Re: [PATCH 2/4] xfs: convert delayed extents to unwritten when zeroing post eof blocks

[Darrick J. Wong]

On Mon, Mar 11, 2024 at 08:22:53PM +0800, Zhang Yi wrote:
> From: Zhang Yi <yi.zhang@xxxxxxxxxx>
> 
> Current clone operation could be non-atomic if the destination of a file
> is beyond EOF, user could get a file with corrupted (zeroed) data on
> crash.
> 
> The problem is about to pre-alloctions. If you write some data into a
> file [A, B) (the position letters are increased one by one), and xfs
> could pre-allocate some blocks, then we get a delayed extent [A, D).
> Then the writeback path allocate blocks and convert this delayed extent
> [A, C) since lack of enough contiguous physical blocks, so the extent
> [C, D) is still delayed. After that, both the in-memory and the on-disk
> file size are B. If we clone file range into [E, F) from another file,
> xfs_reflink_zero_posteof() would call iomap_zero_range() to zero out the
> range [B, E) beyond EOF and flush range. Since [C, D) is still a delayed
> extent, it will be zeroed and the file's in-memory && on-disk size will
> be updated to D after flushing and before doing the clone operation.
> This is wrong, because user can user can see the size change and read
> zeros in the middle of the clone operation.
> 
> We need to keep the in-memory and on-disk size before the clone
> operation starts, so instead of writing zeroes through the page cache
> for delayed ranges beyond EOF, we convert these ranges to unwritten and
> invalidating any cached data over that range beyond EOF.
> 
> Suggested-by: Dave Chinner <david@xxxxxxxxxxxxx>
> Signed-off-by: Zhang Yi <yi.zhang@xxxxxxxxxx>
> ---
>  fs/xfs/xfs_iomap.c | 29 +++++++++++++++++++++++++++++
>  1 file changed, 29 insertions(+)
> 
> diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
> index ccf83e72d8ca..2b2aace25355 100644
> --- a/fs/xfs/xfs_iomap.c
> +++ b/fs/xfs/xfs_iomap.c
> @@ -957,6 +957,7 @@ xfs_buffered_write_iomap_begin(
>  	struct xfs_mount	*mp = ip->i_mount;
>  	xfs_fileoff_t		offset_fsb = XFS_B_TO_FSBT(mp, offset);
>  	xfs_fileoff_t		end_fsb = xfs_iomap_end_fsb(mp, offset, count);
> +	xfs_fileoff_t		eof_fsb = XFS_B_TO_FSBT(mp, XFS_ISIZE(ip));
>  	struct xfs_bmbt_irec	imap, cmap;
>  	struct xfs_iext_cursor	icur, ccur;
>  	xfs_fsblock_t		prealloc_blocks = 0;
> @@ -1035,6 +1036,22 @@ xfs_buffered_write_iomap_begin(
>  	}
>  
>  	if (imap.br_startoff <= offset_fsb) {
> +		/*
> +		 * For zeroing out delayed allocation extent, we trim it if
> +		 * it's partial beyonds EOF block, or convert it to unwritten
> +		 * extent if it's all beyonds EOF block.
> +		 */
> +		if ((flags & IOMAP_ZERO) &&
> +		    isnullstartblock(imap.br_startblock)) {
> +			if (offset_fsb > eof_fsb)
> +				goto convert_delay;
> +			if (end_fsb > eof_fsb) {
> +				end_fsb = eof_fsb + 1;
> +				xfs_trim_extent(&imap, offset_fsb,
> +						end_fsb - offset_fsb);
> +			}
> +		}
> +
>  		/*
>  		 * For reflink files we may need a delalloc reservation when
>  		 * overwriting shared extents.   This includes zeroing of
> @@ -1158,6 +1175,18 @@ xfs_buffered_write_iomap_begin(
>  	xfs_iunlock(ip, lockmode);
>  	return xfs_bmbt_to_iomap(ip, iomap, &imap, flags, 0, seq);
>  
> +convert_delay:
> +	end_fsb = min(end_fsb, imap.br_startoff + imap.br_blockcount);
> +	xfs_iunlock(ip, lockmode);
> +	truncate_pagecache_range(inode, offset, XFS_FSB_TO_B(mp, end_fsb));
> +	error = xfs_iomap_write_direct(ip, offset_fsb, end_fsb - offset_fsb,
> +				       flags, &imap, &seq);

I expected this to be a direct call to xfs_bmapi_convert_delalloc.
What was the reason not for using that?

--D

> +	if (error)
> +		return error;
> +
> +	trace_xfs_iomap_alloc(ip, offset, count, XFS_DATA_FORK, &imap);
> +	return xfs_bmbt_to_iomap(ip, iomap, &imap, flags, IOMAP_F_NEW, seq);
> +
>  found_cow:
>  	seq = xfs_iomap_inode_sequence(ip, 0);
>  	if (imap.br_startoff <= offset_fsb) {
> -- 
> 2.39.2
> 
>