On Tue, Sep 2, 2025 at 8:04 AM Brian Foster <bfoster@xxxxxxxxxx> 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@xxxxxxxxxx>
> ---
> 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
>
>
