linux-kernel - Re: [PATCH v6 03/23] mm: Check against orig_pte for finish

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Date:   Thu, 16 Dec 2021 18:45:07 +1100
From:   Alistair Popple <apopple@...dia.com>
To:     Peter Xu <peterx@...hat.com>
CC:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Hugh Dickins <hughd@...gle.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Jerome Glisse <jglisse@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [PATCH v6 03/23] mm: Check against orig_pte for finish_fault()

On Thursday, 16 December 2021 6:06:54 PM AEDT Peter Xu wrote:

[...]

> I wondered how it could have worked - I thought e.g. pte_alloc_one() will
> always return a pgtable page will all zero-filled, whose allocation should
> require __GFP_ZERO anyway.  But then I quickly noticed that pte_alloc_one() is
> per-arch too..  That explains, because per-arch can re-initialize the default
> pte values.

Yes, I have wondered the same things before as well. It's all a little bit of
fun some of this stuff.

> I thought this patch can greatly simplify things but I overlooked the
> pte_none() check you mentioned.  So it seems I have no good choice but add that
> flag back.
> 
> There's another alternative is we do pte_clear() on vmf->orig_pte as the new
> way to initialize it.  I believe it should work too for s390 and xtensa.
> 
> Any preference?

I prefer the later approach (initialising to pte_clear) as it seems cleaner,
and pte_none(pte_clear()) is true for every architecture afaik.

> Thanks,
> 
> > 
> > > > 
> > > > Thanks,
> > > > 
> > > > > 
> > > > > > This change prepares us to be able to call do_fault() on !none ptes.  For
> > > > > > example, we should allow that to happen for pte marker so that we can restore
> > > > > > information out of the pte markers.
> > > > > > 
> > > > > > Signed-off-by: Peter Xu <peterx@...hat.com>
> > > > > > ---
> > > > > >  mm/memory.c | 2 +-
> > > > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > > > > 
> > > > > > diff --git a/mm/memory.c b/mm/memory.c
> > > > > > index 04662b010005..d5966d9e24c3 100644
> > > > > > --- a/mm/memory.c
> > > > > > +++ b/mm/memory.c
> > > > > > @@ -4052,7 +4052,7 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
> > > > > >  				      vmf->address, &vmf->ptl);
> > > > > >  	ret = 0;
> > > > > >  	/* Re-check under ptl */
> > > > > > -	if (likely(pte_none(*vmf->pte)))
> > > > > > +	if (likely(pte_same(*vmf->pte, vmf->orig_pte)))
> > > > > >  		do_set_pte(vmf, page, vmf->address);
> > > > > >  	else
> > > > > >  		ret = VM_FAULT_NOPAGE;
> > > > > > 
> > > > > 
> > > > > 
> > > > > 
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> > > > 
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