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

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Message-ID: <20120813130906.GA24248@dhcp22.suse.cz>
Date:	Mon, 13 Aug 2012 15:09:06 +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 Mon 13-08-12 20:10:41, Hillf Danton wrote:
> On Sun, Aug 12, 2012 at 5:31 PM, Michal Hocko <mhocko@...e.cz> wrote:
> > 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.
> 
> 
> What if another case of vma_prio_tree_foreach in try_to_unmap_file
> is correct?

That one is surely correct (linear_page_index converts the page offset).
Anyway do you actually have any _real_ objection to the patch? I think
the generated discussion is not worth the whole discussion.
 
> > 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

-- 
Michal Hocko
SUSE Labs
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