Re: [PATCH net-next v3 10/18] nvme/host: Use sendmsg(MSG_SPLICE_PAGES) rather then sendpage

[Nathan Chancellor <nathan@kernel.org>]

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

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