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    SubjectRe: [PATCH AUTOSEL 5.10 03/45] SUNRPC: Handle TCP socket sends with kernel_sendpage() again
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    > On Feb 8, 2021, at 2:34 PM, Trond Myklebust <trondmy@hammerspace.com> wrote:
    >
    > 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);

    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



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