linux-kernel - Re: [PATCH v10 06/25] mm: make pte_unmap

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Message-ID: <20180501130452.GA118722@rodete-laptop-imager.corp.google.com>
Date:   Tue, 1 May 2018 22:04:52 +0900
From:   Minchan Kim <minchan@...nel.org>
To:     Laurent Dufour <ldufour@...ux.vnet.ibm.com>
Cc:     akpm@...ux-foundation.org, mhocko@...nel.org, peterz@...radead.org,
        kirill@...temov.name, ak@...ux.intel.com, 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>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        kemi.wang@...el.com, sergey.senozhatsky.work@...il.com,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        David Rientjes <rientjes@...gle.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Ganesh Mahendran <opensource.ganesh@...il.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,
        paulmck@...ux.vnet.ibm.com, Tim Chen <tim.c.chen@...ux.intel.com>,
        linuxppc-dev@...ts.ozlabs.org, x86@...nel.org
Subject: Re: [PATCH v10 06/25] mm: make pte_unmap_same compatible with SPF

On Mon, Apr 30, 2018 at 04:07:30PM +0200, Laurent Dufour wrote:
> On 23/04/2018 08:31, Minchan Kim wrote:
> > On Tue, Apr 17, 2018 at 04:33:12PM +0200, Laurent Dufour wrote:
> >> pte_unmap_same() is making the assumption that the page table are still
> >> around because the mmap_sem is held.
> >> This is no more the case when running a speculative page fault and
> >> additional check must be made to ensure that the final page table are still
> >> there.
> >>
> >> This is now done by calling pte_spinlock() to check for the VMA's
> >> consistency while locking for the page tables.
> >>
> >> This is requiring passing a vm_fault structure to pte_unmap_same() which is
> >> containing all the needed parameters.
> >>
> >> As pte_spinlock() may fail in the case of a speculative page fault, if the
> >> VMA has been touched in our back, pte_unmap_same() should now return 3
> >> cases :
> >> 	1. pte are the same (0)
> >> 	2. pte are different (VM_FAULT_PTNOTSAME)
> >> 	3. a VMA's changes has been detected (VM_FAULT_RETRY)
> >>
> >> The case 2 is handled by the introduction of a new VM_FAULT flag named
> >> VM_FAULT_PTNOTSAME which is then trapped in cow_user_page().
> > 
> > I don't see such logic in this patch.
> > Maybe you introduces it later? If so, please comment on it.
> > Or just return 0 in case of 2 without introducing VM_FAULT_PTNOTSAME.
> 
> Late in the series, pte_spinlock() will check for the VMA's changes and may
> return 1. This will be then required to handle the 3 cases presented above.
> 
> I can move this handling later in the series, but I wondering if this will make
> it more easier to read.

Just nit:
During reviewing this patch, I was just curious you introduced new thing
here but I couldn't find any site where use it. It makes review hard. :(
That's why I said to you that please commet on it if you will use new thing
late in this series.
If you think as-is is better for review, it would be better to mention it
explicitly.

Thanks.