NFSD: Implement FATTR4_CLONE_BLKSIZE attribute

From: Chuck Lever <[email protected]>

RFC 7862 states that if an NFS server implements a CLONE operation, it MUST also implement FATTR4_CLONE_BLKSIZE. NFSD implements CLONE, but does not implement FATTR4_CLONE_BLKSIZE.

Note that in Section 12.2, RFC 7862 claims that FATTR4_CLONE_BLKSIZE is RECOMMENDED, not REQUIRED. Likely this is because a minor version is not permitted to add a REQUIRED attribute. Confusing.

We assume this attribute reports a block size as a count of bytes, as RFC 7862 does not specify a unit.

Reported-by Roland Mainz <[email protected]>

Suggested-by Christoph Hellwig <[email protected]>

Reviewed-by Roland Mainz <[email protected]>

Cc [email protected] # v6.7+

Signed-off-by Chuck Lever <[email protected]>

---
 fs/nfsd/nfs4xdr.c | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

fs/nfsd/nfs4xdr.c +17 -0

index e67420729ecd..9eb8e5704622 100644
@@ -3391,6 +3391,23 @@ static __be32 nfsd4_encode_fattr4_suppattr_exclcreat(struct xdr_stream *xdr,
 	return nfsd4_encode_bitmap4(xdr, supp[0], supp[1], supp[2]);
 }
 
+/*
+ * Copied from generic_remap_checks/generic_remap_file_range_prep.
+ *
+ * These generic functions use the file system's s_blocksize, but
+ * individual file systems aren't required to use
+ * generic_remap_file_range_prep. Until there is a mechanism for
+ * determining a particular file system's (or file's) clone block
+ * size, this is the best NFSD can do.
+ */
+static __be32 nfsd4_encode_fattr4_clone_blksize(struct xdr_stream *xdr,
+						const struct nfsd4_fattr_args *args)
+{
+	struct inode *inode = d_inode(args->dentry);
+
+	return nfsd4_encode_uint32_t(xdr, inode->i_sb->s_blocksize);
+}
+
 #ifdef CONFIG_NFSD_V4_SECURITY_LABEL
 static __be32 nfsd4_encode_fattr4_sec_label(struct xdr_stream *xdr,
 					    const struct nfsd4_fattr_args *args)

Quoted 10 lines · On Wed, 2025-05-07 at 10:45 -0400, [email protected] wrote:

From: Chuck Lever <[email protected]>

RFC 7862 states that if an NFS server implements a CLONE operation, it MUST also implement FATTR4_CLONE_BLKSIZE. NFSD implements CLONE, but does not implement FATTR4_CLONE_BLKSIZE.

Note that in Section 12.2, RFC 7862 claims that FATTR4_CLONE_BLKSIZE is RECOMMENDED, not REQUIRED. Likely this is because a minor version is not permitted to add a REQUIRED attribute. Confusing.

Isn't CLONE itself an optional operation? It wouldn't make sense to REQUIRE this attribute on servers that don't support CLONE, so I think it makes sense that it should be optional. Anyway, I'm just being pedantic.

Quoted 11 lines · On Wed, 2025-05-07 at 10:45 -0400, [email protected] wrote:

From: Chuck Lever <[email protected]>

RFC 7862 states that if an NFS server implements a CLONE operation, it MUST also implement FATTR4_CLONE_BLKSIZE. NFSD implements CLONE, but does not implement FATTR4_CLONE_BLKSIZE.

Note that in Section 12.2, RFC 7862 claims that FATTR4_CLONE_BLKSIZE is RECOMMENDED, not REQUIRED. Likely this is because a minor version is not permitted to add a REQUIRED attribute. Confusing.

Isn't CLONE itself an optional operation? It wouldn't make sense to REQUIRE this attribute on servers that don't support CLONE, so I think it makes sense that it should be optional. Anyway, I'm just being pedantic.