linux-kernel - Re: [PATCH] arm64: Expose TASK

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Date:   Fri, 9 Sep 2016 10:14:53 -0400
From:   Christopher Covington <cov@...eaurora.org>
To:     Richard Weinberger <richard.weinberger@...il.com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Mark Rutland <mark.rutland@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        James Hogan <james.hogan@...tec.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Jon Masters <jcm@...hat.com>,
        Jeremy Linton <jeremy.linton@....com>
Subject: Re: [PATCH] arm64: Expose TASK_SIZE to userspace via auxv

Hi Richard,

On 08/18/2016 08:17 AM, Richard Weinberger wrote:
> On Wed, Aug 17, 2016 at 1:12 PM, Christopher Covington
> <cov@...eaurora.org> wrote:
>>
>>
>> On August 17, 2016 6:30:06 AM EDT, Catalin Marinas <catalin.marinas@....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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