btrfs: refactor btrfs_check_can_nocow() into two variants

The function btrfs_check_can_nocow() now has two completely different
call patterns.

For nowait variant, callers don't need to do any cleanup.  While for
wait variant, callers need to release the lock if they can do nocow
write.

This is somehow confusing, and is already a problem for the exported
btrfs_check_can_nocow().

So this patch will separate the different patterns into different
functions.
For nowait variant, the function will be called check_nocow_nolock().
For wait variant, the function pair will be btrfs_check_nocow_lock()
btrfs_check_nocow_unlock().
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.h

@@ -3033,8 +3033,9 @@ int btrfs_dirty_pages(struct inode *inode, struct page **pages,
 		      size_t num_pages, loff_t pos, size_t write_bytes,
 		      struct extent_state **cached);
 int btrfs_fdatawrite_range(struct inode *inode, loff_t start, loff_t end);
-int btrfs_check_can_nocow(struct btrfs_inode *inode, loff_t pos,
-			  size_t *write_bytes, bool nowait);
+int btrfs_check_nocow_lock(struct btrfs_inode *inode, loff_t pos,
+			   size_t *write_bytes);
+void btrfs_check_nocow_unlock(struct btrfs_inode *inode);
 
 /* tree-defrag.c */
 int btrfs_defrag_leaves(struct btrfs_trans_handle *trans,

fs/btrfs/file.c

@@ -1533,29 +1533,8 @@ lock_and_cleanup_extent_if_need(struct btrfs_inode *inode, struct page **pages,
 	return ret;
 }
 
-/*
- * Check if we can do nocow write into the range [@pos, @pos + @write_bytes)
- *
- * @pos:	 File offset
- * @write_bytes: The length to write, will be updated to the nocow writeable
- *		 range
- * @nowait:	 Whether this function could sleep
- *
- * This function will flush ordered extents in the range to ensure proper
- * nocow checks for (nowait == false) case.
- *
- * Return:
- * >0		and update @write_bytes if we can do nocow write
- *  0		if we can't do nocow write
- * -EAGAIN	if we can't get the needed lock or there are ordered extents
- * 		for * (nowait == true) case
- * <0		if other error happened
- *
- * NOTE: For wait (nowait == false) calls, callers need to release the drew
- * write lock of inode->root->snapshot_lock when return value > 0.
- */
-int btrfs_check_can_nocow(struct btrfs_inode *inode, loff_t pos,
-			  size_t *write_bytes, bool nowait)
+static int check_can_nocow(struct btrfs_inode *inode, loff_t pos,
+			   size_t *write_bytes, bool nowait)
 {
 	struct btrfs_fs_info *fs_info = inode->root->fs_info;
 	struct btrfs_root *root = inode->root;
@@ -1563,6 +1542,9 @@ int btrfs_check_can_nocow(struct btrfs_inode *inode, loff_t pos,
 	u64 num_bytes;
 	int ret;
 
+	if (!(inode->flags & (BTRFS_INODE_NODATACOW | BTRFS_INODE_PREALLOC)))
+		return 0;
+
 	if (!nowait && !btrfs_drew_try_write_lock(&root->snapshot_lock))
 		return -EAGAIN;
 
@@ -1605,6 +1587,42 @@ int btrfs_check_can_nocow(struct btrfs_inode *inode, loff_t pos,
 	return ret;
 }
 
