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 03:07:26PM +0300, Vladimir Davydov wrote:
> @@ -849,30 +836,23 @@ static int page_referenced_one(struct page *page, struct vm_area_struct *vma,
>  		if (pmd_page(*pmd) != page)
>  			goto unlock_pmd;
>  
> -		if (vma->vm_flags & VM_LOCKED) {
> -			pra->vm_flags |= VM_LOCKED;
> -			ret = SWAP_FAIL; /* To break the loop */
> -			goto unlock_pmd;
> -		}
> -
> -		if (pmdp_clear_flush_young_notify(vma, address, pmd))
> -			referenced++;
> -		spin_unlock(ptl);
> +		pte = (pte_t *)pmd;

pmd_t and pte_t are not always compatible. We shouldn't pretend they are.
And we shouldn't use pte_unmap_unlock() to unlock pmd table.

What about interface like this (I'm not sure about helper's name):

void page_check_address_transhuge(struct page *page, struct mm_struct *mm,
                                   unsigned long address,
                                   pmd_t **pmdp, pte_t **ptep,
				   spinlock_t **ptlp);

page_check_address_transhuge(page, mm, address, &pmd, &pte, &ptl);
if (pmd) {
	/* handle pmd... */
} else if (pte) {
	/* handle pte... */
} else {
	return SWAP_AGAIN;
}

/* common stuff */

if (pmd)
	spin_unlock(ptl);
else 
	pte_unmap_unlock(pte, ptl);

/* ... */

The helper shouldn't set pmd if the page is tracked to pte.

-- 
 Kirill A. Shutemov

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