Re: [PATCH 3/6] iomap: support incremental iomap_iter advances

[Christoph Hellwig <hch@infradead.org>]

On Fri, Dec 13, 2024 at 09:36:07AM -0500, Brian Foster wrote:
> Note that the semantics for operations that use incremental advances
> is slightly different than traditional operations. Operations that
> advance the iter directly are expected to return success or failure
> (i.e. 0 or negative error code) in iter.processed rather than the
> number of bytes processed.

While the uses of the incremental advance later look nice, this bit
is pretty ugly.  I wonder if we could just move overy everything to
the incremental advance model, even if it isn't all that incremental,
that is always call iomap_iter_advance from the processing loop and
eventually remove the call in iomap_iter() entirely?

> @@ -36,7 +36,7 @@ static inline int iomap_iter_advance(struct iomap_iter *iter, s64 count)
>  		return -EIO;
>  	iter->pos += count;
>  	iter->len -= count;
> -	if (!iter->len || (!count && !stale))
> +	if (!iter->len || (!count && !stale && iomap_length(iter)))

This probably warrantd a comment even with the existing code, but really
needs one now.

> + * @iter_spos: The original start pos for the current iomap. Used for
> + *	incremental iter advance.

Maybe spell out the usage as iter_start_pos in the field name as spos
reads a little weird?