[PATCH 110/119] xfs: fail ->bmap for reflink inodes

["Darrick J. Wong" <darrick.wong@xxxxxxxxxx>]

From: Christoph Hellwig <hch@xxxxxx>

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@xxxxxx>
[darrick.wong@xxxxxxxxxx: add a more descriptive changelog]
Signed-off-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
---
 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 31205fa..83fd028 100644
--- a/fs/xfs/xfs_aops.c
+++ b/fs/xfs/xfs_aops.c
@@ -1827,6 +1827,17 @@ 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
+        * bypasseÑ 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);