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

Goldwyn Rodrigues
2020-02-26T02:55:38Z

On 12:53 25/02, Christoph Hellwig wrote:
> On Fri, Feb 21, 2020 at 10:21:04AM +0530, Ritesh Harjani wrote:
> > > 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.
>
> In addition to that copied is also iov_iter_reexpand call. We don't
> really need the re-expand in case of errors, and in fact we also
> have the iov_iter_revert call before jumping out, so this will
> need a little bit more of an audit and properly documented in the
> commit log.
>
> >
> > 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?
>
> We'd need something there iff we care about a good number of written
> in case of the error. Goldwyn, can you explain what you need this
> number for in btrfs? Maybe with a pointer to the current code base?

btrfs needs to account for the bytes "processed", failed or
uptodate. This is currently performed in
fs/btrfs/inode.c:__end_write_update_ordered().

For the current development version, how I am using it is in my git
branch btrfs-iomap-dio [1]. The related commit besides this patch
is:

9aeb2b31d10b ("btrfs: Use ->iomap_end() instead of btrfs_dio_data")

[1] https://github.com/goldwynr/linux/tree/btrfs-iomap-dio

--
Goldwyn