Re: [PATCH 6/6] iomap: rework iomap_write_begin() to return folio offset and length

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

On Wed, Apr 30, 2025 at 03:01:12PM -0400, Brian Foster wrote:
> iomap_write_begin() returns a folio based on current pos and
> remaining length in the iter, and each caller then trims the
> pos/length to the given folio. Clean this up a bit and let
> iomap_write_begin() return the trimmed range along with the folio.
> 
> Signed-off-by: Brian Foster <bfoster@redhat.com>
> ---
>  fs/iomap/buffered-io.c | 26 +++++++++++++++-----------
>  1 file changed, 15 insertions(+), 11 deletions(-)
> 
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index d3b30ebad9ea..2fde268c39fc 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -793,15 +793,22 @@ static int iomap_write_begin_inline(const struct iomap_iter *iter,
>  	return iomap_read_inline_data(iter, folio);
>  }
>  
> -static int iomap_write_begin(struct iomap_iter *iter, size_t len,
> -		struct folio **foliop)
> +/*
> + * Grab and prepare a folio for write based on iter state. Returns the folio,
> + * offset, and length. Callers can optionally pass a max length *plen,
> + * otherwise init to zero.
> + */
> +static int iomap_write_begin(struct iomap_iter *iter, struct folio **foliop,
> +		size_t *poffset, u64 *plen)

Hmm, is this offset and length supposed to be bytes within the folio?
I find it a little odd that plen would be a u64 then, unless we're
preparing for folios that huge?  Or is that just to avoid integer
truncation issues?

--D

>  {
>  	const struct iomap_folio_ops *folio_ops = iter->iomap.folio_ops;
>  	const struct iomap *srcmap = iomap_iter_srcmap(iter);
>  	loff_t pos = iter->pos;
> +	u64 len = min_t(u64, SIZE_MAX, iomap_length(iter));
>  	struct folio *folio;
>  	int status = 0;
>  
> +	len = *plen > 0 ? min_t(u64, len, *plen) : len;
>  	BUG_ON(pos + len > iter->iomap.offset + iter->iomap.length);
>  	if (srcmap != &iter->iomap)
>  		BUG_ON(pos + len > srcmap->offset + srcmap->length);
> @@ -833,8 +840,7 @@ static int iomap_write_begin(struct iomap_iter *iter, size_t len,
>  		}
>  	}
>  
> -	if (pos + len > folio_pos(folio) + folio_size(folio))
> -		len = folio_pos(folio) + folio_size(folio) - pos;
> +	pos = iomap_trim_folio_range(iter, folio, poffset, &len);
>  
>  	if (srcmap->type == IOMAP_INLINE)
>  		status = iomap_write_begin_inline(iter, folio);
> @@ -847,6 +853,7 @@ static int iomap_write_begin(struct iomap_iter *iter, size_t len,
>  		goto out_unlock;
>  
>  	*foliop = folio;
> +	*plen = len;
>  	return 0;
>  
>  out_unlock:
> @@ -967,7 +974,7 @@ static int iomap_write_iter(struct iomap_iter *iter, struct iov_iter *i)
>  			break;
>  		}
>  
> -		status = iomap_write_begin(iter, bytes, &folio);
> +		status = iomap_write_begin(iter, &folio, &offset, &bytes);
>  		if (unlikely(status)) {
>  			iomap_write_failed(iter->inode, iter->pos, bytes);
>  			break;
> @@ -975,7 +982,7 @@ static int iomap_write_iter(struct iomap_iter *iter, struct iov_iter *i)
>  		if (iter->iomap.flags & IOMAP_F_STALE)
>  			break;
>  
> -		pos = iomap_trim_folio_range(iter, folio, &offset, &bytes);
> +		pos = iter->pos;
>  
>  		if (mapping_writably_mapped(mapping))
>  			flush_dcache_folio(folio);
> @@ -1295,14 +1302,12 @@ static int iomap_unshare_iter(struct iomap_iter *iter)
>  		bool ret;
>  
>  		bytes = min_t(u64, SIZE_MAX, bytes);
> -		status = iomap_write_begin(iter, bytes, &folio);
> +		status = iomap_write_begin(iter, &folio, &offset, &bytes);
>  		if (unlikely(status))
>  			return status;
>  		if (iomap->flags & IOMAP_F_STALE)
>  			break;
>  
> -		iomap_trim_folio_range(iter, folio, &offset, &bytes);
> -
>  		ret = iomap_write_end(iter, bytes, bytes, folio);
>  		__iomap_put_folio(iter, bytes, folio);
>  		if (WARN_ON_ONCE(!ret))
> @@ -1367,7 +1372,7 @@ static int iomap_zero_iter(struct iomap_iter *iter, bool *did_zero)
>  		bool ret;
>  
>  		bytes = min_t(u64, SIZE_MAX, bytes);
> -		status = iomap_write_begin(iter, bytes, &folio);
> +		status = iomap_write_begin(iter, &folio, &offset, &bytes);
>  		if (status)
>  			return status;
>  		if (iter->iomap.flags & IOMAP_F_STALE)
> @@ -1376,7 +1381,6 @@ static int iomap_zero_iter(struct iomap_iter *iter, bool *did_zero)
>  		/* warn about zeroing folios beyond eof that won't write back */
>  		WARN_ON_ONCE(folio_pos(folio) > iter->inode->i_size);
>  
> -		iomap_trim_folio_range(iter, folio, &offset, &bytes);
>  		folio_zero_range(folio, offset, bytes);
>  		folio_mark_accessed(folio);
>  
> -- 
> 2.49.0
> 
>