Re: [PATCH 39/40] xfs: don't bother storing merkle tree blocks for zeroed data blocks

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On 2024-03-17 09:33:34, Darrick J. Wong wrote:
> From: Darrick J. Wong <djwong@xxxxxxxxxx>
> 
> Now that fsverity tells our merkle tree io functions about what a hash
> of a data block full of zeroes looks like, we can use this information
> to avoid writing out merkle tree blocks for sparse regions of the file.
> For verified gold master images this can save quite a bit of overhead.
> 
> Signed-off-by: Darrick J. Wong <djwong@xxxxxxxxxx>

Looks good to me:
Reviewed-by: Andrey Albershteyn <aalbersh@xxxxxxxxxx>

> ---
>  fs/xfs/xfs_verity.c |   37 ++++++++++++++++++++++++++++++++++---
>  1 file changed, 34 insertions(+), 3 deletions(-)
> 
> 
> diff --git a/fs/xfs/xfs_verity.c b/fs/xfs/xfs_verity.c
> index abd95bc1ba6e..ba96e7049f61 100644
> --- a/fs/xfs/xfs_verity.c
> +++ b/fs/xfs/xfs_verity.c
> @@ -619,6 +619,20 @@ xfs_verity_read_merkle(
>  	xfs_verity_merkle_key_to_disk(&name, block->offset);
>  
>  	error = xfs_attr_get(&args);
> +	if (error == -ENOATTR) {
> +		u8		*p;
> +		unsigned int	i;
> +
> +		/*
> +		 * No attribute found.  Synthesize a buffer full of the zero
> +		 * digests on the assumption that we elided them at write time.
> +		 */
> +		for (i = 0, p = new_mk->data;
> +		     i < block->size;
> +		     i += req->digest_size, p += req->digest_size)
> +			memcpy(p, req->zero_digest, req->digest_size);
> +		error = 0;
> +	}
>  	if (error)
>  		goto out_new_mk;
>  
> @@ -676,12 +690,29 @@ xfs_verity_write_merkle(
>  		.value			= (void *)buf,
>  		.valuelen		= size,
>  	};
> -	const char			*p = buf + size - 1;
> +	const char			*p;
> +	unsigned int			i;
>  
> -	/* Don't store trailing zeroes. */
> +	/*
> +	 * If this is a block full of hashes of zeroed blocks, don't bother
> +	 * storing the block.  We can synthesize them later.
> +	 */
> +	for (i = 0, p = buf;
> +	     i < size;
> +	     i += req->digest_size, p += req->digest_size)
> +		if (memcmp(p, req->zero_digest, req->digest_size))
> +			break;
> +	if (i == size)
> +		return 0;
> +
> +	/*
> +	 * Don't store trailing zeroes.  Store at least one byte so that the
> +	 * block cannot be mistaken for an elided one.
> +	 */
> +	p = buf + size - 1;
>  	while (p >= (const char *)buf && *p == 0)
>  		p--;
> -	args.valuelen = p - (const char *)buf + 1;
> +	args.valuelen = max(1, p - (const char *)buf + 1);
>  
>  	xfs_verity_merkle_key_to_disk(&name, pos);
>  	return xfs_attr_set(&args);
> 

-- 
- Andrey





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