linux-kernel - Re: [PATCH v2] mm/smaps: Don't access young/dirty bit if pte unpresent

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Date:   Mon, 08 Aug 2022 13:23:46 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Yang Shi <shy828301@...il.com>,
        David Hildenbrand <david@...hat.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Konstantin Khlebnikov <khlebnikov@...nvz.org>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH v2] mm/smaps: Don't access young/dirty bit if pte unpresent

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

> These bits should only be valid when the ptes are present.  Introducing two
> booleans for it and set it to false when !pte_present() for both pte and
> pmd accountings.
>
> The bug is found during code reading and no real world issue reported, but
> logically such an error can cause incorrect readings for either smaps or
> smaps_rollup output on quite a few fields.
>
> For example, it could cause over-estimate on values like Shared_Dirty,
> Private_Dirty, Referenced.  Or it could also cause under-estimate on values
> like LazyFree, Shared_Clean, Private_Clean.
>
> Cc: Konstantin Khlebnikov <khlebnikov@...nvz.org>
> Cc: Huang Ying <ying.huang@...el.com>
> Fixes: b1d4d9e0cbd0 ("proc/smaps: carefully handle migration entries")
> Fixes: c94b6923fa0a ("/proc/PID/smaps: Add PMD migration entry parsing")
> Reviewed-by: Vlastimil Babka <vbabka@...e.cz>
> Reviewed-by: David Hildenbrand <david@...hat.com>
> Reviewed-by: Yang Shi <shy828301@...il.com>
> Signed-off-by: Peter Xu <peterx@...hat.com>

LGTM, Thanks!

Reviewed-by: "Huang, Ying" <ying.huang@...el.com>

> ---
>  fs/proc/task_mmu.c | 14 ++++++++------
>  1 file changed, 8 insertions(+), 6 deletions(-)
>
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 9913f3be9fd2..d56c65f98d00 100644
> --- 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
> @@ -572,11 +573,13 @@ static void smaps_pmd_entry(pmd_t *pmd, 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 (pmd_present(*pmd)) {
>  		/* FOLL_DUMP will return -EFAULT on huge zero page */
>  		page = follow_trans_huge_pmd(vma, addr, pmd, FOLL_DUMP);
> +		young = pmd_young(*pmd);
> +		dirty = pmd_dirty(*pmd);
>  	} else if (unlikely(thp_migration_supported() && is_swap_pmd(*pmd))) {
>  		swp_entry_t entry = pmd_to_swp_entry(*pmd);
>  
> @@ -596,8 +599,7 @@ static void smaps_pmd_entry(pmd_t *pmd, unsigned long addr,
>  	else
>  		mss->file_thp += HPAGE_PMD_SIZE;
>  
> -	smaps_account(mss, page, true, pmd_young(*pmd), pmd_dirty(*pmd),
> -		      locked, migration);
> +	smaps_account(mss, page, true, young, dirty, locked, migration);
>  }
>  #else
>  static void smaps_pmd_entry(pmd_t *pmd, unsigned long addr,