linux-kernel - Re: [PATCH v8 03/23] mm: Check against orig_pte for finish

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Message-ID: <878rsbawd2.fsf@nvdebian.thelocal>
Date:   Tue, 12 Apr 2022 12:05:41 +1000
From:   Alistair Popple <apopple@...dia.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        David Hildenbrand <david@...hat.com>,
        Hugh Dickins <hughd@...gle.com>,
        Jerome Glisse <jglisse@...hat.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Axel Rasmussen <axelrasmussen@...gle.com>
Subject: Re: [PATCH v8 03/23] mm: Check against orig_pte for finish_fault()

Looks ok to me now and should work for all architectures.

Reviewed-by: Alistair Popple <apopple@...dia.com>

Peter Xu <peterx@...hat.com> writes:

> We used to check against none pte in finish_fault(), with the assumption
> that the orig_pte is always none pte.
>
> 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.
>
> Let's change the "pte_none" check into detecting changes since we fetched
> orig_pte.  One trivial thing to take care of here is, when pmd==NULL for
> the pgtable we may not initialize orig_pte at all in handle_pte_fault().
>
> By default orig_pte will be all zeros however the problem is not all
> architectures are using all-zeros for a none pte.  pte_clear() will be the
> right thing to use here so that we'll always have a valid orig_pte value
> for the whole handle_pte_fault() call.
>
> Signed-off-by: Peter Xu <peterx@...hat.com>
> ---
>  mm/memory.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 3f396241a7db..b1af996b09ca 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4241,7 +4241,7 @@ 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;
> @@ -4709,6 +4709,13 @@ static vm_fault_t handle_pte_fault(struct vm_fault *vmf)
>  		 * concurrent faults and from rmap lookups.
>  		 */
>  		vmf->pte = NULL;
> +		/*
> +		 * Always initialize orig_pte.  This matches with below
> +		 * code to have orig_pte to be the none pte if pte==NULL.
> +		 * This makes the rest code to be always safe to reference
> +		 * it, e.g. in finish_fault() we'll detect pte changes.
> +		 */
> +		pte_clear(vmf->vma->vm_mm, vmf->address, &vmf->orig_pte);
>  	} else {
>  		/*
>  		 * If a huge pmd materialized under us just retry later.  Use