately after the allocation
> +	 * if it's a metadata extent or fallocate extent (for these cases we
> +	 * don't create ordered extents).
> +	 */
> +	atomic_t reservations;
> +
>  	/* Lock for free space tree operations. */
>  	struct mutex free_space_lock;
>  
> @@ -3499,6 +3510,9 @@ int btrfs_should_throttle_delayed_refs(struct btrfs_trans_handle *trans,
>  				       struct btrfs_root *root);
>  int btrfs_check_space_for_delayed_refs(struct btrfs_trans_handle *trans,
>  				       struct btrfs_root *root);
> +void btrfs_dec_block_group_reservations(struct btrfs_fs_info *fs_info,
> +					 const u64 start);
> +void btrfs_wait_block_group_reservations(struct btrfs_block_group_cache *bg);
>  void btrfs_put_block_group(struct btrfs_block_group_cache *cache);
>  int btrfs_run_delayed_refs(struct btrfs_trans_handle *trans,
>  			   struct btrfs_root *root, unsigned long count);
> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> index 0544f70..1e009bc 100644
> --- a/fs/btrfs/extent-tree.c
> +++ b/fs/btrfs/extent-tree.c
> @@ -6175,6 +6175,57 @@ int btrfs_exclude_logged_extents(struct btrfs_root *log,
>  	return 0;
>  }
>  
> +static void
> +btrfs_inc_block_group_reservations(struct btrfs_block_group_cache *bg)
> +{
> +	atomic_inc(&bg->reservations);
> +}
> +
> +void btrfs_dec_block_group_reservations(struct btrfs_fs_info *fs_info,
> +					const u64 start)
> +{
> +	struct btrfs_block_group_cache *bg;
> +
> +	bg = btrfs_lookup_block_group(fs_info, start);
> +	ASSERT(bg);
> +	if (atomic_dec_and_test(&bg->reservations))
> +		wake_up_atomic_t(&bg->reservations);
> +	btrfs_put_block_group(bg);
> +}
> +
> +static int btrfs_wait_bg_reservations_atomic_t(atomic_t *a)
> +{
> +	schedule();
> +	return 0;
> +}
> +
> +void btrfs_wait_block_group_reservations(struct btrfs_block_group_cache *bg)
> +{
> +	struct btrfs_space_info *space_info = bg->space_info;
> +
> +	ASSERT(bg->ro);
> +
> +	if (!(bg->flags & BTRFS_BLOCK_GROUP_DATA))
> +		return;
> +
> +	/*
> +	 * Our block group is read only but before we set it to read only,
> +	 * some task might have had allocated an extent from it already, but it
> +	 * has not yet created a respective ordered extent (and added it to a
> +	 * root's list of ordered extents).
> +	 * Therefore wait for any task currently allocating extents, since the
> +	 * block group's reservations counter is incremented while a read lock
> +	 * on the groups' semaphore is held and decremented after releasing
> +	 * the read access on that semaphore and creating the ordered extent.
> +	 */
> +	down_write(&space_info->groups_sem);
> +	up_write(&space_info->groups_sem);
> +
> +	wait_on_atomic_t(&bg->reservations,
> +			 btrfs_wait_bg_reservations_atomic_t,
> +			 TASK_UNINTERRUPTIBLE);
> +}
> +
>  /**
>   * btrfs_update_reserved_bytes - update the block_group and space info counters
>   * @cache:	The cache we are manipulating
> @@ -7434,6 +7485,7 @@ checks:
>  			btrfs_add_free_space(block_group, offset, num_bytes);
>  			goto loop;
>  		}
> +		btrfs_inc_block_group_reservations(block_group);

Metadata doesn't have ordered extent, we can check like

	if (is_data)
		btrfs_inc_block_group_reservations(block_group);
	

>  
>  		/* we are all good, lets return */
>  		ins->objectid = search_start;
> @@ -7615,8 +7667,10 @@ again:
>  	WARN_ON(num_bytes < root->sectorsize);
>  	ret = find_free_extent(root, num_bytes, empty_size, hint_byte, ins,
>  			       flags, delalloc);
> -
> -	if (ret == -ENOSPC) {
> +	if (!ret && !is_data) {
> +		btrfs_dec_block_group_reservations(root->fs_info,
> +						   ins->objectid);
> +	} else if (ret == -ENOSPC) {

Oh, you did this way, then it's fine.

