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

From: Linus Torvalds <torvalds@linux-foundation.org>
To: Matthew Wilcox <willy@infradead.org>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>,
	Peter Zijlstra <peterz@infradead.org>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Joonsoo Kim <iamjoonsoo.kim@lge.com>,
	Mel Gorman <mgorman@techsingularity.net>,
	Tony Luck <tony.luck@intel.com>, Vlastimil Babka <vbabka@suse.cz>,
	Michal Hocko <mhocko@kernel.org>,
	"hillf.zj" <hillf.zj@alibaba-inc.com>,
	Hugh Dickins <hughd@google.com>, Oleg Nesterov <oleg@redhat.com>,
	Rik van Riel <riel@redhat.com>,
	Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
	Vladimir Davydov <vdavydov.dev@gmail.com>,
	Ingo Molnar <mingo@kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-mm <linux-mm@kvack.org>,
	"the arch/x86 maintainers" <x86@kernel.org>
Subject: Re: [mm 4.15-rc8] Random oopses under memory pressure.
Date: Fri, 19 Jan 2018 10:42:18 -0800	[thread overview]
Message-ID: <CA+55aFwWCeFrhN+WJDD8u9nqBzmvknXk428Q0dVwwXAvwhg_-w@mail.gmail.com> (raw)
In-Reply-To: <20180119125503.GA2897@bombadil.infradead.org>

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