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From:   Chuck Lever <chuck.lever@oracle.com>
To:     Trond Myklebust <trondmy@hammerspace.com>
CC:     "sashal@kernel.org" <sashal@kernel.org>,
        "stable@vger.kernel.org" <stable@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
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        "daire@dneg.com" <daire@dneg.com>,
        "netdev@vger.kernel.org" <netdev@vger.kernel.org>
Subject: Re: [PATCH AUTOSEL 5.10 03/45] SUNRPC: Handle TCP socket sends with
 kernel_sendpage() again
Thread-Topic: [PATCH AUTOSEL 5.10 03/45] SUNRPC: Handle TCP socket sends with
 kernel_sendpage() again
Thread-Index: AQHW7ss9RlJPDpfZPEapNWcB2Zz3OqpOxR4AgAAEFQA=
Date:   Mon, 8 Feb 2021 19:48:39 +0000
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> On Feb 8, 2021, at 2:34 PM, Trond Myklebust <trondmy@hammerspace.com> wro=
te:
>=20
> On Tue, 2021-01-19 at 20:25 -0500, Sasha Levin wrote:
>> From: Chuck Lever <chuck.lever@oracle.com>
>>=20
>> [ Upstream commit 4a85a6a3320b4a622315d2e0ea91a1d2b013bce4 ]
>>=20
>> 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.
>>=20
>> 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.
>>=20
>> 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().
>>=20
>> Reported-by: Daire Byrne <daire@dneg.com>
>> BugLink: https://bugzilla.kernel.org/show_bug.cgi?id=3D209439
>> 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(-)
>>=20
>> 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 */
>>  }
>> =20
>> +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 =3D=3D xdr->head[0].iov_len)
-               flags =3D 0;
-       len =3D kernel_sendpage(sock, headpage, headoffset,
-                                 xdr->head[0].iov_len, flags);
-       if (len !=3D xdr->head[0].iov_len)
-               goto out;
-       slen -=3D xdr->head[0].iov_len;
-       if (slen =3D=3D 0)
-               goto out;


--
Chuck Lever