Re: [PATCH] Teach do_mpage_readpage() about unwritten buffers

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 3 Jul 2007 11:10:19 +1000 David Chinner <dgc@sgi.com> wrote:

> Teach do_mpage_readpage() about unwritten extents so we can
> always map them in get_blocks rather than they are are holes on
> read. Allows setup_swap_extents() to use preallocated files on XFS
> filesystems for swap files without ever needing to convert them.
> 
> Signed-Off-By: Dave Chinner <dgc@sgi.com>
> 
> ---
>  fs/mpage.c                  |    5 +++--
>  fs/xfs/linux-2.6/xfs_aops.c |   13 +++----------
>  2 files changed, 6 insertions(+), 12 deletions(-)
> 
> Index: 2.6.x-xfs-new/fs/mpage.c
> ===================================================================
> --- 2.6.x-xfs-new.orig/fs/mpage.c	2007-05-29 16:17:59.000000000 +1000
> +++ 2.6.x-xfs-new/fs/mpage.c	2007-06-27 22:39:35.568852270 +1000
> @@ -207,7 +207,8 @@ do_mpage_readpage(struct bio *bio, struc
>  	 * Map blocks using the result from the previous get_blocks call first.
>  	 */
>  	nblocks = map_bh->b_size >> blkbits;
> -	if (buffer_mapped(map_bh) && block_in_file > *first_logical_block &&
> +	if (buffer_mapped(map_bh) && !buffer_unwritten(map_bh) &&
> +			block_in_file > *first_logical_block &&
>  			block_in_file < (*first_logical_block + nblocks)) {
>  		unsigned map_offset = block_in_file - *first_logical_block;
>  		unsigned last = nblocks - map_offset;
> @@ -242,7 +243,7 @@ do_mpage_readpage(struct bio *bio, struc
>  			*first_logical_block = block_in_file;
>  		}
>  
> -		if (!buffer_mapped(map_bh)) {
> +		if (!buffer_mapped(map_bh) || buffer_unwritten(map_bh)) {
>  			fully_mapped = 0;
>  			if (first_hole == blocks_per_page)
>  				first_hole = page_block;
> Index: 2.6.x-xfs-new/fs/xfs/linux-2.6/xfs_aops.c
> ===================================================================
> --- 2.6.x-xfs-new.orig/fs/xfs/linux-2.6/xfs_aops.c	2007-06-05 22:14:39.000000000 +1000
> +++ 2.6.x-xfs-new/fs/xfs/linux-2.6/xfs_aops.c	2007-06-27 22:39:29.545636749 +1000
> @@ -1340,16 +1340,9 @@ __xfs_get_blocks(
>  		return 0;
>  
>  	if (iomap.iomap_bn != IOMAP_DADDR_NULL) {
> -		/*
> -		 * For unwritten extents do not report a disk address on
> -		 * the read case (treat as if we're reading into a hole).
> -		 */
> -		if (create || !(iomap.iomap_flags & IOMAP_UNWRITTEN)) {
> -			xfs_map_buffer(bh_result, &iomap, offset,
> -				       inode->i_blkbits);
> -		}
> -		if (create && (iomap.iomap_flags & IOMAP_UNWRITTEN)) {
> -			if (direct)
> +		xfs_map_buffer(bh_result, &iomap, offset, inode->i_blkbits);
> +		if (iomap.iomap_flags & IOMAP_UNWRITTEN) {
> +			if (create && direct)
>  				bh_result->b_private = inode;
>  			set_buffer_unwritten(bh_result);
>  		}

Seems sane, although one does wonder whether it's a worthy tradeoff.  We
add additional overhead to readpage[s]() just to avoid some IO during
mkswap?

Also, I wonder what ext4 does in this situation?