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Date: Mon, 31 Jan 2011 22:36:16 -0800
Message-ID: <AANLkTinjoq1bdDLSbTaSi4eJZ73jbdAsvJOyZy93xYQT@mail.gmail.com>
Subject: Re: [PATCH] mlock: operate on any regions with protection != PROT_NONE
From: Michel Lespinasse <walken@google.com>
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Sender: owner-linux-mm@kvack.org
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>, Tao Ma <tm@tao.ma>, KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>, Rik van Riel <riel@redhat.com>, Hugh Dickins <hughd@google.com>
List-ID: <linux-mm.kvack.org>

On Mon, Jan 31, 2011 at 9:59 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Tue, Feb 1, 2011 at 11:03 AM, Michel Lespinasse <walken@google.com> wrote:
>>
>> I am proposing to let mlock ignore vma protection in all cases except
>> PROT_NONE.
>
> What's so special about PROT_NONE? If you want to mlock something
> without actually being able to then fault that in, why not?
>
> IOW, why wouldn't it be right to just make FOLL_FORCE be unconditional in mlock?

I agree this would be the most logical thing to do, but I'm afraid
people would complain about it as it'd be yet another behavior change.

I don't have the entire context here, but PROT_NONE regions are
actually common in modern userspace. It seems that for most shared
libraries, ld.so creates 4 vmas, one of them being just under 2MB and
with PROT_NONE protection. From my shell, if I do cat /proc/$$/maps, I
see:
[...]
7f41ebdb5000-7f41ebdc5000 r-xp 00000000 fc:03 669498
  /usr/lib/zsh/4.3.10/zsh/computil.so
7f41ebdc5000-7f41ebfc4000 ---p 00010000 fc:03 669498
  /usr/lib/zsh/4.3.10/zsh/computil.so
7f41ebfc4000-7f41ebfc5000 r--p 0000f000 fc:03 669498
  /usr/lib/zsh/4.3.10/zsh/computil.so
7f41ebfc5000-7f41ebfc6000 rw-p 00010000 fc:03 669498
  /usr/lib/zsh/4.3.10/zsh/computil.so
7f41ebfc6000-7f41ebfce000 r-xp 00000000 fc:03 669508
  /usr/lib/zsh/4.3.10/zsh/parameter.so
7f41ebfce000-7f41ec1ce000 ---p 00008000 fc:03 669508
  /usr/lib/zsh/4.3.10/zsh/parameter.so
7f41ec1ce000-7f41ec1cf000 r--p 00008000 fc:03 669508
  /usr/lib/zsh/4.3.10/zsh/parameter.so
7f41ec1cf000-7f41ec1d0000 rw-p 00009000 fc:03 669508
  /usr/lib/zsh/4.3.10/zsh/parameter.so
[...]

I don't know why userspace does that, but these regions currently
never get any page into RSS, even when processes call mlockall(). I am
told that we need to preserve this property.

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.

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