Re: [PATCH RFC 20/29] xfs: disable preallocations for fsverity Merkle tree writes

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

On Mon, Jul 28, 2025 at 10:30:24PM +0200, Andrey Albershteyn wrote:
> While writing Merkle tree, file is read-only and there's no further
> writes except Merkle tree building. The file is truncated beforehand to
> remove any preallocated extents.
> 
> The Merkle tree is the only data XFS will write. As we don't want XFS to
> truncate file after we done writing, let's also skip truncation on
> fsverity files. Therefore, we also need to disable preallocations while
> writing merkle tree as we don't want any unused extents past the tree.
> 
> Signed-off-by: Andrey Albershteyn <aalbersh@kernel.org>
> ---
>  fs/xfs/xfs_iomap.c | 13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
> index ff05e6b1b0bb..00ec1a738b39 100644
> --- a/fs/xfs/xfs_iomap.c
> +++ b/fs/xfs/xfs_iomap.c
> @@ -32,6 +32,8 @@
>  #include "xfs_rtbitmap.h"
>  #include "xfs_icache.h"
>  #include "xfs_zone_alloc.h"
> +#include "xfs_fsverity.h"
> +#include <linux/fsverity.h>

What do these includes pull in for the iflags tests below?

>  #define XFS_ALLOC_ALIGN(mp, off) \
>  	(((off) >> mp->m_allocsize_log) << mp->m_allocsize_log)
> @@ -1849,7 +1851,9 @@ xfs_buffered_write_iomap_begin(
>  		 * Determine the initial size of the preallocation.
>  		 * We clean up any extra preallocation when the file is closed.
>  		 */
> -		if (xfs_has_allocsize(mp))
> +		if (xfs_iflags_test(ip, XFS_VERITY_CONSTRUCTION))
> +			prealloc_blocks = 0;
> +		else if (xfs_has_allocsize(mp))
>  			prealloc_blocks = mp->m_allocsize_blocks;
>  		else if (allocfork == XFS_DATA_FORK)
>  			prealloc_blocks = xfs_iomap_prealloc_size(ip, allocfork,
> @@ -1976,6 +1980,13 @@ xfs_buffered_write_iomap_end(
>  	if (flags & IOMAP_FAULT)
>  		return 0;
>  
> +	/*
> +	 * While writing Merkle tree to disk we would not have any other
> +	 * delayed allocations
> +	 */
> +	if (xfs_iflags_test(XFS_I(inode), XFS_VERITY_CONSTRUCTION))
> +		return 0;

I assume XFS_VERITY_CONSTRUCTION doesn't get set until after we've
locked the inode, flushed the dirty pagecache, and truncated the file to
EOF?  In which case I guess this is ok -- we're never going to have new
delalloc reservations, and verity data can't be straddling the EOF
folio, no matter how large it is.  Right?

--D

> +
>  	/* Nothing to do if we've written the entire delalloc extent */
>  	start_byte = iomap_last_written_block(inode, offset, written);
>  	end_byte = round_up(offset + length, i_blocksize(inode));
> 
> -- 
> 2.50.0
> 
>