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

[Fengwei Yin]

 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.