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Subject: Re: POWER: Unexpected fault when writing to brk-allocated memory
To:     "Kirill A. Shutemov" <kirill@shutemov.name>
Cc:     Nicholas Piggin <npiggin@gmail.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        linuxppc-dev@lists.ozlabs.org, linux-mm <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Dave Hansen <dave.hansen@intel.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-arch@vger.kernel.org, Ingo Molnar <mingo@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
References: <20171105231850.5e313e46@roar.ozlabs.ibm.com>
 <871slcszfl.fsf@linux.vnet.ibm.com>
 <20171106174707.19f6c495@roar.ozlabs.ibm.com>
 <24b93038-76f7-33df-d02e-facb0ce61cd2@redhat.com>
 <20171106192524.12ea3187@roar.ozlabs.ibm.com>
 <d52581f4-8ca4-5421-0862-3098031e29a8@linux.vnet.ibm.com>
 <546d4155-5b7c-6dba-b642-29c103e336bc@redhat.com>
 <20171107160705.059e0c2b@roar.ozlabs.ibm.com>
 <20171107111543.ep57evfxxbwwlhdh@node.shutemov.name>
 <c5586546-1e7e-0f0f-a8b3-680fadb38dcf@redhat.com>
 <20171107114422.bgnm5k6w2zqjoazc@node.shutemov.name>
From:   Florian Weimer <fweimer@redhat.com>
Message-ID: <7fc1641b-361c-2ee2-c510-f7c64d173bf8@redhat.com>
Date:   Tue, 7 Nov 2017 14:05:42 +0100
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On 11/07/2017 12:44 PM, Kirill A. Shutemov wrote:
> On Tue, Nov 07, 2017 at 12:26:12PM +0100, Florian Weimer wrote:
>> On 11/07/2017 12:15 PM, Kirill A. Shutemov wrote:
>>
>>>> First of all, using addr and MAP_FIXED to develop our heuristic can
>>>> never really give unchanged ABI. It's an in-band signal. brk() is a
>>>> good example that steadily keeps incrementing address, so depending
>>>> on malloc usage and address space randomization, you will get a brk()
>>>> that ends exactly at 128T, then the next one will be >
>>>> DEFAULT_MAP_WINDOW, and it will switch you to 56 bit address space.
>>>
>>> No, it won't. You will hit stack first.
>>
>> That's not actually true on POWER in some cases.  See the process maps I
>> posted here:
>>
>>    <https://marc.info/?l=linuxppc-embedded&m=150988538106263&w=2>
> 
> Hm? I see that in all three cases the [stack] is the last mapping.
> Do I miss something?

Hah, I had not noticed.  Occasionally, the order of heap and stack is 
reversed.  This happens in approximately 15% of the runs.

See the attached example.

Thanks,
Florian

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