linux-kernel - Re: [PATCH net-next v3 10/18] nvme/host: Use sendmsg(MSG_SPLICE

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Date:   Fri, 30 Jun 2023 09:10:43 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     David Howells <dhowells@...hat.com>,
        Aurelien Aptel <aaptel@...dia.com>, netdev@...r.kernel.org,
        Alexander Duyck <alexander.duyck@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        David Ahern <dsahern@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        Jens Axboe <axboe@...nel.dk>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Sagi Grimberg <sagi@...mberg.me>,
        Willem de Bruijn <willemb@...gle.com>,
        Keith Busch <kbusch@...nel.org>, Jens Axboe <axboe@...com>,
        Christoph Hellwig <hch@....de>,
        Chaitanya Kulkarni <kch@...dia.com>,
        linux-nvme@...ts.infradead.org, llvm@...ts.linux.dev
Subject: Re: [PATCH net-next v3 10/18] nvme/host: Use
 sendmsg(MSG_SPLICE_PAGES) rather then sendpage

On Thu, Jun 29, 2023 at 04:43:18PM -0700, Jakub Kicinski wrote:
> On Thu, 29 Jun 2023 22:34:59 +0100 David Howells wrote:
> >                 if (!sendpage_ok(page))
> > -                       msg.msg_flags &= ~MSG_SPLICE_PAGES,
> > +                       msg.msg_flags &= ~MSG_SPLICE_PAGES;
> 
> 😵️
> 
> Let me CC llvm@ in case someone's there is willing to make 
> the compiler warn about this.
> 

Turns out clang already has a warning for this, -Wcomma:

  drivers/nvme/host/tcp.c:1017:38: error: possible misuse of comma operator here [-Werror,-Wcomma]
   1017 |                         msg.msg_flags &= ~MSG_SPLICE_PAGES,
        |                                                           ^
  drivers/nvme/host/tcp.c:1017:4: note: cast expression to void to silence warning
   1017 |                         msg.msg_flags &= ~MSG_SPLICE_PAGES,
        |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        |                         (void)(                           )
  1 error generated.

Let me do some wider build testing to see if it is viable to turn this
on for the whole kernel because it seems worth it, at least in this
case. There are a lot of cases where a warning won't be emitted (see the
original upstream review for a list: https://reviews.llvm.org/D3976) but
something is better than nothing, right? :)

Cheers,
Nathan