Btrfs: introduce btrfs_{start, end}_nocow_write() for each subvolume

If the snapshot creation happened after the nocow write but before the dirty
data flush, we would fail to flush the dirty data because of no space.

So we must keep track of when those nocow write operations start and when they
end, if there are nocow writers, the snapshot creators must wait. In order
to implement this function, I introduce btrfs_{start, end}_nocow_write(),
which is similar to mnt_{want,drop}_write().

These two functions are only used for nocow file write operations.

Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Josef Bacik <jbacik@fb.com>

1 files changed, 30 insertions, 5 deletions

diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 1ae45bd..57bc9f3 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -611,6 +611,23 @@ fail:
 	return ret;
 }
 
+static void btrfs_wait_nocow_write(struct btrfs_root *root)
+{
+	s64 writers;
+	DEFINE_WAIT(wait);
+
+	do {
+		prepare_to_wait(&root->subv_writers->wait, &wait,
+				TASK_UNINTERRUPTIBLE);
+
+		writers = percpu_counter_sum(&root->subv_writers->counter);
+		if (writers)
+			schedule();
+
+		finish_wait(&root->subv_writers->wait, &wait);
+	} while (writers);
+}
+
 static int create_snapshot(struct btrfs_root *root, struct inode *dir,
 			   struct dentry *dentry, char *name, int namelen,
 			   u64 *async_transid, bool readonly,
@@ -624,15 +641,21 @@ static int create_snapshot(struct btrfs_root *root, struct inode *dir,
 	if (!root->ref_cows)
 		return -EINVAL;
 
+	atomic_inc(&root->will_be_snapshoted);
+	smp_mb__after_atomic_inc();
+	btrfs_wait_nocow_write(root);
+
 	ret = btrfs_start_delalloc_inodes(root, 0);
 	if (ret)
-		return ret;
+		goto out;
 
 	btrfs_wait_ordered_extents(root, -1);
 
 	pending_snapshot = kzalloc(sizeof(*pending_snapshot), GFP_NOFS);
-	if (!pending_snapshot)
-		return -ENOMEM;
+	if (!pending_snapshot) {
+		ret = -ENOMEM;
+		goto out;
+	}
 
 	btrfs_init_block_rsv(&pending_snapshot->block_rsv,
 			     BTRFS_BLOCK_RSV_TEMP);
@@ -649,7 +672,7 @@ static int create_snapshot(struct btrfs_root *root, struct inode *dir,
 					&pending_snapshot->qgroup_reserved,
 					false);
 	if (ret)
-		goto out;
+		goto free;
 
 	pending_snapshot->dentry = dentry;
 	pending_snapshot->root = root;
@@ -700,8 +723,10 @@ fail:
 	btrfs_subvolume_release_metadata(BTRFS_I(dir)->root,
 					 &pending_snapshot->block_rsv,
 					 pending_snapshot->qgroup_reserved);
-out:
+free:
 	kfree(pending_snapshot);
+out:
+	atomic_dec(&root->will_be_snapshoted);
 	return ret;
 }