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

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Date:	Wed, 4 Jan 2012 15:50:42 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Cc:	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, 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?

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

> +	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.


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