linux-kernel - Re: [PATCH v2 14/20] mm: Provide speculative fault infrastructure

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Message-Id: <3da42409-6313-d710-5293-47426fac4808@linux.vnet.ibm.com>
Date:   Wed, 30 Aug 2017 09:18:59 +0530
From:   Anshuman Khandual <khandual@...ux.vnet.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Laurent Dufour <ldufour@...ux.vnet.ibm.com>
Cc:     "Kirill A. Shutemov" <kirill@...temov.name>,
        paulmck@...ux.vnet.ibm.com, akpm@...ux-foundation.org,
        ak@...ux.intel.com, mhocko@...nel.org, dave@...olabs.net,
        jack@...e.cz, Matthew Wilcox <willy@...radead.org>,
        benh@...nel.crashing.org, mpe@...erman.id.au, paulus@...ba.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, hpa@...or.com,
        Will Deacon <will.deacon@....com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        haren@...ux.vnet.ibm.com, khandual@...ux.vnet.ibm.com,
        npiggin@...il.com, bsingharora@...il.com,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        linuxppc-dev@...ts.ozlabs.org, x86@...nel.org
Subject: Re: [PATCH v2 14/20] mm: Provide speculative fault infrastructure

On 08/29/2017 05:34 PM, Peter Zijlstra wrote:
> On Tue, Aug 29, 2017 at 09:59:30AM +0200, Laurent Dufour wrote:
>> On 27/08/2017 02:18, Kirill A. Shutemov wrote:
>>>> +
>>>> +	if (unlikely(!vma->anon_vma))
>>>> +		goto unlock;
>>> It deserves a comment.
>> You're right I'll add it in the next version.
>> For the record, the root cause is that __anon_vma_prepare() requires the
>> mmap_sem to be held because vm_next and vm_prev must be safe.
> But should that test not be:
> 
> 	if (unlikely(vma_is_anonymous(vma) && !vma->anon_vma))
> 		goto unlock;

This makes more sense. We are backing off from speculative path
because struct anon_vma has not been created for this anonymous
vma and we cannot do that without holding mmap_sem. This should
have nothing to do with vma->vm_ops availability.