linux-kernel - Re: [PATCH v2 2/5] mm: Clear vmf->pte after pte_unmap

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Date:   Wed, 8 Sep 2021 01:12:06 +0000
From:   Liam Howlett <liam.howlett@...cle.com>
To:     Peter Xu <peterx@...hat.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Yang Shi <shy828301@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        Jerome Glisse <jglisse@...hat.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Miaohe Lin <linmiaohe@...wei.com>,
        David Hildenbrand <david@...hat.com>,
        Alistair Popple <apopple@...dia.com>
Subject: Re: [PATCH v2 2/5] mm: Clear vmf->pte after pte_unmap_same() returns

* Peter Xu <peterx@...hat.com> [210902 16:17]:
> pte_unmap_same() will always unmap the pte pointer.  After the unmap, vmf->pte
> will not be valid any more, we should clear it.
> 
> It was safe only because no one is accessing vmf->pte after pte_unmap_same()
> returns, since the only caller of pte_unmap_same() (so far) is do_swap_page(),
> where vmf->pte will in most cases be overwritten very soon.
> 
> Directly pass in vmf into pte_unmap_same() and then we can also avoid the long
> parameter list too, which should be a nice cleanup.
> 
> Reviewed-by: Miaohe Lin <linmiaohe@...wei.com>
> Reviewed-by: David Hildenbrand <david@...hat.com>
> Signed-off-by: Peter Xu <peterx@...hat.com>
> ---
>  mm/memory.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 25fc46e87214..7b095f07c4ef 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2724,19 +2724,19 @@ EXPORT_SYMBOL_GPL(apply_to_existing_page_range);
>   * proceeding (but do_wp_page is only called after already making such a check;
>   * and do_anonymous_page can safely check later on).
>   */
> -static inline int pte_unmap_same(struct mm_struct *mm, pmd_t *pmd,
> -				pte_t *page_table, pte_t orig_pte)
> +static inline int pte_unmap_same(struct vm_fault *vmf)
>  {
>  	int same = 1;
>  #if defined(CONFIG_SMP) || defined(CONFIG_PREEMPTION)
>  	if (sizeof(pte_t) > sizeof(unsigned long)) {
> -		spinlock_t *ptl = pte_lockptr(mm, pmd);
> +		spinlock_t *ptl = pte_lockptr(vmf->vma->vm_mm, vmf->pmd);
>  		spin_lock(ptl);
> -		same = pte_same(*page_table, orig_pte);
> +		same = pte_same(*vmf->pte, vmf->orig_pte);
>  		spin_unlock(ptl);
>  	}
>  #endif
> -	pte_unmap(page_table);
> +	pte_unmap(vmf->pte);
> +	vmf->pte = NULL;
>  	return same;
>  }
>  
> @@ -3487,7 +3487,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>  	vm_fault_t ret = 0;
>  	void *shadow = NULL;
>  
> -	if (!pte_unmap_same(vma->vm_mm, vmf->pmd, vmf->pte, vmf->orig_pte))
> +	if (!pte_unmap_same(vmf))
>  		goto out;
>  
>  	entry = pte_to_swp_entry(vmf->orig_pte);
> -- 
> 2.31.1
> 

Reviewed-by: Liam R. Howlett <Liam.Howlett@...cle.com>