linux-kernel - Re: [PATCH 4/4] mm: prepare page_referenced() and page

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Date:	Thu, 5 Nov 2015 15:53:54 +0300
From:	Vladimir Davydov <vdavydov@...tuozzo.com>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
CC:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Hugh Dickins <hughd@...gle.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	Minchan Kim <minchan@...nel.org>,
	<linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>
Subject: Re: [PATCH 4/4] mm: prepare page_referenced() and page_idle to new
 THP refcounting

On Thu, Nov 05, 2015 at 02:36:06PM +0200, Kirill A. Shutemov wrote:
> 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.

Out of curiosity, is it OK that __page_check_address can call
pte_unmap_unlock on pte returned by huge_pte_offset, which isn't really
pte, but pmd or pud?

> 
> 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);

spin_unlock(ptl);
if (pte)
	pte_unmap(pte);

would look neater IMO. Other than that, I think it's OK. At least, it
looks better and less error-prone than duplicating such a huge chunk of
code IMO.

Thanks,
Vladimir

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