Re: [PATCH 2/2] ZERO PAGE based on pte_special

[Minchan Kim <minchan.kim@gmail.com>]

Hi, Kame. 

On Thu, 16 Jul 2009 18:04:24 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:

> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> 
> ZERO_PAGE for anonymous private mapping is useful when an application
> requires large continuous memory but write sparsely or some other usages.
> It was removed in 2.6.24 but this patch tries to re-add it.
> (Because there are some use cases..)
> 
> In past, ZERO_PAGE was removed because heavy cache line contention in
> ZERO_PAGE's refcounting, this version of ZERO_PAGE avoid to refcnt it.
> Then, implementation is changed as following.
> 
>   - Use of ZERO_PAGE is limited to PRIVATE mapping. Then, VM_MAYSHARE is
>     checked as VM_SHARED.
> 
>   - pte_special(), _PAGE_SPECIAL bit in pte is used for indicating ZERO_PAGE.
> 
>   - vm_normal_page() eats FOLL_XXX flag. If FOLL_NOZERO is set,
>     NULL is returned even if ZERO_PAGE is found.
> 
>   - __get_user_pages() eats one more flag as GUP_FLAGS_IGNORE_ZERO. If set,
>     __get_user_page() returns NULL even if ZERO_PAGE is found.
> 
> Note:
>   - no changes to get_user_pages(). ZERO_PAGE can be returned when
>     vma is ANONYMOUS && PRIVATE and the access is READ.
> 
> Changelog v3->v4
>  - FOLL_NOZERO is directly passed to vm_normal_page()
> 
> Changelog v2->v3
>  - totally renewed.
>  - use pte_special()
>  - added new argument to vm_normal_page().
>  - MAYSHARE is checked.
> 
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> ---
>  fs/proc/task_mmu.c |    8 +--
>  include/linux/mm.h |    3 -
>  mm/fremap.c        |    2 
>  mm/internal.h      |    1 
>  mm/memory.c        |  136 +++++++++++++++++++++++++++++++++++++++++------------
>  mm/mempolicy.c     |    8 +--
>  mm/migrate.c       |    6 +-
>  mm/mlock.c         |    2 
>  mm/rmap.c          |    6 +-
>  9 files changed, 128 insertions(+), 44 deletions(-)
> 
> Index: mmotm-2.6.31-Jul15/mm/memory.c
> ===================================================================
> --- mmotm-2.6.31-Jul15.orig/mm/memory.c
> +++ mmotm-2.6.31-Jul15/mm/memory.c
> @@ -442,6 +442,27 @@ static inline int is_cow_mapping(unsigne
>  }
>  
>  /*
> + * Can we use ZERO_PAGE at fault ? or Can we do the FOLL_ANON optimization ?
> + */
> +static inline int use_zero_page(struct vm_area_struct *vma)
> +{
> +	/*
> +	 * We don't want to optimize FOLL_ANON for make_pages_present()
> +	 * when it tries to page in a VM_LOCKED region. As to VM_SHARED,
> +	 * we want to get the page from the page tables to make sure
> +	 * that we serialize and update with any other user of that
> +	 * mapping. At doing page fault, VM_MAYSHARE should be also check for
> +	 * avoiding possible changes to VM_SHARED.
> +	 */
> +	if (vma->vm_flags & (VM_LOCKED | VM_SHARED | VM_MAYSHARE))
> +		return 0;
> +	/*
> +	 * And if we have a fault routine, it's not an anonymous region.
> +	 */
> +	return !vma->vm_ops || !vma->vm_ops->fault;
> +}
> +
> +/*
>   * vm_normal_page -- This function gets the "struct page" associated with a pte.
>   *
>   * "Special" mappings do not wish to be associated with a "struct page" (either
> @@ -488,16 +509,33 @@ static inline int is_cow_mapping(unsigne
>  #else
>  # define HAVE_PTE_SPECIAL 0
>  #endif
> +
> +#ifdef CONFIG_SUPPORT_ANON_ZERO_PAGE
> +# define HAVE_ANON_ZERO 1
> +#else
> +# define HAVE_ANON_ZERO 0
> +#endif
>  struct page *vm_normal_page(struct vm_area_struct *vma, unsigned long addr,
> -				pte_t pte)
> +			    pte_t pte, unsigned int flags)
>  {
>  	unsigned long pfn = pte_pfn(pte);
>  
