linux-kernel - Re: [PATCH 1/4] pagemap: avoid splitting thp when reading /proc/pid/pagemap

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Date:	Thu, 05 Jan 2012 11:28:31 -0500
From:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>, linux-mm@...ck.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>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] pagemap: avoid splitting thp when reading /proc/pid/pagemap

On Wed, Jan 04, 2012 at 03:50:42PM -0800, Andrew Morton wrote:
> On Wed, 21 Dec 2011 17:23:45 -0500
> Naoya Horiguchi <n-horiguchi@...jp.nec.com> wrote:
> 
> > Thp split is not necessary if we explicitly check whether pmds are
> > mapping thps or not. This patch introduces the check and the code
> > to generate pagemap entries for pmds mapping thps, which results in
> > less performance impact of pagemap on thp.
> > 
> >
> > ...
> 
> The type choices seem odd:
> 
> > +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> > +static u64 thp_pte_to_pagemap_entry(pte_t pte, int offset)
> > +{
> > +	u64 pme = 0;
> 
> Why are these u64?

I guess (I just copied this type choice from other *pte_to_pagemap_entry()
type functions) it's because each entry in /proc/pid/pagemap is in fixed
sized (64 bit) format as described in the comment above pagemap_read().

> Should we have a pme_t, matching pte_t, pmd_t, etc?

Yes, it makes code's meaning clearer.

> 
> > +	if (pte_present(pte))
> > +		pme = PM_PFRAME(pte_pfn(pte) + offset)
> > +			| PM_PSHIFT(PAGE_SHIFT) | PM_PRESENT;
> > +	return pme;
> > +}
> > +#else
> > +static inline u64 thp_pte_to_pagemap_entry(pte_t pte, int offset)
> > +{
> > +	return 0;
> > +}
> > +#endif
> > +
> >  static int pagemap_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
> >  			     struct mm_walk *walk)
> >  {
> > @@ -665,14 +684,34 @@ static int pagemap_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
> >  	struct pagemapread *pm = walk->private;
> >  	pte_t *pte;
> >  	int err = 0;
> > -
> > -	split_huge_page_pmd(walk->mm, pmd);
> > +	u64 pfn = PM_NOT_PRESENT;
> 
> Again, why a u64?  pfn's are usually unsigned long.

I think variable's name 'pfn' is wrong rather than type choice
because this variable stores pagemap entry which is not a pure pfn.
There's room for improvement, so I'll try it in the next turn.

Thanks,
Naoya
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