linux-kernel - Re: [PATCH v3 3/3] hugetlbfs: add swap entry check in follow_hugetlb

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Message-ID: <1365544834-c6v2jpoo-mutt-n-horiguchi@ah.jp.nec.com>
Date:	Tue, 09 Apr 2013 18:00:34 -0400
From:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>, Hugh Dickins <hughd@...gle.com>,
	Rik van Riel <riel@...hat.com>,
	Konstantin Khlebnikov <khlebnikov@...nvz.org>,
	Michal Hocko <mhocko@...e.cz>,
	HATAYAMA Daisuke <d.hatayama@...fujitsu.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 3/3] hugetlbfs: add swap entry check in
 follow_hugetlb_page()

On Mon, Apr 08, 2013 at 04:57:44PM -0400, KOSAKI Motohiro wrote:
> > -               if (absent ||
> > +               /*
> > +                * 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.
> > +                */
> > +               if (absent || is_swap_pte(huge_ptep_get(pte)) ||
> >                     ((flags & FOLL_WRITE) && !pte_write(huge_ptep_get(pte)))) {
> >                         int ret;
> 
> Your comment describe what the code is. However we want the comment describe
> why. In migration case, calling hugetlb_fault() is natural. but in
> hwpoison case, it is
> needed more explanation.

We should call hugetlb_fault() when we encounter any kind of swap
type entry. It's consistent with handling of normal pages.

> Why can't we call is_hugetlb_hwpoisoned() directly?

We can use it, but I like to make code simple.

I rewrite the comment here, how about this?

-		if (absent ||
+		/*
+		 * We need call hugetlb_fault for both hugepages under migration
+		 * (in which case hugetlb_fault waits for the migration,) and
+		 * hwpoisoned hugepages (in which case we need to prevent the
+		 * caller from accessing to them.) In order to do this, we use
+		 * here is_swap_pte instead of is_hugetlb_entry_migration and
+		 * is_hugetlb_entry_hwpoisoned. This is because it simply covers
+		 * both cases, and because we can't follow correct pages directly
+		 * from any kind of swap entries.
+		 */
+		if (absent || is_swap_pte(huge_ptep_get(pte)) ||
 		    ((flags & FOLL_WRITE) && !pte_write(huge_ptep_get(pte)))) {
 			int ret;

Thanks,
Naoya
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