linux-kernel - Re: [PATCH] smaps should deal with huge zero page exactly same as normal zero page

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Date:	Thu, 09 Oct 2014 09:36:30 -0700
From:	Dave Hansen <dave.hansen@...el.com>
To:	Fengwei Yin <yfw.kernel@...il.com>,
	LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org
CC:	fengguang.wu@...el.com,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH] smaps should deal with huge zero page exactly same as
 normal zero page

On 10/09/2014 02:19 AM, Fengwei Yin wrote:
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 80ca4fb..8550b27 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -476,7 +476,7 @@ static void smaps_pte_entry(pte_t ptent, unsigned long addr,
>  			mss->nonlinear += ptent_size;
>  	}
>  
> -	if (!page)
> +	if (!page || is_huge_zero_page(page))
>  		return;

This really seems like a bit of a hack.  A normal (small) zero page
won't make it to this point because of the vm_normal_page() check in
smaps_pte_entry() which hits the _PAGE_SPECIAL bit in the pte.

Is there a reason we can't set _PAGE_SPECIAL on the huge_zero_page ptes?
 If we did that, we wouldn't need a special case here.

If we can't do that for some reason, can we at least teach
vm_normal_page() about the huge_zero_page in some other way?

>  	if (PageAnon(page))
> @@ -516,7 +516,8 @@ static int smaps_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
>  	if (pmd_trans_huge_lock(pmd, vma, &ptl) == 1) {
>  		smaps_pte_entry(*(pte_t *)pmd, addr, HPAGE_PMD_SIZE, walk);
>  		spin_unlock(ptl);
> -		mss->anonymous_thp += HPAGE_PMD_SIZE;
> +		if (!is_huge_zero_pmd(*pmd))
> +			mss->anonymous_thp += HPAGE_PMD_SIZE;
>  		return 0;
>  	}

How about we just move this hunk in to smaps_pte_entry()?  Something
along these lines:

...
        if (PageAnon(page)) {
                mss->anonymous += ptent_size;
+		if (PageTransHuge(page))
+			mss->anonymous_thp += ptent_size;
	}

If we do that, plus teaching vm_normal_page() about huge_zero_pages, it
will help keep the hacks and the extra code due to huge pages to a miniumum.

> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index 63579cb..758f569 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -34,6 +34,10 @@ extern int change_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
>  			unsigned long addr, pgprot_t newprot,
>  			int prot_numa);
>  
> +extern bool is_huge_zero_page(struct page *page);
> +
> +extern bool is_huge_zero_pmd(pmd_t pmd);
> +
>  enum transparent_hugepage_flag {
>  	TRANSPARENT_HUGEPAGE_FLAG,
>  	TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG,
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index d9a21d06..bedc3ae 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -173,12 +173,12 @@ static int start_khugepaged(void)
>  static atomic_t huge_zero_refcount;
>  static struct page *huge_zero_page __read_mostly;
>  
> -static inline bool is_huge_zero_page(struct page *page)
> +bool is_huge_zero_page(struct page *page)
>  {
>  	return ACCESS_ONCE(huge_zero_page) == page;
>  }
>  
> -static inline bool is_huge_zero_pmd(pmd_t pmd)
> +bool is_huge_zero_pmd(pmd_t pmd)
>  {
>  	return is_huge_zero_page(pmd_page(pmd));
>  }

^^^ And all these exports.
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