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

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Date:   Fri, 5 Aug 2022 11:52:18 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Yang Shi <shy828301@...il.com>,
        Nadav Amit <nadav.amit@...il.com>,
        David Hildenbrand <david@...hat.com>,
        Konstantin Khlebnikov <khlebnikov@...nvz.org>
Subject: Re: [PATCH] mm/smaps: Don't access young/dirty bit if pte unpresent

On Thu, Aug 04, 2022 at 03:40:50PM -0700, Andrew Morton wrote:
> On Wed,  3 Aug 2022 18:03:29 -0400 Peter Xu <peterx@...hat.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?

Will do in the future.

For this specific one I'll add some more details:

  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.

Meanwhile I think I overlooked the pmd handling which seems to have the
same issue but done in another commit..  Will repost soon.

Thanks,

> 
> 
> > --- 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
> 

-- 
Peter Xu