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

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Message-ID: <20230630091442.172ec67f@kernel.org>
Date: Fri, 30 Jun 2023 09:14:42 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Nathan Chancellor <nathan@...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 Fri, 30 Jun 2023 09:10:43 -0700 Nathan Chancellor wrote:
> > 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? :)

Ah, neat. Misleading indentation is another possible angle, I reckon,
but not sure if that's enabled/possible to enable for the entire kernel
either :( We test-build with W=1 in networking, FWIW, so W=1 would be
enough for us.