Re: [PATCH v2 14/22] xfs: disable preallocations for fsverity Merkle tree writes

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

On Mon, Jan 12, 2026 at 03:51:30PM +0100, 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 | 28 +++++++++++++++++++++++++---
>  1 file changed, 25 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
> index 04f39ea158..61aab5617f 100644
> --- a/fs/xfs/xfs_iomap.c
> +++ b/fs/xfs/xfs_iomap.c
> @@ -1938,7 +1938,9 @@
>  		 * 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,
> @@ -2065,6 +2067,13 @@
>  	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;
> +
>  	/* 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));
> @@ -2109,6 +2118,7 @@
>  	bool			shared = false;
>  	unsigned int		lockmode = XFS_ILOCK_SHARED;
>  	u64			seq;
> +	int			iomap_flags;
>  
>  	ASSERT(!(flags & (IOMAP_WRITE | IOMAP_ZERO)));
>  
> @@ -2128,8 +2138,20 @@
>  	if (error)
>  		return error;
>  	trace_xfs_iomap_found(ip, offset, length, XFS_DATA_FORK, &imap);
> -	return xfs_bmbt_to_iomap(ip, iomap, &imap, flags,
> -				 shared ? IOMAP_F_SHARED : 0, seq);
> +	iomap_flags = shared ? IOMAP_F_SHARED : 0;
> +
> +	/*
> +	 * We can not use fsverity_active() here. fsverity_active() checks for
> +	 * verity info attached to inode. This info is based on data from a
> +	 * verity descriptor. But to read verity descriptor we need to go
> +	 * through read iomap path (this function). So, when descriptor is read
> +	 * we will not set IOMAP_F_BEYOND_EOF and descriptor page will be empty
> +	 * (post EOF hole).
> +	 */
> +	if ((offset >= XFS_FSVERITY_REGION_START) && IS_VERITY(inode))

(Ok so XFS_FSVERITY_REGION_START is in units of bytes.  Maybe cast it to
loff_t to make that clearer?)

#define XFS_FSVERITY_REGION_START ((loff_t)1ULL << 53)

> +		iomap_flags |= IOMAP_F_BEYOND_EOF;

Ok, this answers my question.  IOMAP_F_BEYOND_EOF is indeed only
intended to be set for mappings who are completely beyond EOF.  IOWs,
for the fsverity metadata but not the regular file data.

Looks fine to me,
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>

--D


> +
> +	return xfs_bmbt_to_iomap(ip, iomap, &imap, flags, iomap_flags, seq);
>  }
>  
>  const struct iomap_ops xfs_read_iomap_ops = {
> 
> -- 
> - Andrey
> 
>