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

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Fri, Oct 10, 2014 at 09:21:08PM +0800, Fengwei Yin wrote:
> On Thu, Oct 09, 2014 at 09:36:30AM -0700, Dave Hansen wrote:
> > 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?
> I suppose _PAGE_SPECIAL can't work. Two reasons:
> 1. Not all arch have HAVE_PTE_SPECIAL set. So always need another way to
>    handle the arch which has no PTE_SPECIAL.
> 2. _PAGE_SPECIAL is just for PTE now. If want to add it for huge page,
>    we need to introduce pmd_mkspecial() thing which I don't think it's
>    worth to do now (unless you want it. :)).
> 
> Yes. We could move the check to vm_normal_page(). But it still needs
> export functions from huge_memory.c.
> 
> Please check the new patch.
> 
> > 
> > >  	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;
> > 	}
> Done.
> > 
> > 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.
> 
> A new function is_huge_zero_pfn() is added to mm/huge_memory.c
> and exported.
> 
> Thanks.

> From 4e7bdd5bc22874175982ab50303eab32843c753c Mon Sep 17 00:00:00 2001
> From: Fengwei Yin <yfw.kernel@gmail.com>
> Date: Thu, 9 Oct 2014 22:20:58 +0800
> Subject: [PATCH] smaps should deal with huge zero page exactly same as normal
>  zero page.
> 

Description?

> Signed-off-by: Fengwei Yin <yfw.kernel@gmail.com>
> ---
>  fs/proc/task_mmu.c      | 6 ++++--
>  include/linux/huge_mm.h | 2 ++
>  mm/huge_memory.c        | 5 +++++
>  mm/memory.c             | 4 ++++
>  4 files changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index c341568..fb19c0c 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -471,8 +471,11 @@ static void smaps_pte_entry(pte_t ptent, unsigned long addr,
>  	if (!page)
>  		return;
>  
> -	if (PageAnon(page))
> +	if (PageAnon(page)) {
>  		mss->anonymous += ptent_size;
> +		if (PageTransHuge(page))
> +			mss->anonymous_thp += HPAGE_PMD_SIZE;
> +	}
>  
>  	if (page->index != pgoff)
>  		mss->nonlinear += ptent_size;
> @@ -508,7 +511,6 @@ 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;
>  		return 0;
>  	}
>  
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index 63579cb..9bf6263 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -34,6 +34,8 @@ 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_pfn(unsigned long pfn);
> +
>  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 f8ffd94..71ca4ed 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -183,6 +183,11 @@ static inline bool is_huge_zero_pmd(pmd_t pmd)
>  	return is_huge_zero_page(pmd_page(pmd));
>  }
>  
> +inline bool is_huge_zero_pfn(unsigned long pfn)

There's no way the function can be inlined.

Otherwise looks good to me.

-- 
 Kirill A. Shutemov

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