+static int check_nocow_nolock(struct btrfs_inode *inode, loff_t pos,
+			      size_t *write_bytes)
+{
+	return check_can_nocow(inode, pos, write_bytes, true);
+}
+
+/*
+ * Check if we can do nocow write into the range [@pos, @pos + @write_bytes)
+ *
+ * @pos:	 File offset
+ * @write_bytes: The length to write, will be updated to the nocow writeable
+ *		 range
+ *
+ * This function will flush ordered extents in the range to ensure proper
+ * nocow checks.
+ *
+ * Return:
+ * >0		and update @write_bytes if we can do nocow write
+ *  0		if we can't do nocow write
+ * -EAGAIN	if we can't get the needed lock or there are ordered extents
+ * 		for * (nowait == true) case
+ * <0		if other error happened
+ *
+ * NOTE: Callers need to release the lock by btrfs_check_nocow_unlock().
+ */
+int btrfs_check_nocow_lock(struct btrfs_inode *inode, loff_t pos,
+			   size_t *write_bytes)
+{
+	return check_can_nocow(inode, pos, write_bytes, false);
+}
+
+void btrfs_check_nocow_unlock(struct btrfs_inode *inode)
+{
+	btrfs_drew_write_unlock(&inode->root->snapshot_lock);
+}
+
 static noinline ssize_t btrfs_buffered_write(struct kiocb *iocb,
 					       struct iov_iter *i)
 {
@@ -1612,7 +1630,6 @@ static noinline ssize_t btrfs_buffered_write(struct kiocb *iocb,
 	loff_t pos = iocb->ki_pos;
 	struct inode *inode = file_inode(file);
 	struct btrfs_fs_info *fs_info = btrfs_sb(inode->i_sb);
-	struct btrfs_root *root = BTRFS_I(inode)->root;
 	struct page **pages = NULL;
 	struct extent_changeset *data_reserved = NULL;
 	u64 release_bytes = 0;
@@ -1668,10 +1685,8 @@ static noinline ssize_t btrfs_buffered_write(struct kiocb *iocb,
 		ret = btrfs_check_data_free_space(inode, &data_reserved, pos,
 						  write_bytes);
 		if (ret < 0) {
-			if ((BTRFS_I(inode)->flags & (BTRFS_INODE_NODATACOW |
-						      BTRFS_INODE_PREALLOC)) &&
-			    btrfs_check_can_nocow(BTRFS_I(inode), pos,
-						  &write_bytes, false) > 0) {
+			if (btrfs_check_nocow_lock(BTRFS_I(inode), pos,
+						   &write_bytes) > 0) {
 				/*
 				 * For nodata cow case, no need to reserve
 				 * data space.
@@ -1700,7 +1715,7 @@ static noinline ssize_t btrfs_buffered_write(struct kiocb *iocb,
 						data_reserved, pos,
 						write_bytes);
 			else
-				btrfs_drew_write_unlock(&root->snapshot_lock);
+				btrfs_check_nocow_unlock(BTRFS_I(inode));
 			break;
 		}
 
@@ -1804,7 +1819,7 @@ static noinline ssize_t btrfs_buffered_write(struct kiocb *iocb,
 
 		release_bytes = 0;
 		if (only_release_metadata)
-			btrfs_drew_write_unlock(&root->snapshot_lock);
+			btrfs_check_nocow_unlock(BTRFS_I(inode));
 
 		if (only_release_metadata && copied > 0) {
 			lockstart = round_down(pos,
@@ -1831,7 +1846,7 @@ static noinline ssize_t btrfs_buffered_write(struct kiocb *iocb,
 
 	if (release_bytes) {
 		if (only_release_metadata) {
-			btrfs_drew_write_unlock(&root->snapshot_lock);
+			btrfs_check_nocow_unlock(BTRFS_I(inode));
 			btrfs_delalloc_release_metadata(BTRFS_I(inode),
 					release_bytes, true);
 		} else {
@@ -1946,10 +1961,8 @@ static ssize_t btrfs_file_write_iter(struct kiocb *iocb,
 		 * We will allocate space in case nodatacow is not set,
 		 * so bail
 		 */
-		if (!(BTRFS_I(inode)->flags & (BTRFS_INODE_NODATACOW |
-					      BTRFS_INODE_PREALLOC)) ||
-		    btrfs_check_can_nocow(BTRFS_I(inode), pos, &nocow_bytes,
-					  true) <= 0) {
+		if (check_nocow_nolock(BTRFS_I(inode), pos, &nocow_bytes)
+		    <= 0) {
 			inode_unlock(inode);
 			return -EAGAIN;
 		}

fs/btrfs/inode.c

@@ -4534,10 +4534,8 @@ int btrfs_truncate_block(struct inode *inode, loff_t from, loff_t len,
 	ret = btrfs_check_data_free_space(inode, &data_reserved, block_start,
 					  blocksize);
 	if (ret < 0) {
-		if ((BTRFS_I(inode)->flags & (BTRFS_INODE_NODATACOW |
-					      BTRFS_INODE_PREALLOC)) &&
-		    btrfs_check_can_nocow(BTRFS_I(inode), block_start,
-					  &write_bytes, false) > 0) {
+		if (btrfs_check_nocow_lock(BTRFS_I(inode), block_start,
+					   &write_bytes) > 0) {
 			/* For nocow case, no need to reserve data space */
 			only_release_metadata = true;
 		} else {
@@ -4638,7 +4636,7 @@ int btrfs_truncate_block(struct inode *inode, loff_t from, loff_t len,
 	put_page(page);
 out:
 	if (only_release_metadata)
-		btrfs_drew_write_unlock(&BTRFS_I(inode)->root->snapshot_lock);
+		btrfs_check_nocow_unlock(BTRFS_I(inode));
 	extent_changeset_free(data_reserved);
 	return ret;
 }