linux-kernel - Re: [mm 4.15-rc8] Random oopses under memory pressure.

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Message-ID: <CA+55aFwWCeFrhN+WJDD8u9nqBzmvknXk428Q0dVwwXAvwhg_-w@mail.gmail.com>
Date:   Fri, 19 Jan 2018 10:42:18 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     "Kirill A. Shutemov" <kirill@...temov.name>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Tony Luck <tony.luck@...el.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Michal Hocko <mhocko@...nel.org>,
        "hillf.zj" <hillf.zj@...baba-inc.com>,
        Hugh Dickins <hughd@...gle.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Rik van Riel <riel@...hat.com>,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        "the arch/x86 maintainers" <x86@...nel.org>
Subject: Re: [mm 4.15-rc8] Random oopses under memory pressure.

On Fri, Jan 19, 2018 at 4:55 AM, Matthew Wilcox <willy@...radead.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