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Subject: Re: [PATCH] arm64: Expose TASK_SIZE to userspace via auxv
To: Richard Weinberger <richard.weinberger@gmail.com>
References: <20160816183231.21179-1-cov@codeaurora.org>
 <20160817103006.GB20762@e104818-lin.cambridge.arm.com>
 <CED1E802-65C5-48B9-8165-46684BE81164@codeaurora.org>
 <CAFLxGvx2YB9D1rECjDrALYWoEtGk=4WL9SKXEGgpxH1c_4Sfxg@mail.gmail.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        James Hogan <james.hogan@imgtec.com>,
        "linux-arm-kernel@lists.infradead.org" 
        <linux-arm-kernel@lists.infradead.org>,
        LKML <linux-kernel@vger.kernel.org>, Jon Masters <jcm@redhat.com>,
        Jeremy Linton <jeremy.linton@arm.com>
From: Christopher Covington <cov@codeaurora.org>
Message-ID: <857a7f1d-ecdb-21e1-6043-1bb6dce3aad6@codeaurora.org>
Date: Fri, 9 Sep 2016 10:14:53 -0400
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Hi Richard,

On 08/18/2016 08:17 AM, Richard Weinberger wrote:
> On Wed, Aug 17, 2016 at 1:12 PM, Christopher Covington
> <cov@codeaurora.org> wrote:
>>
>>
>> On August 17, 2016 6:30:06 AM EDT, Catalin Marinas <catalin.marinas@arm.com> wrote:
>>> On Tue, Aug 16, 2016 at 02:32:29PM -0400, Christopher Covington wrote:
>>>> Some userspace applications need to know the maximum virtual address
>>> they can
>>>> use (TASK_SIZE).
>>>
>>> Just curious, what are the cases needing TASK_SIZE in user space?
>>
>> Checkpoint/Restore In Userspace and the Mozilla Javascript Engine https://bugzilla.mozilla.org/show_bug.cgi?id=1143022 are the specific cases I've run into. I've heard LuaJIT might have a similar situation. In general I think making allocations from the top down is a shortcut for finding a large unused region of memory.
> 
> I think this makes sense for all archs.
> At lest UserModeLinux on x86 also needs to know bottom and top
> addresses of the usable
> address space.
> Currently it figures by scanning and catching SIGSEGV.

For the bottom, can you use /proc/sys/vm/mmap_min_addr?

Cov

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