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Date:   Thu, 4 Aug 2022 15:40:50 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Peter Xu <peterx@redhat.com>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Yang Shi <shy828301@gmail.com>,
        Nadav Amit <nadav.amit@gmail.com>,
        David Hildenbrand <david@redhat.com>,
        Konstantin Khlebnikov <khlebnikov@openvz.org>
Subject: Re: [PATCH] mm/smaps: Don't access young/dirty bit if pte unpresent
Message-Id: <20220804154050.e6ab9f8c38e2445acc5b57b1@linux-foundation.org>
In-Reply-To: <20220803220329.46299-1-peterx@redhat.com>
References: <20220803220329.46299-1-peterx@redhat.com>
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On Wed,  3 Aug 2022 18:03:29 -0400 Peter Xu <peterx@redhat.com> wrote:

> These bits should only be valid when the ptes are present.  Introducing two
> booleans for it and set it to false when !pte_present().

Please (always) describe the user visible runtime effects of the bug
which is being fixed?


> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -527,10 +527,12 @@ static void smaps_pte_entry(pte_t *pte, unsigned long addr,
>  	struct vm_area_struct *vma = walk->vma;
>  	bool locked = !!(vma->vm_flags & VM_LOCKED);
>  	struct page *page = NULL;
> -	bool migration = false;
> +	bool migration = false, young = false, dirty = false;
>  
>  	if (pte_present(*pte)) {
>  		page = vm_normal_page(vma, addr, *pte);
> +		young = pte_young(*pte);
> +		dirty = pte_dirty(*pte);
>  	} else if (is_swap_pte(*pte)) {
>  		swp_entry_t swpent = pte_to_swp_entry(*pte);
>  
> @@ -560,8 +562,7 @@ static void smaps_pte_entry(pte_t *pte, unsigned long addr,
>  	if (!page)
>  		return;
>  
> -	smaps_account(mss, page, false, pte_young(*pte), pte_dirty(*pte),
> -		      locked, migration);
> +	smaps_account(mss, page, false, young, dirty, locked, migration);
>  }
>  
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> -- 
> 2.32.0