[PATCH 4.10 14/81] xfs: reject all unaligned direct writes to reflinked files

Greg Kroah-Hartman
2017-04-06T08:44:21Z

From: Christoph Hellwig <hch@lst.de>

commit 54a4ef8af4e0dc5c983d17fcb9cf5fd25666d94e upstream.

We currently fall back from direct to buffered writes if we detect a
remaining shared extent in the iomap_begin callback. But by the time
iomap_begin is called for the potentially unaligned end block we might
have already written most of the data to disk, which we'd now write
again using buffered I/O. To avoid this reject all writes to reflinked
files before starting I/O so that we are guaranteed to only write the
data once.

The alternative would be to unshare the unaligned start and/or end block
before doing the I/O. I think that's doable, and will actually be
required to support reflinks on DAX file system. But it will take a
little more time and I'd rather get rid of the double write ASAP.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
fs/xfs/xfs_file.c | 9 +++++++++
fs/xfs/xfs_iomap.c | 12 +-----------
fs/xfs/xfs_trace.h | 2 +-
3 files changed, 11 insertions(+), 12 deletions(-)

--- a/fs/xfs/xfs_file.c
+++ b/fs/xfs/xfs_file.c
@@ -527,6 +527,15 @@ xfs_file_dio_aio_write(
if ((iocb->ki_pos & mp->m_blockmask) ||
((iocb->ki_pos + count) & mp->m_blockmask)) {
unaligned_io = 1;
+
+ /*
+ * We can't properly handle unaligned direct I/O to reflink
+ * files yet, as we can't unshare a partial block.
+ */
+ if (xfs_is_reflink_inode(ip)) {
+ trace_xfs_reflink_bounce_dio_write(ip, iocb->ki_pos, count);
+ return -EREMCHG;
+ }
iolock = XFS_IOLOCK_EXCL;
} else {
iolock = XFS_IOLOCK_SHARED;
--- a/fs/xfs/xfs_iomap.c
+++ b/fs/xfs/xfs_iomap.c
@@ -1026,17 +1026,7 @@ xfs_file_iomap_begin(
if (error)
goto out_unlock;

- /*
- * We're here because we're trying to do a directio write to a
- * region that isn't aligned to a filesystem block. If the
- * extent is shared, fall back to buffered mode to handle the
- * RMW.
- */
- if (!(flags & IOMAP_REPORT) && shared) {
- trace_xfs_reflink_bounce_dio_write(ip, &imap);
- error = -EREMCHG;
- goto out_unlock;
- }
+ ASSERT((flags & IOMAP_REPORT) || !shared);
}

if ((flags & (IOMAP_WRITE | IOMAP_ZERO)) && xfs_is_reflink_inode(ip)) {
--- a/fs/xfs/xfs_trace.h
+++ b/fs/xfs/xfs_trace.h
@@ -3250,7 +3250,7 @@ DEFINE_INODE_IREC_EVENT(xfs_reflink_conv
DEFINE_RW_EVENT(xfs_reflink_reserve_cow);
DEFINE_RW_EVENT(xfs_reflink_allocate_cow_range);

-DEFINE_INODE_IREC_EVENT(xfs_reflink_bounce_dio_write);
+DEFINE_SIMPLE_IO_EVENT(xfs_reflink_bounce_dio_write);
DEFINE_IOMAP_EVENT(xfs_reflink_find_cow_mapping);
DEFINE_INODE_IREC_EVENT(xfs_reflink_trim_irec);