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

[Trond Myklebust]

On Mon, 2021-02-08 at 19:48 +0000, Chuck Lever wrote:
> 
> 
> > On Feb 8, 2021, at 2:34 PM, Trond Myklebust <
> > trondmy@xxxxxxxxxxxxxxx> wrote:
> > 
> > On Tue, 2021-01-19 at 20:25 -0500, Sasha Levin wrote:
> > > From: Chuck Lever <chuck.lever@xxxxxxxxxx>
> > > 
> > > [ 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@xxxxxxxx>
> > > BugLink: https://bugzilla.kernel.org/show_bug.cgi?id=209439
> > > Signed-off-by: Chuck Lever <chuck.lever@xxxxxxxxxx>
> > > Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
> > > ---
> > >  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);
> 
> Thanks for your review!
> 
> > I'm having trouble with this line. This looks like it is trying to
> > push
> > a slab page into kernel_sendpage().
> 
> The head and tail kvec's in rq_res are not kmalloc'd, they are
> backed by pages in rqstp->rq_pages[].
> 
> 
> > What guarantees that the nfsd
> > thread won't call kfree() before the socket layer is done
> > transmitting
> > the page?
> 
> If I understand correctly what Neil told us last week, the page
> reference count on those pages is set up so that one of
> svc_xprt_release() or the network layer does the final put_page(),
> in a safe fashion.
> 
> Before da1661b93bf4 ("SUNRPC: Teach server to use xprt_sock_sendmsg
> for socket sends"), the original svc_send_common() code did this:
> 
> -       /* send head */
> -       if (slen == xdr->head[0].iov_len)
> -               flags = 0;
> -       len = kernel_sendpage(sock, headpage, headoffset,
> -                                 xdr->head[0].iov_len, flags);
> -       if (len != xdr->head[0].iov_len)
> -               goto out;
> -       slen -= xdr->head[0].iov_len;
> -       if (slen == 0)
> -               goto out;
> 
> 
> 

OK, so then only the argument kvec can be allocated on the slab (thanks
to  svc_deferred_recv)? Is that correct?

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