linux-kernel - Re: [patch] hugetlb: correct page offset index for sharing pmd

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Message-ID: <20120812093110.GB18057@dhcp22.suse.cz>
Date:	Sun, 12 Aug 2012 11:31:11 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Hillf Danton <dhillf@...il.com>
Cc:	Mel Gorman <mgorman@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	David Rientjes <rientjes@...gle.com>
Subject: Re: [patch] hugetlb: correct page offset index for sharing pmd

On Sun 12-08-12 12:08:21, Hillf Danton wrote:
> On Fri, Aug 10, 2012 at 9:48 PM, Michal Hocko <mhocko@...e.cz> wrote:
> 
> > It's been compile tested because it only restores the previous code with
> > a simple and obvious bug fix.
> 
> It helps more if you elaborate on such a simple and obvious bug and
> enrich your change log accordingly?

Hmmm, to be honest I really don't care much about this change. It is just
that your previous patch (0c176d5) made the code more confusing and this
aims at fixing that.

But anyway. I will post this to Andrew unless somebody has any
objections.
---
>From d07b88a70ee1dbcc96502c48cde878931e7deb38 Mon Sep 17 00:00:00 2001
From: Michal Hocko <mhocko@...e.cz>
Date: Fri, 10 Aug 2012 15:03:07 +0200
Subject: [PATCH] hugetlb: do not use vma_hugecache_offset for
 vma_prio_tree_foreach

0c176d5 (mm: hugetlb: fix pgoff computation when unmapping page
from vma) fixed pgoff calculation but it has replaced it by
vma_hugecache_offset which is not approapriate for offsets used for
vma_prio_tree_foreach because that one expects index in page units
rather than in huge_page_shift.
Using vma_hugecache_offset is not incorrect because the pgoff will fit
into the same vmas but it is confusing so the standard PAGE_SHIFT based
index calculation is used instead.

Cc: Hillf Danton <dhillf@...il.com>
Cc: Mel Gorman <mel@....ul.ie>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc: Andrea Arcangeli <aarcange@...hat.com>
Cc: David Rientjes <rientjes@...gle.com>
Signed-off-by: Michal Hocko <mhocko@...e.cz>
---
 mm/hugetlb.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index c39e4be..a74ea31 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -2462,7 +2462,8 @@ static int unmap_ref_private(struct mm_struct *mm, struct vm_area_struct *vma,
 	 * from page cache lookup which is in HPAGE_SIZE units.
 	 */
 	address = address & huge_page_mask(h);
-	pgoff = vma_hugecache_offset(h, vma, address);
+	pgoff = ((address - vma->vm_start) >> PAGE_SHIFT) +
+			vma->vm_pgoff;
 	mapping = vma->vm_file->f_dentry->d_inode->i_mapping;
 
 	/*
-- 
1.7.10.4


-- 
Michal Hocko
SUSE Labs
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