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, 17 insertions, 4 deletions

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index d88f2dc4d045..006cf9f0e852 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -1410,6 +1410,10 @@ static noinline int check_can_nocow(struct inode *inode, loff_t pos,
 	u64 num_bytes;
 	int ret;
 
+	ret = btrfs_start_nocow_write(root);
+	if (!ret)
+		return -ENOSPC;
+
 	lockstart = round_down(pos, root->sectorsize);
 	lockend = round_up(pos + *write_bytes, root->sectorsize) - 1;
 
@@ -1427,11 +1431,13 @@ static noinline int check_can_nocow(struct inode *inode, loff_t pos,
 
 	num_bytes = lockend - lockstart + 1;
 	ret = can_nocow_extent(inode, lockstart, &num_bytes, NULL, NULL, NULL);
-	if (ret <= 0)
+	if (ret <= 0) {
 		ret = 0;
-	else
+		btrfs_end_nocow_write(root);
+	} else {
 		*write_bytes = min_t(size_t, *write_bytes ,
 				     num_bytes - pos + lockstart);
+	}
 
 	unlock_extent(&BTRFS_I(inode)->io_tree, lockstart, lockend);
 
@@ -1520,6 +1526,8 @@ static noinline ssize_t __btrfs_buffered_write(struct file *file,
 			if (!only_release_metadata)
 				btrfs_free_reserved_data_space(inode,
 							       reserve_bytes);
+			else
+				btrfs_end_nocow_write(root);
 			break;
 		}
 
@@ -1608,6 +1616,9 @@ again:
 		}
 
 		release_bytes = 0;
+		if (only_release_metadata)
+			btrfs_end_nocow_write(root);
+
 		if (only_release_metadata && copied > 0) {
 			u64 lockstart = round_down(pos, root->sectorsize);
 			u64 lockend = lockstart +
@@ -1634,10 +1645,12 @@ again:
 	kfree(pages);
 
 	if (release_bytes) {
-		if (only_release_metadata)
+		if (only_release_metadata) {
+			btrfs_end_nocow_write(root);
 			btrfs_delalloc_release_metadata(inode, release_bytes);
-		else
+		} else {
 			btrfs_delalloc_release_space(inode, release_bytes);
+		}
 	}
 
 	return num_written ? num_written : ret;