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@xxxxxx>
---
fs/xfs/xfs_file.c | 7 +++++++
fs/xfs/xfs_iomap.c | 12 +-----------
2 files changed, 8 insertions(+), 11 deletions(-)
diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
index f5effa6..873cd42 100644
--- a/fs/xfs/xfs_file.c
+++ b/fs/xfs/xfs_file.c
@@ -532,6 +532,13 @@ 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))
+ return -EREMCHG;
iolock = XFS_IOLOCK_EXCL;
} else {
iolock = XFS_IOLOCK_SHARED;
diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
index 0d14742..78105db 100644
--- 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)) {
--
2.1.4
--
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