[PATCH 67/71] xfs: fail ->bmap for reflink inodes

[Darrick J. Wong]

From: Christoph Hellwig <hch at lst.de>

Have xfs_vm_bmap return zero for reflinked files.  This hack prevents
using a file with shared blocks as a swap file, because we don't want
to deal with CoW when we're (probably) low on memory.

Signed-off-by: Christoph Hellwig <hch at lst.de>
[darrick.wong at oracle.com: add a more descriptive changelog]
Signed-off-by: Darrick J. Wong <darrick.wong at oracle.com>
---
 fs/xfs/xfs_aops.c |   11 +++++++++++
 1 file changed, 11 insertions(+)


diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
index a85bc62..f564d02 100644
--- a/fs/xfs/xfs_aops.c
+++ b/fs/xfs/xfs_aops.c
@@ -1553,6 +1553,17 @@ xfs_vm_bmap(
 
 	trace_xfs_vm_bmap(XFS_I(inode));
 	xfs_ilock(ip, XFS_IOLOCK_SHARED);
+
+	/*
+	 * The swap code (ab-)uses ->bmap to get a block mapping and then
+	 * bypasseѕ the file system for actual I/O.  We really can't allow
+	 * that on reflinks inodes, so we have to skip out here.  And yes,
+	 * 0 is the magic code for a bmap error..
+	 */
+	if (xfs_is_reflink_inode(ip)) {
+		xfs_iunlock(ip, XFS_IOLOCK_SHARED);
+		return 0;
+	}
 	filemap_write_and_wait(mapping);
 	xfs_iunlock(ip, XFS_IOLOCK_SHARED);
 	return generic_block_bmap(mapping, block, xfs_get_blocks);