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

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Message-ID: <6260997.DYpkEd5BTb@nvdebian>
Date:   Thu, 16 Dec 2021 16:01:47 +1100
From:   Alistair Popple <apopple@...dia.com>
To:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        Peter Xu <peterx@...hat.com>
CC:     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>, <peterx@...hat.com>,
        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 Monday, 15 November 2021 6:55:02 PM AEDT Peter Xu wrote:
> We used to check against none pte and in those cases orig_pte should always be
> none pte anyway.

Is that always true? From what I can see in handle_pte_fault() orig_pte only
gets initialised in the !pmd_none() case so might not be pte_none.

> 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;
>