linux-kernel - Re: [patch v2] hugetlb: correct page offset index for sharing pmd

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Message-Id: <20120815160342.5b77bd3b.akpm@linux-foundation.org>
Date:	Wed, 15 Aug 2012 16:03:42 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Hillf Danton <dhillf@...il.com>
Cc:	Michal Hocko <mhocko@...e.cz>, Mel Gorman <mgorman@...e.de>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [patch v2] hugetlb: correct page offset index for sharing pmd

On Mon, 6 Aug 2012 21:37:45 +0800
Hillf Danton <dhillf@...il.com> wrote:

> On Mon, Aug 6, 2012 at 9:24 PM, Michal Hocko <mhocko@...e.cz> wrote:
> > On Sat 04-08-12 14:08:31, Hillf Danton wrote:
> >> The computation of page offset index is incorrect to be used in scanning
> >> prio tree, as huge page offset is required, and is fixed with well
> >> defined routine.
> >>
> >> Changes from v1
> >>       o s/linear_page_index/linear_hugepage_index/ for clearer code
> >>       o hp_idx variable added for less change
> >>
> >>
> >> Signed-off-by: Hillf Danton <dhillf@...il.com>
> >> ---
> >>
> >> --- a/arch/x86/mm/hugetlbpage.c       Fri Aug  3 20:34:58 2012
> >> +++ b/arch/x86/mm/hugetlbpage.c       Fri Aug  3 20:40:16 2012
> >> @@ -62,6 +62,7 @@ static void huge_pmd_share(struct mm_str
> >>  {
> >>       struct vm_area_struct *vma = find_vma(mm, addr);
> >>       struct address_space *mapping = vma->vm_file->f_mapping;
> >> +     pgoff_t hp_idx;
> >>       pgoff_t idx = ((addr - vma->vm_start) >> PAGE_SHIFT) +
> >>                       vma->vm_pgoff;
> >
> > So we have two indexes now. That is just plain ugly!
> >
> 
> Two indexes result in less code change here and no change
> in page_table_shareable. Plus linear_hugepage_index tells
> clearly readers that hp_idx and idx are different.
> 
> Anyway I have no strong opinion to keep
> page_table_shareable unchanged, but prefer less changes.

Don't be too concerned about the size of a change - it's the end result
which matters.  If a larger patch results in a better end result, then
do the larger patch.

Also, please add some details to the changelog: the patch is fixing a
bug but we aren't told about the end-user-visible effects of that bug. 
This is important information.

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