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Date: Wed, 21 Jun 2017 20:53:51 +0300
From: Cyrill Gorcunov <gorcunov@gmail.com>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Hugh Dickins <hughd@google.com>, Andrey Vagin <avagin@openvz.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Pavel Emelyanov <xemul@virtuozzo.com>,
        Dmitry Safonov <dsafonov@virtuozzo.com>,
        Andrew Morton <akpm@linuxfoundation.org>,
        Adrian Reber <areber@redhat.com>
Subject: Re: [criu] 1M guard page ruined restore
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On Wed, Jun 21, 2017 at 07:15:45PM +0200, Oleg Nesterov wrote:
> On 06/21, Oleg Nesterov wrote:
> >
> > On 06/21, Cyrill Gorcunov wrote:
> > >
> > > On Wed, Jun 21, 2017 at 05:57:30PM +0200, Oleg Nesterov wrote:
> > > > >
> > > > > 	p = fake_grow_down;
> > > > > 	*p-- = 'c';
> > > >
> > > > I guess this works? I mean, *p-- = 'c' should not fail...
> > >
> > > It fails.
> > 
> > Hmm. Impossible ;) could you add the additional printf's to re-check?
> 
> Yes, please...

I meant of course second store, sorry for confusion :)

> 
> > > 	p = fake_grow_down;
> > > 	*p-- = 'c';
> > 
> > once again, I can't believe this STORE can fail...
> > 
> > > 	*p = 'b';
> > 
> > Ah. I forgot about another kernel "feature" ;) not related to the recent guard
> > page changes...
> 
> On the second thought it is actually connected, and it seems we really need to
> remove this code in do_page_fault... I'll re-check tomorrow, need to run away.
> 
> > Could you test the patch below?
> > 
> > Oleg.
> > 
> > 
> > diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> > index 8ad91a0..edc5d68 100644
> > --- a/arch/x86/mm/fault.c
> > +++ b/arch/x86/mm/fault.c
> > @@ -1416,7 +1416,7 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code,
> >  		 * and pusha to work. ("enter $65535, $31" pushes
> >  		 * 32 pointers and then decrements %sp by 65535.)
> >  		 */
> > -		if (unlikely(address + 65536 + 32 * sizeof(unsigned long) < regs->sp)) {
> > +if (0)		if (unlikely(address + 65536 + 32 * sizeof(unsigned long) < regs->sp)) {
> >  			bad_area(regs, error_code, address);
> >  			return;
> >  		}

It does the trick

[root@fc2 ~]# ~/st2
start_addr 7fce7c3f8000
start_addr 7fce7c529000
test_addr 7fce7c527000
grow_down 7fce7c526000

	Cyrill