Re: [PATCH v4 02/10] iomap: split out iomap check and reset logic from iter advance

["Darrick J. Wong" <djwong@kernel.org>]

On Tue, Feb 04, 2025 at 08:30:36AM -0500, Brian Foster wrote:
> In preparation for more granular iomap_iter advancing, break out
> some of the logic associated with higher level iteration from
> iomap_advance_iter(). Specifically, factor the iomap reset code into
> a separate helper and lift the iomap.length check into the calling
> code, similar to how ->iomap_end() calls are handled.
> 
> Signed-off-by: Brian Foster <bfoster@redhat.com>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> ---
>  fs/iomap/iter.c | 49 ++++++++++++++++++++++++++-----------------------
>  1 file changed, 26 insertions(+), 23 deletions(-)
> 
> diff --git a/fs/iomap/iter.c b/fs/iomap/iter.c
> index 3790918646af..731ea7267f27 100644
> --- a/fs/iomap/iter.c
> +++ b/fs/iomap/iter.c
> @@ -7,6 +7,13 @@
>  #include <linux/iomap.h>
>  #include "trace.h"
>  
> +static inline void iomap_iter_reset_iomap(struct iomap_iter *iter)
> +{
> +	iter->processed = 0;
> +	memset(&iter->iomap, 0, sizeof(iter->iomap));
> +	memset(&iter->srcmap, 0, sizeof(iter->srcmap));
> +}
> +
>  /*
>   * Advance to the next range we need to map.
>   *
> @@ -14,32 +21,24 @@
>   * processed - it was aborted because the extent the iomap spanned may have been
>   * changed during the operation. In this case, the iteration behaviour is to
>   * remap the unprocessed range of the iter, and that means we may need to remap
> - * even when we've made no progress (i.e. iter->processed = 0). Hence the
> - * "finished iterating" case needs to distinguish between
> - * (processed = 0) meaning we are done and (processed = 0 && stale) meaning we
> - * need to remap the entire remaining range.
> + * even when we've made no progress (i.e. count = 0). Hence the "finished
> + * iterating" case needs to distinguish between (count = 0) meaning we are done
> + * and (count = 0 && stale) meaning we need to remap the entire remaining range.
>   */
> -static inline int iomap_iter_advance(struct iomap_iter *iter)
> +static inline int iomap_iter_advance(struct iomap_iter *iter, s64 count)
>  {
>  	bool stale = iter->iomap.flags & IOMAP_F_STALE;
>  	int ret = 1;
>  
> -	/* handle the previous iteration (if any) */
> -	if (iter->iomap.length) {
> -		if (iter->processed < 0)
> -			return iter->processed;
> -		if (WARN_ON_ONCE(iter->processed > iomap_length(iter)))
> -			return -EIO;
> -		iter->pos += iter->processed;
> -		iter->len -= iter->processed;
> -		if (!iter->len || (!iter->processed && !stale))
> -			ret = 0;
> -	}
> +	if (count < 0)
> +		return count;
> +	if (WARN_ON_ONCE(count > iomap_length(iter)))
> +		return -EIO;
> +	iter->pos += count;
> +	iter->len -= count;
> +	if (!iter->len || (!count && !stale))
> +		ret = 0;
>  
> -	/* clear the per iteration state */
> -	iter->processed = 0;
> -	memset(&iter->iomap, 0, sizeof(iter->iomap));
> -	memset(&iter->srcmap, 0, sizeof(iter->srcmap));

Are there any consequences to not resetting the iter if
iter->iomap.length is zero?  I think the answer is "no" because callers
are supposed to initialize the iter with zeroes and filesystems are
never supposed to return zero-length iomaps from ->begin_iomap, right?

If the answers are "no" and "yes" then
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>

--D

>  	return ret;
>  }
>  
> @@ -82,10 +81,14 @@ int iomap_iter(struct iomap_iter *iter, const struct iomap_ops *ops)
>  			return ret;
>  	}
>  
> +	/* advance and clear state from the previous iteration */
>  	trace_iomap_iter(iter, ops, _RET_IP_);
> -	ret = iomap_iter_advance(iter);
> -	if (ret <= 0)
> -		return ret;
> +	if (iter->iomap.length) {
> +		ret = iomap_iter_advance(iter, iter->processed);
> +		iomap_iter_reset_iomap(iter);
> +		if (ret <= 0)
> +			return ret;
> +	}
>  
>  	ret = ops->iomap_begin(iter->inode, iter->pos, iter->len, iter->flags,
>  			       &iter->iomap, &iter->srcmap);
> -- 
> 2.48.1
> 
>