Re: [PATCH 4/4] mm: prepare page_referenced() and page_idle to new THP refcounting

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Thu, Nov 05, 2015 at 07:03:24PM +0300, Vladimir Davydov wrote:
> On Tue, Nov 03, 2015 at 05:26:15PM +0200, Kirill A. Shutemov wrote:
> ...
> > @@ -812,60 +812,104 @@ static int page_referenced_one(struct page *page, struct vm_area_struct *vma,
> >  	spinlock_t *ptl;
> >  	int referenced = 0;
> >  	struct page_referenced_arg *pra = arg;
> > +	pgd_t *pgd;
> > +	pud_t *pud;
> > +	pmd_t *pmd;
> > +	pte_t *pte;
> >  
> > -	if (unlikely(PageTransHuge(page))) {
> > -		pmd_t *pmd;
> > -
> > -		/*
> > -		 * rmap might return false positives; we must filter
> > -		 * these out using page_check_address_pmd().
> > -		 */
> > -		pmd = page_check_address_pmd(page, mm, address, &ptl);
> > -		if (!pmd)
> > +	if (unlikely(PageHuge(page))) {
> > +		/* when pud is not present, pte will be NULL */
> > +		pte = huge_pte_offset(mm, address);
> > +		if (!pte)
> >  			return SWAP_AGAIN;
> >  
> > -		if (vma->vm_flags & VM_LOCKED) {
> > +		ptl = huge_pte_lockptr(page_hstate(page), mm, pte);
> > +		goto check_pte;
> > +	}
> > +
> > +	pgd = pgd_offset(mm, address);
> > +	if (!pgd_present(*pgd))
> > +		return SWAP_AGAIN;
> > +	pud = pud_offset(pgd, address);
> > +	if (!pud_present(*pud))
> > +		return SWAP_AGAIN;
> > +	pmd = pmd_offset(pud, address);
> > +
> > +	if (pmd_trans_huge(*pmd)) {
> > +		int ret = SWAP_AGAIN;
> > +
> > +		ptl = pmd_lock(mm, pmd);
> > +		if (!pmd_present(*pmd))
> > +			goto unlock_pmd;
> > +		if (unlikely(!pmd_trans_huge(*pmd))) {
> >  			spin_unlock(ptl);
> > +			goto map_pte;
> > +		}
> > +
> > +		if (pmd_page(*pmd) != page)
> > +			goto unlock_pmd;
> > +
> > +		if (vma->vm_flags & VM_LOCKED) {
> >  			pra->vm_flags |= VM_LOCKED;
> > -			return SWAP_FAIL; /* To break the loop */
> > +			ret = SWAP_FAIL; /* To break the loop */
> > +			goto unlock_pmd;
> >  		}
> >  
> >  		if (pmdp_clear_flush_young_notify(vma, address, pmd))
> >  			referenced++;
> > -
> >  		spin_unlock(ptl);
> > +		goto found;
> > +unlock_pmd:
> > +		spin_unlock(ptl);
> > +		return ret;
> >  	} else {
> > -		pte_t *pte;
> > -
> > -		/*
> > -		 * rmap might return false positives; we must filter
> > -		 * these out using page_check_address().
> > -		 */
> > -		pte = page_check_address(page, mm, address, &ptl, 0);
> > -		if (!pte)
> > +		pmd_t pmde = *pmd;
> > +		barrier();
> 
> This is supposed to be
> 
> 		pmd_t pmde = READ_ONCE(*pmd);
> 
> Right?

See e37c69827063. If I read this correctly, barrier() is less overhead for
some archs.

> 
> I don't understand why we need a barrier here. Why can't we just do
> 
> 	} else if (!pmd_present(*pmd))
> 		reutnr SWAP_AGAIN;
> 
> ?

See f72e7dcdd252 too.

> > +		if (!pmd_present(pmde) || pmd_trans_huge(pmde))
> >  			return SWAP_AGAIN;
> > +	}
-- 
 Kirill A. Shutemov

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