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

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Date: Fri, 7 Jul 2023 13:45:55 -0700
From: Nick Desaulniers <ndesaulniers@...gle.com>
To: Nathan Chancellor <nathan@...nel.org>
Cc: Jakub Kicinski <kuba@...nel.org>, 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, Jun 30, 2023 at 12:28 PM Nathan Chancellor <nathan@...nel.org> wrote:
>
> On Fri, Jun 30, 2023 at 09:14:42AM -0700, Jakub Kicinski wrote:
> > 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? :)
>
> Well, that was a pipe dream :/ In ARCH=arm multi_v7_defconfig alone,
> there are 289 unique instances of the warning (although a good number
> have multiple instances per line, so it is not quite as bad as it seems,
> but still bad):
>
> $ rg -- -Wcomma arm-multi_v7_defconfig.log | sort | uniq -c | wc -l
> 289
>
> https://gist.github.com/nathanchance/907867e0a7adffc877fd39fd08853801

It's definitely interesting to take a look at some of these cases.
Some are pretty funny IMO.

>
> Probably not a good sign of the signal to noise ratio, I looked through
> a good handful and all the cases I saw were not interesting... Perhaps
> the warning could be tuned further to become useful for the kernel but
> in its current form, it is definitely a no-go :/
>
> > Ah, neat. Misleading indentation is another possible angle, I reckon,
> > but not sure if that's enabled/possible to enable for the entire kernel
>
> Yeah, I was surprised there was no warning for misleading indentation...
> it is a part of -Wall for both clang and GCC, so it is on for the
> kernel, it just appears not to trigger in this case.
>
> > either :( We test-build with W=1 in networking, FWIW, so W=1 would be
> > enough for us.
>
> Unfortunately, even in its current form, it is way too noisy for W=1, as
> the qualifier for W=1 is "do not occur too often". Probably could be
> placed under W=2 but it still has the problem of wading through every
> instance and it is basically a no-op because nobody tests with W=2.
>
> Cheers,
> Nathan
>


-- 
Thanks,
~Nick Desaulniers