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Subject: Re: [PATCH v2] iomap: return partial I/O count on error in
 iomap_dio_bio_actor
To:     Goldwyn Rodrigues <rgoldwyn@suse.com>, linux-ext4@vger.kernel.org
Cc:     linux-fsdevel@vger.kernel.org, hch@infradead.org,
        darrick.wong@oracle.com
References: <20200220152355.5ticlkptc7kwrifz@fiona>
From:   Ritesh Harjani <riteshh@linux.ibm.com>
Date:   Fri, 21 Feb 2020 10:21:04 +0530
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On 2/20/20 8:53 PM, Goldwyn Rodrigues wrote:
> In case of a block device error, written parameter in iomap_end()
> is zero as opposed to the amount of submitted I/O.
> Filesystems such as btrfs need to account for the I/O in ordered
> extents, even if it resulted in an error. Having (incomplete)
> submitted bytes in written gives the filesystem the amount of data
> which has been submitted before the error occurred, and the
> filesystem code can choose how to use it.
> 
> The final returned error for iomap_dio_rw() is set by
> iomap_dio_complete().
> 
> Partial writes in direct I/O are considered an error. So,
> ->iomap_end() using written == 0 as error must be changed
> to written < length. In this case, ext4 is the only user.
> 
> Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
> ---
>   fs/ext4/inode.c      | 2 +-
>   fs/iomap/direct-io.c | 2 +-
>   2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index e60aca791d3f..e50e7414351a 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -3475,7 +3475,7 @@ static int ext4_iomap_end(struct inode *inode, loff_t offset, loff_t length,
>   	 * the I/O. Any blocks that may have been allocated in preparation for
>   	 * the direct I/O will be reused during buffered I/O.
>   	 */
> -	if (flags & (IOMAP_WRITE | IOMAP_DIRECT) && written == 0)
> +	if (flags & (IOMAP_WRITE | IOMAP_DIRECT) && written < length)
>   		return -ENOTBLK;
>   
>   	return 0;
> diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
> index 41c1e7c20a1f..01865db1bd09 100644
> --- a/fs/iomap/direct-io.c
> +++ b/fs/iomap/direct-io.c
> @@ -264,7 +264,7 @@ iomap_dio_bio_actor(struct inode *inode, loff_t pos, loff_t length,
>   		size_t n;
>   		if (dio->error) {
>   			iov_iter_revert(dio->submit.iter, copied);
> -			copied = ret = 0;
> +			ret = 0;
>   			goto out;
>   		}

But if I am seeing this correctly, even after there was a dio->error
if you return copied > 0, then the loop in iomap_dio_rw will continue
for next iteration as well. Until the second time it won't copy
anything since dio->error is set and from there I guess it may return
0 which will break the loop.

Is this the correct flow? Shouldn't the while loop doing
iomap_apply in iomap_dio_rw should also break in case of
dio->error? Or did I miss anything?


-ritesh