linux-kernel - Re: [RFC v5 01/11] mm: Dont assume page-table invariance during faults

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Message-Id: <e1d576f4-82f9-04f1-3387-19fb72bdb161@linux.vnet.ibm.com>
Date:   Tue, 8 Aug 2017 14:11:44 +0200
From:   Laurent Dufour <ldufour@...ux.vnet.ibm.com>
To:     Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
        paulmck@...ux.vnet.ibm.com, peterz@...radead.org,
        akpm@...ux-foundation.org, kirill@...temov.name,
        ak@...ux.intel.com, mhocko@...nel.org, dave@...olabs.net,
        jack@...e.cz, Matthew Wilcox <willy@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        haren@...ux.vnet.ibm.com, npiggin@...il.com, bsingharora@...il.com,
        Tim Chen <tim.c.chen@...ux.intel.com>
Subject: Re: [RFC v5 01/11] mm: Dont assume page-table invariance during
 faults

On 08/08/2017 11:45, Anshuman Khandual wrote:
> On 06/16/2017 11:22 PM, Laurent Dufour wrote:
>> From: Peter Zijlstra <peterz@...radead.org>
>>
>> One of the side effects of speculating on faults (without holding
>> mmap_sem) is that we can race with free_pgtables() and therefore we
>> cannot assume the page-tables will stick around.
>>
>> Remove the relyance on the pte pointer.
> 
> Looking into other parts of the series, it seemed like now we have
> sequence lock both at MM and VMA level but then after that we still
> need to take page table lock before handling page faults (in turn
> manipulating PTE which includes swap in paths as well). Is not that
> true ?

Page table locking is still required as several VMAs can reference the same
page table.