Re: [PATCH] nfsd: reset write verifier when async COPY writeback fails

[Jeff Layton <jlayton@kernel.org>]

On Fri, 2026-05-22 at 15:44 -0400, Chuck Lever wrote:
> From: Chuck Lever <chuck.lever@oracle.com>
> 
> Async COPY captures nn->writeverf at request time and reports it to
> the client via CB_OFFLOAD after the worker kthread completes. When
> the post-copy vfs_fsync_range() or filemap_check_wb_err() in
> _nfsd_copy_file_range() reports an error, the worker correctly
> leaves NFSD4_COPY_F_COMMITTED clear so that CB_OFFLOAD encodes
> wr_stable_how as NFS_UNSTABLE, but the server's write verifier is
> not rotated.
> 
> A client that receives NFS_UNSTABLE in CB_OFFLOAD follows up with
> COMMIT to make the copied data durable. With the verifier
> unchanged, COMMIT returns the same value the client just received
> via CB_OFFLOAD, and the client concludes the copy is durable --
> silently dropping the data whose writeback in fact failed. This
> violates the UNSTABLE+COMMIT durability contract (RFC 7862 section
> 15.1, RFC 8881 section 18.32) and matches the bug just fixed in
> nfsd_vfs_write() and nfsd_commit().
> 
> Rotate nn->writeverf at the writeback-failure site. The async COPY
> worker has no svc_rqst, so commit_reset_write_verifier() is not
> available here; calling nfsd_reset_write_verifier() directly
> mirrors the trace-less reset already used by
> nfsd_file_check_write_error() for the same purpose.
> 
> Fixes: eac0b17a77fb ("NFSD add vfs_fsync after async copy is done")
> Cc: stable@vger.kernel.org
> Assisted-by: kres:claude-opus-4-7
> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
> ---
>  fs/nfsd/nfs4proc.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/fs/nfsd/nfs4proc.c b/fs/nfsd/nfs4proc.c
> index 8561540ab2db..26f09e2ac4f3 100644
> --- a/fs/nfsd/nfs4proc.c
> +++ b/fs/nfsd/nfs4proc.c
> @@ -1972,6 +1972,8 @@ static ssize_t _nfsd_copy_file_range(struct nfsd4_copy *copy,
>  			status = filemap_check_wb_err(dst->f_mapping, since);
>  		if (!status)
>  			set_bit(NFSD4_COPY_F_COMMITTED, &copy->cp_flags);
> +		else
> +			nfsd_reset_write_verifier(copy->cp_nn);
>  	}
>  	return bytes_copied;
>  }

commit_reset_write_verifier() filters out EAGAIN and ENOSPC. Do we need
the same filter here?

-- 
Jeff Layton <jlayton@kernel.org>