On Wed, Feb 01, 2017 at 10:42:36PM +0100, Christoph Hellwig wrote:
> 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>
> Reviewed-by: Brian Foster <bfoster@xxxxxxxxxx>
> ---
> 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 bbb9eb6..06236bf 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -527,6 +527,13 @@ 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 767208f..d17aa3b 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);
Can you either port the tracepoint to the new hunk or just kill it
entirely?
(If you opt for killing it, I'll just remove it from xfs_trace.h when I
import this patch.)
Otherwise,
Reviewed-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
--D
> - 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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