[PATCH 4.9 17/72] xfs: reject all unaligned direct writes to reflinked files

Greg Kroah-Hartman
2017-04-06T09:18:53Z

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>
[slight changes in context due to the new direct I/O code in 4.10+]
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
fs/xfs/xfs_aops.c | 45 ---------------------------------------------
fs/xfs/xfs_file.c | 9 +++++++++
fs/xfs/xfs_trace.h | 2 +-
3 files changed, 10 insertions(+), 46 deletions(-)

--- a/fs/xfs/xfs_aops.c
+++ b/fs/xfs/xfs_aops.c
@@ -1263,44 +1263,6 @@ xfs_map_trim_size(
bh_result->b_size = mapping_size;
}

-/* Bounce unaligned directio writes to the page cache. */
-static int
-xfs_bounce_unaligned_dio_write(
- struct xfs_inode *ip,
- xfs_fileoff_t offset_fsb,
- struct xfs_bmbt_irec *imap)
-{
- struct xfs_bmbt_irec irec;
- xfs_fileoff_t delta;
- bool shared;
- bool x;
- int error;
-
- irec = *imap;
- if (offset_fsb > irec.br_startoff) {
- delta = offset_fsb - irec.br_startoff;
- irec.br_blockcount -= delta;
- irec.br_startblock += delta;
- irec.br_startoff = offset_fsb;
- }
- error = xfs_reflink_trim_around_shared(ip, &irec, &shared, &x);
- if (error)
- return error;
-
- /*
- * We're here because we're trying to do a directio write to a
- * region that isn't aligned to a filesystem block. If any part
- * of the extent is shared, fall back to buffered mode to handle
- * the RMW. This is done by returning -EREMCHG ("remote addr
- * changed"), which is caught further up the call stack.
- */
- if (shared) {
- trace_xfs_reflink_bounce_dio_write(ip, imap);
- return -EREMCHG;
- }
- return 0;
-}
-
STATIC int
__xfs_get_blocks(
struct inode *inode,
@@ -1438,13 +1400,6 @@ __xfs_get_blocks(
if (imap.br_startblock != HOLESTARTBLOCK &&
imap.br_startblock != DELAYSTARTBLOCK &&
(create || !ISUNWRITTEN(&imap))) {
- if (create && direct && !is_cow) {
- error = xfs_bounce_unaligned_dio_write(ip, offset_fsb,
- &imap);
- if (error)
- return error;
- }
-
xfs_map_buffer(inode, bh_result, &imap, offset);
if (ISUNWRITTEN(&imap))
set_buffer_unwritten(bh_result);
--- a/fs/xfs/xfs_file.c
+++ b/fs/xfs/xfs_file.c
@@ -554,6 +554,15 @@ 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_trace.h
+++ b/fs/xfs/xfs_trace.h
@@ -3353,7 +3353,7 @@ 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);