linux-kernel - Re: [PATCHv4 19/24] thp, mm: use migration entries to freeze page counts on split

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Message-ID: <87bnjalc9g.fsf@linux.vnet.ibm.com>
Date:	Mon, 30 Mar 2015 21:15:47 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
Cc:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Hugh Dickins <hughd@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Rik van Riel <riel@...hat.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Christoph Lameter <cl@...two.org>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Steve Capper <steve.capper@...aro.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>,
	Jerome Marchand <jmarchan@...hat.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCHv4 19/24] thp, mm: use migration entries to freeze page counts on split

"Kirill A. Shutemov" <kirill@...temov.name> writes:

> On Mon, Mar 30, 2015 at 08:38:08PM +0530, Aneesh Kumar K.V wrote:
>> ....
>> ....
>>  +static void freeze_page(struct anon_vma *anon_vma, struct page *page)
>> > +{
>> > +	struct anon_vma_chain *avc;
>> > +	struct vm_area_struct *vma;
>> > +	pgoff_t pgoff = page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT);
>> 
>> So this get called only with head page, We also do
>> BUG_ON(PageTail(page)) in the caller.  But
>> 
>> 
>> > +	unsigned long addr, haddr;
>> > +	unsigned long mmun_start, mmun_end;
>> > +	pgd_t *pgd;
>> > +	pud_t *pud;
>> > +	pmd_t *pmd;
>> > +	pte_t *start_pte, *pte;
>> > +	spinlock_t *ptl;
>> ......
>> 
>> 
>> > +
>> > +static void unfreeze_page(struct anon_vma *anon_vma, struct page *page)
>> > +{
>> > +	struct anon_vma_chain *avc;
>> > +	pgoff_t pgoff = page_to_pgoff(page);
>> 
>> Why ? Can this get called for tail pages ?
>
> It cannot. pgoff is offset of head page (and therefore whole compound
> page) within rmapping.
>

This we can use 

	pgoff_t pgoff = page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT);
        
similar to what we do in freeze_page(). The difference between
freeze/unfreeze confused me.

-aneesh

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