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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri, 19 Jan 2018 10:42:18 -0800
Message-ID: <CA+55aFwWCeFrhN+WJDD8u9nqBzmvknXk428Q0dVwwXAvwhg_-w@mail.gmail.com>
Subject: Re: [mm 4.15-rc8] Random oopses under memory pressure.
To:     Matthew Wilcox <willy@infradead.org>
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        Peter Zijlstra <peterz@infradead.org>,
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On Fri, Jan 19, 2018 at 4:55 AM, Matthew Wilcox <willy@infradead.org> wrote:
>
> So really we should be casting 'b' and 'a' to uintptr_t to be fully
> compliant with the spec.

That's an unnecessary technicality.

Any compiler that doesn't get pointer inequality testing right is not
worth even worrying about. We wouldn't want to use such a compiler,
because it's intentionally generating garbage just to f*ck with us.
Why would you go along with that?

So the only real issue is that pointer subtraction case.

I actually asked (long long ago) for an optinal compiler warning for
"pointer subtraction with non-power-of-2 sizes". Not because of it
being undefined, but simply because it's expensive. The
divide->multiply thing doesn't always work, and a real divide is
really quite expensive on many architectures.

We *should* be careful about it. I guess sparse could be made to warn,
but I'm afraid that we have so many of these things that a warning
isn't reasonable.

And most of the time, a pointer difference really is inside the same
array. The operation doesn't tend to make sense otherwise.

           Linus