Re: [PATCH 5/7] xfs: fill dirty folios on zero range of unwritten mappings

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

On Thu, Jun 05, 2025 at 01:33:55PM -0400, Brian Foster wrote:
> Use the iomap folio batch mechanism to select folios to zero on zero
> range of unwritten mappings. Trim the resulting mapping if the batch
> is filled (unlikely for current use cases) to distinguish between a
> range to skip and one that requires another iteration due to a full
> batch.
> 
> Signed-off-by: Brian Foster <bfoster@redhat.com>
> ---
>  fs/xfs/xfs_iomap.c | 23 +++++++++++++++++++++++
>  1 file changed, 23 insertions(+)
> 
> diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
> index b5cf5bc6308d..63054f7ead0e 100644
> --- a/fs/xfs/xfs_iomap.c
> +++ b/fs/xfs/xfs_iomap.c
> @@ -1691,6 +1691,8 @@ xfs_buffered_write_iomap_begin(
>  	struct iomap		*iomap,
>  	struct iomap		*srcmap)
>  {
> +	struct iomap_iter	*iter = container_of(iomap, struct iomap_iter,
> +						     iomap);

/me has been wondering more and more if we should just pass the iter
directly to iomap_begin rather than make them play these container_of
tricks... OTOH I think the whole point of this:

	ret = ops->iomap_begin(iter->inode, iter->pos, iter->len, iter->flags,
			       &iter->iomap, &iter->srcmap);

is to "avoid" allowing the iomap users to mess with the internals of the
iomap iter...

>  	struct xfs_inode	*ip = XFS_I(inode);
>  	struct xfs_mount	*mp = ip->i_mount;
>  	xfs_fileoff_t		offset_fsb = XFS_B_TO_FSBT(mp, offset);
> @@ -1762,6 +1764,7 @@ xfs_buffered_write_iomap_begin(
>  	 */
>  	if (flags & IOMAP_ZERO) {
>  		xfs_fileoff_t eof_fsb = XFS_B_TO_FSB(mp, XFS_ISIZE(ip));
> +		u64 end;
>  
>  		if (isnullstartblock(imap.br_startblock) &&
>  		    offset_fsb >= eof_fsb)
> @@ -1769,6 +1772,26 @@ xfs_buffered_write_iomap_begin(
>  		if (offset_fsb < eof_fsb && end_fsb > eof_fsb)
>  			end_fsb = eof_fsb;
>  
> +		/*
> +		 * Look up dirty folios for unwritten mappings within EOF.
> +		 * Providing this bypasses the flush iomap uses to trigger
> +		 * extent conversion when unwritten mappings have dirty
> +		 * pagecache in need of zeroing.
> +		 *
> +		 * Trim the mapping to the end pos of the lookup, which in turn
> +		 * was trimmed to the end of the batch if it became full before
> +		 * the end of the mapping.
> +		 */
> +		if (imap.br_state == XFS_EXT_UNWRITTEN &&
> +		    offset_fsb < eof_fsb) {
> +			loff_t len = min(count,
> +					 XFS_FSB_TO_B(mp, imap.br_blockcount));
> +
> +			end = iomap_fill_dirty_folios(iter, offset, len);

...though I wonder, does this need to happen in
xfs_buffered_write_iomap_begin?  Is it required to hold the ILOCK while
we go look for folios in the mapping?  Or could this become a part of
iomap_write_begin?

--D

> +			end_fsb = min_t(xfs_fileoff_t, end_fsb,
> +					XFS_B_TO_FSB(mp, end));
> +		}
> +
>  		xfs_trim_extent(&imap, offset_fsb, end_fsb - offset_fsb);
>  	}
>  
> -- 
> 2.49.0
> 
>