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Date:   Fri, 19 Jan 2018 04:55:03 -0800
From:   Matthew Wilcox <willy@infradead.org>
To:     "Kirill A. Shutemov" <kirill@shutemov.name>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        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>,
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Subject: Re: [mm 4.15-rc8] Random oopses under memory pressure.
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On Fri, Jan 19, 2018 at 02:49:55AM +0300, Kirill A. Shutemov wrote:
> > So that's why you can't do pointer diffs between two arrays. Not
> > because you can't subtract the two pointers, but because the
> > *division* part of the C pointer diff rules leads to issues.
> 
> Thanks a lot for the explanation!
> 
> I wounder if this may be a problem in other places?
> 
> For instance, perf uses address of a mutex to determinate the lock
> ordering. See mutex_lock_double(). The mutex is embedded into struct
> perf_event_context, which is allocated with kzalloc() so I don't see how
> we can presume that alignment is consistent between them.
> 
> I don't think it's the only example in kernel. Are we just lucky?

If you're just *comparing* the addresses of two objects, GCC doesn't
care what the size of the object is.  ie there's a difference between
'if (b < a)' and 'if ((a - b) < n)'.

But yes, if you go by the strict wording of the standard:

  When two pointers are compared, the result depends on the relative
  locations in the address space of the objects pointed to. [...] In
  all other cases, the behavior is undefined

http://www.open-std.org/jtc1/sc22/WG14/www/docs/n1256.pdf

So really we should be casting 'b' and 'a' to uintptr_t to be fully
compliant with the spec.