linux-kernel - Re: [PATCH v2] futex: Fix regression with read only mappings

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Message-ID: <4E08EAE8.6060805@linux.intel.com>
Date:	Mon, 27 Jun 2011 13:41:12 -0700
From:	Darren Hart <dvhart@...ux.intel.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Shawn Bohrer <sbohrer@...advisors.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	eric.dumazet@...il.com, david@...advisors.com,
	linux-kernel@...r.kernel.org, zvonler@...advisors.com,
	hughd@...gle.com, tglx@...utronix.de, mingo@...e.hu
Subject: Re: [PATCH v2] futex: Fix regression with read only mappings



On 06/27/2011 11:15 AM, Peter Zijlstra wrote:
> On Mon, 2011-06-27 at 11:40 -0500, Shawn Bohrer wrote:
>>>>     if (PageAnon(page_head)) {
>>>
>>> This bit needs a comment too (unless I am the only one to whom this
>> was
>>> non-obvious), maybe:
>>>
>>>               
>>>               /*
>>>                * A read-only anonymous page implies a COW on a
>>>                * MAP_PRIVATE mapping. There is no sane use-case
>>>                * for this scenario, return -EFAULT to userspace.
>>>                */
>>
>> Your comment is wrong.  Unfortunately the code is completly
>> non-obvious to me as well, and I have no idea why it is there.  This
>> little snippet came from Peter's suggested fix in:
>>
>> https://lkml.org/lkml/2011/6/6/368
>>
>> Sadly Peter's gone silent and I'm left wondering if he knew some
>> corner case that should return -EFAULT with a RO anonymous page or if
>> he _thought_ this was preventing RO MAP_PRIVATE mappings.  If it is
>> the latter then this block can be removed because it does NOT do that.
>>>> +           if (ro) {
>>>> +                   err = -EFAULT;
>>>> +                   goto out;
>>>> +           }
>>>> + 
> 
> Peter simply gets too much email.. anyway, the reason I put that there
> is that a RO Anon page will never change and is thus a little pointless
> to use for futex ops.
>

Right, and that was the logic I was trying to document. Shawn, how is my
comment above wrong? A read-only anonymous page but itself doesn't imply
a COW, but it does it does in the context of this code from my reading.


-- 
Darren Hart
Intel Open Source Technology Center
Yocto Project - Linux Kernel
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