linux-kernel - Re: [PATCH 1/6] pagemap: avoid splitting thp when reading /proc/pid/pagemap

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Message-ID: <4F145BE7.1060802@ah.jp.nec.com>
Date:	Mon, 16 Jan 2012 12:18:31 -0500
From:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To:	Andrea Arcangeli <aarcange@...hat.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>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/6] pagemap: avoid splitting thp when reading /proc/pid/pagemap

On Sat, Jan 14, 2012 at 06:00:26PM +0100, Andrea Arcangeli wrote:
> On Thu, Jan 12, 2012 at 02:34:53PM -0500, Naoya Horiguchi wrote:
> > +		if (pmd_trans_splitting(*pmd)) {
> > +			spin_unlock(&walk->mm->page_table_lock);
> > +			wait_split_huge_page(vma->anon_vma, pmd);
> > +		} else {
> > +			for (; addr != end; addr += PAGE_SIZE) {
> > +				unsigned long offset = (addr & ~PAGEMAP_WALK_MASK)
> > +					>> PAGE_SHIFT;
> > +				pfn = thp_pte_to_pagemap_entry(*(pte_t *)pmd,
> > +							       offset);
> 
> What is this that then morphs into a pme (which still has a cast
> inside its creation)? thp_pte_to_pagemap_entry don't seem to be passed
> ptes too. The only case where it is valid to do a cast like that is
> when a function is used by both ptes sand pmds and the code tends to
> work for both with minimal modification to differentiate the two
> cases. Considering the function that gets the cast is called thp_ this
> is hardly the case here.

Agreed.

> Why don't you pass the pmd and then do "if (pmd_present(pmd))
> page_to_pfn(pmd_page(pmd)) ? What's the argument for the cast. I'm
> just reviewing this series and maybe it was covered in previous
> versions.

OK, I can do this by introducing pmd_pte as you commented in another email.

> I don't get this pme thing for something as trivial as above that
> shouldn't require any cast at all.
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