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

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Message-ID: <20120810122730.GA1425@dhcp22.suse.cz>
Date:	Fri, 10 Aug 2012 14:27:30 +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>
Subject: Re: [patch] hugetlb: correct page offset index for sharing pmd

On Fri 10-08-12 20:07:12, Hillf Danton wrote:
> On Fri, Aug 10, 2012 at 5:48 PM, Michal Hocko <mhocko@...e.cz> wrote:
> > On Fri 03-08-12 15:32:35, Michal Hocko wrote:
> >> On Fri 03-08-12 20:56:45, Hillf Danton wrote:
> >> > The computation of page offset index is open coded, and incorrect, to
> >> > be used in scanning prio tree, as huge page offset is required, and is
> >> > fixed with the well defined routine.
> >>
> >> I guess that nobody reported this because if someone really wants to
> >> share he will use aligned address for mmap/shmat and so the index is 0.
> >> Anyway it is worth fixing. Thanks for pointing out!
> >
> > I have looked at the code again and I don't think there is any problem
> > at all. vma_prio_tree_foreach understands page units so it will find
> > appropriate svmas.
> > Or am I missing something?
> 
> Well, what if another case of vma_prio_tree_foreach used by hugetlb
> is correct?

I guess you mean unmap_ref_private and that has been changed by you
(0c176d5 mm: hugetlb: fix pgoff computation when unmapping page from
vma)...  I was wrong at that time when giving my Reviewed-by. The patch
didn't break anything because you still find all relevant vmas because
vma_hugecache_offset just provides a smaller index which is still within
boundaries.
I think that 0c176d52 should be reverted because we do not have to refer
to the head page in this case and as we can see it causes confusion.

> 
> Hillf

-- 
Michal Hocko
SUSE Labs
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