Re: [PATCH v3] NFSD: Implement FATTR4_CLONE_BLKSIZE attribute

[Jeff Layton <jlayton@kernel.org>]

On Wed, 2025-05-07 at 10:45 -0400, cel@kernel.org wrote:
> From: Chuck Lever <chuck.lever@oracle.com>
> 
> 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.
 
> 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 <roland.mainz@nrubsig.org>
> Suggested-by: Christoph Hellwig <hch@infradead.org>
> Reviewed-by: Roland Mainz <roland.mainz@nrubsig.org>
> Cc: stable@vger.kernel.org # v6.7+
> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
> ---
>  fs/nfsd/nfs4xdr.c | 19 ++++++++++++++++++-
>  1 file changed, 18 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/nfsd/nfs4xdr.c b/fs/nfsd/nfs4xdr.c
> index e67420729ecd..9eb8e5704622 100644
> --- a/fs/nfsd/nfs4xdr.c
> +++ b/fs/nfsd/nfs4xdr.c
> @@ -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)
> @@ -3545,7 +3562,7 @@ static const nfsd4_enc_attr nfsd4_enc_fattr4_encode_ops[] = {
>  	[FATTR4_MODE_SET_MASKED]	= nfsd4_encode_fattr4__noop,
>  	[FATTR4_SUPPATTR_EXCLCREAT]	= nfsd4_encode_fattr4_suppattr_exclcreat,
>  	[FATTR4_FS_CHARSET_CAP]		= nfsd4_encode_fattr4__noop,
> -	[FATTR4_CLONE_BLKSIZE]		= nfsd4_encode_fattr4__noop,
> +	[FATTR4_CLONE_BLKSIZE]		= nfsd4_encode_fattr4_clone_blksize,
>  	[FATTR4_SPACE_FREED]		= nfsd4_encode_fattr4__noop,
>  	[FATTR4_CHANGE_ATTR_TYPE]	= nfsd4_encode_fattr4__noop,
>  


Reviewed-by: Jeff Layton <jlayton@kernel.org>