Re: [PATCH 1/4] hugetlb, rmap: always use anon_vma root pointer

[Rik van Riel]

On 09/10/2010 05:50 PM, Andi Kleen wrote:
> On Fri, 10 Sep 2010 10:19:24 -0700
> Linus Torvalds<torvalds@xxxxxxxxxxxxxxxxxxxx>  wrote:
>
> > On Thu, Sep 9, 2010 at 9:23 PM, Naoya Horiguchi
> > <n-horiguchi@xxxxxxxxxxxxx>  wrote:
> > > This patch applies Andrea's fix given by the following patch into
> > > hugepage rmapping code:
> > >
> > >   commit 288468c334e98aacbb7e2fb8bde6bc1adcd55e05
> > >   Author: Andrea Arcangeli<aarcange@xxxxxxxxxx>
> > >   Date:   Mon Aug 9 17:19:09 2010 -0700
> > >
> > > This patch uses anon_vma->root and avoids unnecessary overwriting
> > > when anon_vma is already set up.
> >
> > Btw, why isn't the code in __page_set_anon_rmap() also doing this
> > cleaner version (ie a single "if (PageAnon(page)) return;" up front)?
>
> Perhaps I misunderstand the question, but __page_set_anon_rmap
> should handle Anon pages, shouldn't it?

__page_set_anon_rmap sets the page->mapping to be
a pointer to the anon_vma & PAGE_MAPPING_ANON.

PageAnon tests for page->mapping & PAGE_MAPPING_ANON,
ie. whether page->mapping is already pointing to an
anon_vma.

If it is, __page_set_anon_rmap should leave the page
mapping pointer alone.

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