xfs: only grab shared inode locks for source file during reflink

Reflink and dedupe operations remap blocks from a source file into a
destination file.  The destination file needs exclusive locks on all
levels because we're updating its block map, but the source file isn't
undergoing any block map changes so we can use a shared lock.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>

fs/xfs/xfs_reflink.c

@@ -1202,13 +1202,16 @@ xfs_reflink_remap_blocks(
 
 	/* drange = (destoff, destoff + len); srange = (srcoff, srcoff + len) */
 	while (len) {
+		uint		lock_mode;
+
 		trace_xfs_reflink_remap_blocks_loop(src, srcoff, len,
 				dest, destoff);
+
 		/* Read extent from the source file */
 		nimaps = 1;
-		xfs_ilock(src, XFS_ILOCK_EXCL);
+		lock_mode = xfs_ilock_data_map_shared(src);
 		error = xfs_bmapi_read(src, srcoff, len, &imap, &nimaps, 0);
-		xfs_iunlock(src, XFS_ILOCK_EXCL);
+		xfs_iunlock(src, lock_mode);
 		if (error)
 			goto err;
 		ASSERT(nimaps == 1);
@@ -1260,7 +1263,7 @@ xfs_iolock_two_inodes_and_break_layout(
 
 retry:
 	if (src < dest) {
-		inode_lock(src);
+		inode_lock_shared(src);
 		inode_lock_nested(dest, I_MUTEX_NONDIR2);
 	} else {
 		/* src >= dest */
@@ -1271,7 +1274,7 @@ retry:
 	if (error == -EWOULDBLOCK) {
 		inode_unlock(dest);
 		if (src < dest)
-			inode_unlock(src);
+			inode_unlock_shared(src);
 		error = break_layout(dest, true);
 		if (error)
 			return error;
@@ -1280,11 +1283,11 @@ retry:
 	if (error) {
 		inode_unlock(dest);
 		if (src < dest)
-			inode_unlock(src);
+			inode_unlock_shared(src);
 		return error;
 	}
 	if (src > dest)
-		inode_lock_nested(src, I_MUTEX_NONDIR2);
+		inode_lock_shared_nested(src, I_MUTEX_NONDIR2);
 	return 0;
 }
 
@@ -1324,7 +1327,7 @@ xfs_reflink_remap_range(
 	if (same_inode)
 		xfs_ilock(src, XFS_MMAPLOCK_EXCL);
 	else
-		xfs_lock_two_inodes(src, XFS_MMAPLOCK_EXCL, dest,
+		xfs_lock_two_inodes(src, XFS_MMAPLOCK_SHARED, dest,
 				XFS_MMAPLOCK_EXCL);
 
 	/* Check file eligibility and prepare for block sharing. */
@@ -1393,10 +1396,12 @@ xfs_reflink_remap_range(
 			is_dedupe);
 
 out_unlock:
-	xfs_iunlock(src, XFS_MMAPLOCK_EXCL);
+	xfs_iunlock(dest, XFS_MMAPLOCK_EXCL);
+	if (!same_inode)
+		xfs_iunlock(src, XFS_MMAPLOCK_SHARED);
+	inode_unlock(inode_out);
 	if (!same_inode)
-		xfs_iunlock(dest, XFS_MMAPLOCK_EXCL);
-	unlock_two_nondirectories(inode_in, inode_out);
+		inode_unlock_shared(inode_in);
 	if (ret)
 		trace_xfs_reflink_remap_range_error(dest, ret, _RET_IP_);
 	return ret;

include/linux/fs.h

@@ -748,6 +748,11 @@ static inline void inode_lock_nested(struct inode *inode, unsigned subclass)
 	down_write_nested(&inode->i_rwsem, subclass);
 }
 
+static inline void inode_lock_shared_nested(struct inode *inode, unsigned subclass)
+{
+	down_read_nested(&inode->i_rwsem, subclass);
+}
+
 void lock_two_nondirectories(struct inode *, struct inode*);
 void unlock_two_nondirectories(struct inode *, struct inode*);