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>
Reviewed-by: Andrey Albershteyn <aalbersh@xxxxxxxxxx>
---
fs/xfs/xfs_fsverity.c | 29 ++++++++++++++++++++++++++++-
1 file changed, 28 insertions(+), 1 deletion(-)
diff --git a/fs/xfs/xfs_fsverity.c b/fs/xfs/xfs_fsverity.c
index f6c650e81cb26..e2de99272b7da 100644
--- a/fs/xfs/xfs_fsverity.c
+++ b/fs/xfs/xfs_fsverity.c
@@ -824,6 +824,20 @@ xfs_fsverity_read_merkle(
/* Read the block in from disk and try to store it in the cache. */
xfs_fsverity_init_merkle_args(ip, &name, block->pos, &args);
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;
@@ -875,10 +889,23 @@ xfs_fsverity_write_merkle(
.valuelen = size,
};
const char *p;
+ unsigned int i;
+
+ /*
+ * 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, except for the first byte, which we
- * need to avoid ENODATA errors in the merkle read path.
+ * need to avoid confusion with elided blocks.
*/
p = buf + size - 1;
while (p >= (const char *)buf && *p == 0)
