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
