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

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Date:   Tue, 29 Aug 2017 15:45:50 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     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 Tue, Aug 29, 2017 at 03:18:25PM +0200, Laurent Dufour wrote:
> On 29/08/2017 14:04, 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;
> > 
> > Because !anon vmas will never have ->anon_vma set and you don't want to
> > exclude those.
> 
> Yes in the case we later allow non anonymous vmas to be handled.
> Currently only anonymous vmas are supported so the check is good enough,
> isn't it ?

That wasn't at all clear from reading the code. This makes it clear
->anon_vma is only ever looked at for anonymous.

And like Kirill says, we _really_ should start allowing some (if not
all) vm_ops. Large file based mappings aren't particularly rare.

I'm not sure we want to introduce a white-list or just bite the bullet
and audit all ->fault() implementations. But either works and isn't
terribly difficult, auditing all is more work though.