linux-kernel - Re: [RFC v5 04/11] mm: VMA sequence count

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Date:   Tue, 8 Aug 2017 13:04:06 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Anshuman Khandual <khandual@...ux.vnet.ibm.com>
Cc:     Laurent Dufour <ldufour@...ux.vnet.ibm.com>,
        paulmck@...ux.vnet.ibm.com, 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>,
        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 04/11] mm: VMA sequence count

On Tue, Aug 08, 2017 at 04:29:32PM +0530, Anshuman Khandual wrote:
> On 06/16/2017 11:22 PM, Laurent Dufour wrote:
> > From: Peter Zijlstra <peterz@...radead.org>
> > 
> 
> First of all, please do mention that its adding a new element into the
> vm_area_struct which will act as a sequential lock element and help
> in navigating page fault without mmap_sem lock.

You're not making sense, there is no lock, and the lines below clearly
state we're adding a sequence count.

> 
> > Wrap the VMA modifications (vma_adjust/unmap_page_range) with sequence
> > counts such that we can easily test if a VMA is changed
> 
> Yeah true.
> 
> > 
> > The unmap_page_range() one allows us to make assumptions about
> > page-tables; when we find the seqcount hasn't changed we can assume
> > page-tables are still valid.
> 
> Because unmap_page_range() is the only function which can tear it down ?
> Or is there any other reason for this assumption ?

Yep.

> > 
> > The flip side is that we cannot distinguish between a vma_adjust() and
> > the unmap_page_range() -- where with the former we could have
> > re-checked the vma bounds against the address.
> 
> Distinguished for what purpose ?

It states. If you know its a vma_adjust we could just check if we're
inside the new boundaries and continue. But since we cannot, we have to
assume the worst and bail.