Re: [PATCH v6 15/22] xfs: add fs-verity support

[Eric Biggers <ebiggers@kernel.org>]

On Tue, Mar 31, 2026 at 11:28:16PM +0200, Andrey Albershteyn wrote:
> +	/*
> +	 * If this is a block full of hashes of zeroed blocks, don't bother
> +	 * storing the block. We can synthesize them later.
> +	 *
> +	 * However, do this only in case Merkle tree block == fs block size.
> +	 * Iomap synthesizes these blocks based on holes in the merkle tree. We
> +	 * won't be able to tell if something need to be synthesizes for the
> +	 * range in the fs block. For example, for 4k filesystem block
> +	 *
> +	 *	[ 1k | zero hashes | zero hashes | 1k ]
> +	 *
> +	 * Iomap won't know about these empty blocks.
> +	 */
> +	for (i = 0, p = buf; i < size; i += digest_size, p += digest_size)
> +		if (memcmp(p, zero_digest, digest_size))
> +			break;
> +	if (i == size && size == ip->i_mount->m_sb.sb_blocksize)
> +		return 0;

Might be too subtle, but this could be done more efficiently with just
two calls to memcmp():

	if (size == ip->i_mount->m_sb.sb_blocksize &&
	    /* first digest is zero_digest */
	    memcmp(buf, zero_digest, digest_size) == 0 &&
	    /* every digest is same as previous, thus all are zero_digest */
	    memcmp(buf + digest_size, buf, size - digest_size) == 0)
		return 0;

- Eric