Message-ID: <4E08EAE8.6060805@linux.intel.com>
Date: Mon, 27 Jun 2011 13:41:12 -0700
From: Darren Hart <dvhart@linux.intel.com>
User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.17) Gecko/20110424 Lightning/1.0b2 Thunderbird/3.1.10
MIME-Version: 1.0
To: Peter Zijlstra <peterz@infradead.org>
CC: Shawn Bohrer <sbohrer@rgmadvisors.com>,
        KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
        eric.dumazet@gmail.com, david@rgmadvisors.com,
        linux-kernel@vger.kernel.org, zvonler@rgmadvisors.com,
        hughd@google.com, tglx@linutronix.de, mingo@elte.hu
Subject: Re: [PATCH v2] futex: Fix regression with read only mappings
References: <20110623194949.GA2083@BohrerMBP.rgmadvisors.com>	 <1308931186-28707-1-git-send-email-sbohrer@rgmadvisors.com>	 <4E052DC3.6000902@linux.intel.com>	 <20110627164008.GA2095@BohrerMBP.rgmadvisors.com> <1309198559.6701.108.camel@twins>
In-Reply-To: <1309198559.6701.108.camel@twins>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1893
Lines: 52


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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/