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

[Chuck Lever]

> 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;


--
Chuck Lever