Re: [PATCH 1/3] xfs: reject all unaligned direct writes to reflinked files

[Brian Foster <bfoster@redhat.com>]

On Mon, Dec 05, 2016 at 10:05:22PM +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@lst.de>
> ---

Reviewed-by: Brian Foster <bfoster@redhat.com>

>  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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