From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1755511AbbIYXS7 (ORCPT <rfc822;w@1wt.eu>);
	Fri, 25 Sep 2015 19:18:59 -0400
Received: from www.sr71.net ([198.145.64.142]:43142 "EHLO blackbird.sr71.net"
	rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
	id S1753559AbbIYXS6 (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
	Fri, 25 Sep 2015 19:18:58 -0400
Subject: Re: [PATCH 10/26] x86, pkeys: notify userspace about protection key
 faults
To: Ingo Molnar <mingo@kernel.org>
References: <20150916174903.E112E464@viggo.jf.intel.com>
 <20150916174906.51062FBC@viggo.jf.intel.com>
 <20150924092320.GA26876@gmail.com> <20150924093026.GA29699@gmail.com>
 <560435B4.1010603@sr71.net> <20150925071119.GB15753@gmail.com>
Cc: x86@kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Thomas Gleixner <tglx@linutronix.de>
From: Dave Hansen <dave@sr71.net>
Message-ID: <5605D660.8000009@sr71.net>
Date: Fri, 25 Sep 2015 16:18:56 -0700
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101
 Thunderbird/38.2.0
MIME-Version: 1.0
In-Reply-To: <20150925071119.GB15753@gmail.com>
Content-Type: text/plain; charset=windows-1252
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On 09/25/2015 12:11 AM, Ingo Molnar wrote:
>>> > > Btw., how does pkey support interact with hugepages?
>> > 
>> > Surprisingly little.  I've made sure that everything works with huge pages and 
>> > that the (huge) PTEs and VMAs get set up correctly, but I'm not sure I had to 
>> > touch the huge page code at all.  I have test code to ensure that it works the 
>> > same as with small pages, but everything worked pretty naturally.
> Yeah, so the reason I'm asking about expectations is that this code:
> 
> +       follow_ret = follow_pte(tsk->mm, address, &ptep, &ptl);
> +       if (!follow_ret) {
> +               /*
> +                * On a successful follow, make sure to
> +                * drop the lock.
> +                */
> +               pte = *ptep;
> +               pte_unmap_unlock(ptep, ptl);
> +               ret = pte_pkey(pte);
> 
> is visibly hugepage-unsafe: if a vma is hugepage mapped, there are no ptes, only 
> pmds - and the protection key index lives in the pmd. We don't seem to recover 
> that information properly.

You got me on this one.  I assumed that follow_pte() handled huge pages.
 It does not.

But, the code still worked.  Since follow_pte() fails for all huge
pages, it just falls back to pulling the protection key out of the VMA,
which _does_ work for huge pages.

I've actually removed the PTE walking and I just now use the VMA
directly.  I don't see a ton of additional value from walking the page
tables when we can get what we need from the VMA.