linux-ext4 - Re: [PATCH RFC 2/2] iomap: revert the iomap_iter pos on ->iomap

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Message-ID: <CAJnrk1bmjCB=8o-YOkPScftoXMrgpBKU3vtkMOViEfFQ9LXLfg@mail.gmail.com>
Date: Tue, 2 Sep 2025 14:11:35 -0700
From: Joanne Koong <joannelkoong@...il.com>
To: Brian Foster <bfoster@...hat.com>
Cc: linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org, 
	linux-xfs@...r.kernel.org, jack@...e.cz, djwong@...nel.org
Subject: Re: [PATCH RFC 2/2] iomap: revert the iomap_iter pos on ->iomap_end() error

On Tue, Sep 2, 2025 at 8:04 AM Brian Foster <bfoster@...hat.com> wrote:
>
> An iomap op iteration should not be considered successful if
> ->iomap_end() fails. Most ->iomap_end() callbacks do not return
> errors, and for those that do we return the error to the caller, but
> this is still not sufficient in some corner cases.
>
> For example, if a DAX write to a shared iomap fails at ->iomap_end()
> on XFS, this means the remap of shared blocks from the COW fork to
> the data fork has possibly failed. In turn this means that just
> written data may not be accessible in the file. dax_iomap_rw()
> returns partial success over a returned error code and the operation
> has already advanced iter.pos by the time ->iomap_end() is called.
> This means that dax_iomap_rw() can return more bytes processed than
> have been completed successfully, including partial success instead
> of an error code if the first iteration happens to fail.
>
> To address this problem, first tweak the ->iomap_end() error
> handling logic to run regardless of whether the current iteration
> advanced the iter. Next, revert pos in the error handling path. Add
> a new helper to undo the changes from iomap_iter_advance(). It is
> static to start since the only initial user is in iomap_iter.c.
>
> Signed-off-by: Brian Foster <bfoster@...hat.com>
> ---
>  fs/iomap/iter.c | 20 +++++++++++++++++++-
>  1 file changed, 19 insertions(+), 1 deletion(-)
>
> diff --git a/fs/iomap/iter.c b/fs/iomap/iter.c
> index 7cc4599b9c9b..69c993fe51fa 100644
> --- a/fs/iomap/iter.c
> +++ b/fs/iomap/iter.c
> @@ -27,6 +27,22 @@ int iomap_iter_advance(struct iomap_iter *iter, u64 *count)
>         return 0;
>  }
>
> +/**
> + * iomap_iter_revert - revert the iterator position
> + * @iter: iteration structure
> + * @count: number of bytes to revert
> + *
> + * Revert the iterator position by the specified number of bytes, undoing
> + * the effect of a previous iomap_iter_advance() call. The count must not
> + * exceed the amount previously advanced in the current iter.
> + */
> +static void iomap_iter_revert(struct iomap_iter *iter, u64 count)
> +{
> +       count = min_t(u64, iter->pos - iter->iter_start_pos, count);
> +       iter->pos -= count;
> +       iter->len += count;
> +}
> +
>  static inline void iomap_iter_done(struct iomap_iter *iter)
>  {
>         WARN_ON_ONCE(iter->iomap.offset > iter->pos);
> @@ -80,8 +96,10 @@ int iomap_iter(struct iomap_iter *iter, const struct iomap_ops *ops)
>                                 iomap_length_trim(iter, iter->iter_start_pos,
>                                                   olen),
>                                 advanced, iter->flags, &iter->iomap);
> -               if (ret < 0 && !advanced && !iter->status)
> +               if (ret < 0 && !iter->status) {
> +                       iomap_iter_revert(iter, advanced);
>                         return ret;
> +               }

Should iomap_iter_revert() also be called in the "if (iter->status <
0)" case a few lines below? I think otherwise, that leads to the same
problem in dax_iomap_rw() you pointed out in the commit message.

Thanks,
Joanne
>         }
>
>         /* detect old return semantics where this would advance */
> --
> 2.51.0
>
>