Re: [PATCH v2] iomap: return partial I/O count on error in iomap_dio_bio_actor

Ritesh Harjani
2020-02-21T04:51:18Z

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