Re: [PATCH AUTOSEL 5.10 03/45] SUNRPC: Handle TCP socket sends with kernel_sendpage() again

[Trond Myklebust <trondmy@hammerspace.com>]

On Tue, 2021-01-19 at 20:25 -0500, Sasha Levin wrote:
> From: Chuck Lever <chuck.lever@oracle.com>
> 
> [ Upstream commit 4a85a6a3320b4a622315d2e0ea91a1d2b013bce4 ]
> 
> Daire Byrne reports a ~50% aggregrate throughput regression on his
> Linux NFS server after commit da1661b93bf4 ("SUNRPC: Teach server to
> use xprt_sock_sendmsg for socket sends"), which replaced
> kernel_send_page() calls in NFSD's socket send path with calls to
> sock_sendmsg() using iov_iter.
> 
> Investigation showed that tcp_sendmsg() was not using zero-copy to
> send the xdr_buf's bvec pages, but instead was relying on memcpy.
> This means copying every byte of a large NFS READ payload.
> 
> It looks like TLS sockets do indeed support a ->sendpage method,
> so it's really not necessary to use xprt_sock_sendmsg() to support
> TLS fully on the server. A mechanical reversion of da1661b93bf4 is
> not possible at this point, but we can re-implement the server's
> TCP socket sendmsg path using kernel_sendpage().
> 
> Reported-by: Daire Byrne <daire@dneg.com>
> BugLink: https://bugzilla.kernel.org/show_bug.cgi?id=209439
> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
> Signed-off-by: Sasha Levin <sashal@kernel.org>
> ---
>  net/sunrpc/svcsock.c | 86
> +++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 85 insertions(+), 1 deletion(-)
> 
> diff --git a/net/sunrpc/svcsock.c b/net/sunrpc/svcsock.c
> index c2752e2b9ce34..4404c491eb388 100644
> --- a/net/sunrpc/svcsock.c
> +++ b/net/sunrpc/svcsock.c
> @@ -1062,6 +1062,90 @@ static int svc_tcp_recvfrom(struct svc_rqst
> *rqstp)
>         return 0;       /* record not complete */
>  }
>  
> +static int svc_tcp_send_kvec(struct socket *sock, const struct kvec
> *vec,
> +                             int flags)
> +{
> +       return kernel_sendpage(sock, virt_to_page(vec->iov_base),
> +                              offset_in_page(vec->iov_base),
> +                              vec->iov_len, flags);

I'm having trouble with this line. This looks like it is trying to push
a slab page into kernel_sendpage(). What guarantees that the nfsd
thread won't call kfree() before the socket layer is done transmitting
the page?



-- 
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trond.myklebust@hammerspace.com