linux-kernel - Re: [PATCH 2/2] hugetlbfs: add swap entry check in follow_hugetlb

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Date:	Tue, 02 Apr 2013 10:35:50 -0400
From:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>, Hugh Dickins <hughd@...gle.com>,
	Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	stable@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] hugetlbfs: add swap entry check in
 follow_hugetlb_page()

On Tue, Apr 02, 2013 at 11:24:41AM +0200, Michal Hocko wrote:
> On Fri 29-03-13 13:23:38, Naoya Horiguchi wrote:
> > Hi,
> > 
> > On Fri, Mar 29, 2013 at 02:57:30PM +0100, Michal Hocko wrote:
> > > On Thu 28-03-13 11:42:38, Naoya Horiguchi wrote:
> > > [...]
> > > > @@ -2968,7 +2968,8 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
> > > >  		 * first, for the page indexing below to work.
> > > >  		 */
> > > >  		pte = huge_pte_offset(mm, vaddr & huge_page_mask(h));
> > > > -		absent = !pte || huge_pte_none(huge_ptep_get(pte));
> > > > +		absent = !pte || huge_pte_none(huge_ptep_get(pte)) ||
> > > > +			is_swap_pte(huge_ptep_get(pte));
> > > 
> > > is_swap_pte doesn't seem right. Shouldn't you use is_hugetlb_entry_hwpoisoned
> > > instead?
> > 
> > I tested only hwpoisoned hugepage, but the same can happen for hugepages
> > under migration. So I intended to filter out all types of swap entries.
> > The local variable 'absent' seems to mean whether data on the address
> > is immediately available, so swap type entry isn't included in it.
> 
> OK, I didn't consider huge pages under migration and I was merely worried
> that is_hugetlb_entry_hwpoisoned sounds more appropriate than
> is_swap_pte.
> 
> Could you add a comment which would clarify that is_swap_pte covers both
> migration and hwpoison pages, please? Something like:
> 
> 		/*
> 		 * is_swap_pte test covers both is_hugetlb_entry_hwpoisoned
> 		 * and hugepages under migration in which case
> 		 * hugetlb_fault waits for the migration and bails out
> 		 * properly for HWPosined pages.
> 		 */
> 		 absent = !pte || huge_pte_none(huge_ptep_get(pte)) ||
> 		 	 is_swap_pte(huge_ptep_get(pte));

OK, I'll add this.

> Other than that feel free to add
> Reviewed-by: Michal Hocko <mhocko@...e.cz>

Thank you!
Naoya
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