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Subject: Re: About get_page for compound page
From: Peter Zijlstra <peterz@infradead.org>
To: Fengwei Yin <yfw.kernel@gmail.com>
Cc: LKML <linux-kernel@vger.kernel.org>
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Date: Fri, 20 Jul 2007 08:55:33 +0200
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On Fri, 2007-07-20 at 07:59 +0800, Fengwei Yin wrote:
> On 7/20/07, Peter Zijlstra <peterz@infradead.org> wrote:
> > On Thu, 2007-07-19 at 23:54 +0800, Fengwei Yin wrote:
> > > Hi,
> > > In get_page(), the parameter page is changed by
> > >          page = compound_head(page);
> > > if the page is not the first page of compound pages.
> > >
> > > My question: is this behavior correct?
> >
> > Yes it is, in general the page state of a compound page is kept in the
> > first page of the set.
> >
> Yes. I know this. But
> static inline void get_page(struct page *page)
>                                                      ~~~~~~ changed by
> this function.
> {
>          page = compound_head(page);
>                     ~~~~~~~~~~~~~~~~~~~~~~~
>          VM_BUG_ON(atomic_read(&page->_count) == 0);
>          atomic_inc(&page->_count);
> }
> 
> As I understand, just need update the first page reference count.
> Or the caller aware of this bahavior?
> 
> Or change it to
> static inline void get_page(struct page *page)
> {
>          struct page *head_page = compound_head(page);
>          VM_BUG_ON(atomic_read(&head_page->_count) == 0);
>          atomic_inc(&head_page->_count);
> }

Arguments are local to the function.