netdev - Re: [PATCH RFC] SUNRPC: Use zero-copy to perform socket send operations

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Message-ID: <20201109175551.GC11144@fieldses.org>
Date:   Mon, 9 Nov 2020 12:55:51 -0500
From:   bfields@...ldses.org (J. Bruce Fields)
To:     Chuck Lever <chuck.lever@...cle.com>
Cc:     Trond Myklebust <trondmy@...merspace.com>,
        Linux NFS Mailing List <linux-nfs@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH RFC] SUNRPC: Use zero-copy to perform socket send
 operations

On Mon, Nov 09, 2020 at 12:36:15PM -0500, Chuck Lever wrote:
> > On Nov 9, 2020, at 12:32 PM, Trond Myklebust <trondmy@...merspace.com> wrote:
> > On Mon, 2020-11-09 at 12:12 -0500, Chuck Lever wrote:
> >> I assume you mean the client side only. Those issues aren't a factor
> >> on the server. Not setting SOCK_ZEROCOPY here should be enough to
> >> prevent the use of zero-copy on the client.
> >> 
> >> However, the client loses the benefits of sending a page at a time.
> >> Is there a desire to remedy that somehow?
> > 
> > What about splice reads on the server side?
> 
> On the server, this path formerly used kernel_sendpages(), which I
> assumed is similar to the sendmsg zero-copy mechanism. How does
> kernel_sendpages() mitigate against page instability?

We turn it off when gss integrity or privacy services is used, to
prevent spurious checksum failures (grep for RQ_SPLICE_OK).

But maybe that's not the only problematic case, I don't know.

--b.