linux-kernel - Re: [PATCH 6/6] pagemap: introduce data structure for pagemap entry

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Date:	Wed,  8 Feb 2012 23:16:20 -0500
From:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>, linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Rientjes <rientjes@...gle.com>,
	Andi Kleen <andi@...stfloor.org>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 6/6] pagemap: introduce data structure for pagemap entry

On Thu, Feb 09, 2012 at 11:29:36AM +0900, KAMEZAWA Hiroyuki wrote:
> On Wed,  8 Feb 2012 10:51:42 -0500
> Naoya Horiguchi <n-horiguchi@...jp.nec.com> wrote:
> 
> > Currently a local variable of pagemap entry in pagemap_pte_range()
> > is named pfn and typed with u64, but it's not correct (pfn should
> > be unsigned long.)
> 
> Does this means "the name 'pfn' implies unsigned long, usually. And
> this usage is confusing." ?

Yes, that is one I meant.
And another meaning is that this variable can contain not only page frame
number but also other information about page state. The format of pagemap
entry is described in a comment above pagemap_read() like this

 * Bits 0-55  page frame number (PFN) if present
 * Bits 0-4   swap type if swapped
 * Bits 5-55  swap offset if swapped
 * Bits 55-60 page shift (page size = 1<<page shift)
 * Bit  61    reserved for future use
 * Bit  62    page swapped
 * Bit  63    page present
 *
 * If the page is not present but in swap, then the PFN contains an
 * encoding of the swap file number and the page's offset into the
 * swap. Unmapped pages return a null PFN. This allows determining
 * precisely which pages are mapped (or in swap) and comparing mapped
 * pages between processes.

. So the name 'pfn' does not exactly match what it represents.

> > This patch introduces special type for pagemap entry and replace
> > code with it.
> > 
> > Signed-off-by: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
> > Cc: Andrew Morton <akpm@...ux-foundation.org>
> > 
> 
> Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>

Thank you.
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