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

Goldwyn Rodrigues
2020-02-21T12:48:39Z

On 10:21 21/02, Ritesh Harjani wrote:
>
>
> 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?
>

Yes, We can save an extra iteration by checking for dio->error in the
while loop of iomap_dio_rw().

--
Goldwyn