Others are good.

Reviewed-by: Liu Bo <bo.li.liu@oracle.com>

Thanks,

-liubo

>  		if (!final_tried && ins->offset) {
>  			num_bytes = min(num_bytes >> 1, ins->offset);
>  			num_bytes = round_down(num_bytes, root->sectorsize);
> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
> index 41a5688..0085899 100644
> --- a/fs/btrfs/inode.c
> +++ b/fs/btrfs/inode.c
> @@ -824,6 +824,7 @@ retry:
>  						async_extent->ram_size - 1, 0);
>  			goto out_free_reserve;
>  		}
> +		btrfs_dec_block_group_reservations(root->fs_info, ins.objectid);
>  
>  		/*
>  		 * clear dirty, set writeback and unlock the pages.
> @@ -861,6 +862,7 @@ retry:
>  	}
>  	return;
>  out_free_reserve:
> +	btrfs_dec_block_group_reservations(root->fs_info, ins.objectid);
>  	btrfs_free_reserved_extent(root, ins.objectid, ins.offset, 1);
>  out_free:
>  	extent_clear_unlock_delalloc(inode, async_extent->start,
> @@ -1038,6 +1040,8 @@ static noinline int cow_file_range(struct inode *inode,
>  				goto out_drop_extent_cache;
>  		}
>  
> +		btrfs_dec_block_group_reservations(root->fs_info, ins.objectid);
> +
>  		if (disk_num_bytes < cur_alloc_size)
>  			break;
>  
> @@ -1066,6 +1070,7 @@ out:
>  out_drop_extent_cache:
>  	btrfs_drop_extent_cache(inode, start, start + ram_size - 1, 0);
>  out_reserve:
> +	btrfs_dec_block_group_reservations(root->fs_info, ins.objectid);
>  	btrfs_free_reserved_extent(root, ins.objectid, ins.offset, 1);
>  out_unlock:
>  	extent_clear_unlock_delalloc(inode, start, end, locked_page,
> @@ -7162,6 +7167,8 @@ static struct extent_map *btrfs_new_extent_direct(struct inode *inode,
>  		return ERR_PTR(ret);
>  	}
>  
> +	btrfs_dec_block_group_reservations(root->fs_info, ins.objectid);
> +
>  	em = create_pinned_em(inode, start, ins.offset, start, ins.objectid,
>  			      ins.offset, ins.offset, ins.offset, 0);
>  	if (IS_ERR(em)) {
> @@ -9942,6 +9949,7 @@ static int __btrfs_prealloc_file_range(struct inode *inode, int mode,
>  				btrfs_end_transaction(trans, root);
>  			break;
>  		}
> +		btrfs_dec_block_group_reservations(root->fs_info, ins.objectid);
>  
>  		last_alloc = ins.offset;
>  		ret = insert_reserved_file_extent(trans, inode,
> diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
> index 9c35ea2..2aed15c 100644
> --- a/fs/btrfs/relocation.c
> +++ b/fs/btrfs/relocation.c
> @@ -4253,11 +4253,7 @@ int btrfs_relocate_block_group(struct btrfs_root *extent_root, u64 group_start)
>  	btrfs_info(extent_root->fs_info, "relocating block group %llu flags %llu",
>  	       rc->block_group->key.objectid, rc->block_group->flags);
>  
> -	ret = btrfs_start_delalloc_roots(fs_info, 0, -1);
> -	if (ret < 0) {
> -		err = ret;
> -		goto out;
> -	}
> +	btrfs_wait_block_group_reservations(rc->block_group);
>  	btrfs_wait_ordered_roots(fs_info, -1,
>  				 rc->block_group->key.objectid,
>  				 rc->block_group->key.offset);
> -- 
> 2.7.0.rc3
> 
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