>  	if (HAVE_PTE_SPECIAL) {
>  		if (likely(!pte_special(pte)))
>  			goto check_pfn;
> -		if (!(vma->vm_flags & (VM_PFNMAP | VM_MIXEDMAP)))
> -			print_bad_pte(vma, addr, pte, NULL);
> +
> +		if (vma->vm_flags & (VM_PFNMAP | VM_MIXEDMAP))
> +			return NULL;
> +		/*
> +		 * ZERO PAGE ? If vma is shared or has page fault handler,
> +		 * Using ZERO PAGE is bug.
> +		 */
> +		if (HAVE_ANON_ZERO && use_zero_page(vma)) {
> +			if (flags & FOLL_NOZERO)
> +				return NULL;
> +			return ZERO_PAGE(0);
> +		}
> +		print_bad_pte(vma, addr, pte, NULL);
>  		return NULL;
>  	}
>  
> @@ -591,8 +629,7 @@ copy_one_pte(struct mm_struct *dst_mm, s
>  	if (vm_flags & VM_SHARED)
>  		pte = pte_mkclean(pte);
>  	pte = pte_mkold(pte);
> -
> -	page = vm_normal_page(vma, addr, pte);
> +	page = vm_normal_page(vma, addr, pte, FOLL_NOZERO);
>  	if (page) {
>  		get_page(page);
>  		page_dup_rmap(page, vma, addr);
> @@ -783,7 +820,7 @@ static unsigned long zap_pte_range(struc
>  		if (pte_present(ptent)) {
>  			struct page *page;
>  
> -			page = vm_normal_page(vma, addr, ptent);
> +			page = vm_normal_page(vma, addr, ptent, FOLL_NOZERO);
>  			if (unlikely(details) && page) {
>  				/*
>  				 * unmap_shared_mapping_pages() wants to
> @@ -1141,7 +1178,7 @@ struct page *follow_page(struct vm_area_
>  		goto no_page;
>  	if ((flags & FOLL_WRITE) && !pte_write(pte))
>  		goto unlock;
> -	page = vm_normal_page(vma, address, pte);
> +	page = vm_normal_page(vma, address, pte, flags);
>  	if (unlikely(!page))
>  		goto bad_page;
>  
> @@ -1186,23 +1223,6 @@ no_page_table:
>  	return page;
>  }
>  
> -/* Can we do the FOLL_ANON optimization? */
> -static inline int use_zero_page(struct vm_area_struct *vma)
> -{
> -	/*
> -	 * We don't want to optimize FOLL_ANON for make_pages_present()
> -	 * when it tries to page in a VM_LOCKED region. As to VM_SHARED,
> -	 * we want to get the page from the page tables to make sure
> -	 * that we serialize and update with any other user of that
> -	 * mapping.
> -	 */
> -	if (vma->vm_flags & (VM_LOCKED | VM_SHARED))
> -		return 0;
> -	/*
> -	 * And if we have a fault routine, it's not an anonymous region.
> -	 */
> -	return !vma->vm_ops || !vma->vm_ops->fault;
> -}
>  
>  
>  
> @@ -1216,6 +1236,7 @@ int __get_user_pages(struct task_struct 
>  	int force = !!(flags & GUP_FLAGS_FORCE);
>  	int ignore = !!(flags & GUP_FLAGS_IGNORE_VMA_PERMISSIONS);
>  	int ignore_sigkill = !!(flags & GUP_FLAGS_IGNORE_SIGKILL);
> +	int ignore_zero = !!(flags & GUP_FLAGS_IGNORE_ZERO);
>  
>  	if (nr_pages <= 0)
>  		return 0;
> @@ -1259,7 +1280,12 @@ int __get_user_pages(struct task_struct 
>  				return i ? : -EFAULT;
>  			}
>  			if (pages) {
> -				struct page *page = vm_normal_page(gate_vma, start, *pte);
> +				struct page *page;
> +				/*
> +				 * this is not anon vma...don't haddle zero page
> +				 * related flags.
> +				 */
> +				page = vm_normal_page(gate_vma, start, *pte, 0);
>  				pages[i] = page;
>  				if (page)
>  					get_page(page);
> @@ -1287,8 +1313,13 @@ int __get_user_pages(struct task_struct 
>  		foll_flags = FOLL_TOUCH;
>  		if (pages)
>  			foll_flags |= FOLL_GET;
> -		if (!write && use_zero_page(vma))
> -			foll_flags |= FOLL_ANON;
> +		if (!write) {
> +			if (use_zero_page(vma))
> +				foll_flags |= FOLL_ANON;
> +			else
> +				ignore_zero = 0;
> +		} else
> +			ignore_zero = 0;

Hmm. nested condition is not good for redabililty. 

How about this ?
if (!write && use_zero_page(vma))
	foll_flags |= FOLL_ANON;
else
	ignore_zero = 0;



-- 
Kind regards,
Minchan Kim

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