linux-kernel - Re: POWER: Unexpected fault when writing to brk-allocated memory

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Message-ID: <c5586546-1e7e-0f0f-a8b3-680fadb38dcf@redhat.com>
Date:   Tue, 7 Nov 2017 12:26:12 +0100
From:   Florian Weimer <fweimer@...hat.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>,
        Nicholas Piggin <npiggin@...il.com>
Cc:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        linuxppc-dev@...ts.ozlabs.org, linux-mm <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...capital.net>,
        Dave Hansen <dave.hansen@...el.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-arch@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: POWER: Unexpected fault when writing to brk-allocated memory

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>

Thanks,
Florian