Re: [PATCH 1/3] mm: thp: calculate the mapcount correctly for THP pages during WP faults

[Alex Williamson]

On Fri,  6 May 2016 17:03:58 +0200
Andrea Arcangeli <aarcange@xxxxxxxxxx> wrote:

> This will provide fully accuracy to the mapcount calculation in the
> write protect faults, so page pinning will not get broken by false
> positive copy-on-writes.
> 
> total_mapcount() isn't the right calculation needed in
> reuse_swap_page(), so this introduces a page_trans_huge_mapcount()
> that is effectively the full accurate return value for page_mapcount()
> if dealing with Transparent Hugepages, however we only use the
> page_trans_huge_mapcount() during COW faults where it strictly needed,
> due to its higher runtime cost.
> 
> This also provide at practical zero cost the total_mapcount
> information which is needed to know if we can still relocate the page
> anon_vma to the local vma. If page_trans_huge_mapcount() returns 1 we
> can reuse the page no matter if it's a pte or a pmd_trans_huge
> triggering the fault, but we can only relocate the page anon_vma to
> the local vma->anon_vma if we're sure it's only this "vma" mapping the
> whole THP physical range.
> 
> Kirill A. Shutemov discovered the problem with moving the page
> anon_vma to the local vma->anon_vma in a previous version of this
> patch and another problem in the way page_move_anon_rmap() was called.
> 
> Reviewed-by: "Kirill A. Shutemov" <kirill@xxxxxxxxxxxxx>
> Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> ---

I've been running my original vfio test case on this for over 6 hours
with no sign of error.

Tested-by: Alex Williamson <alex.williamson@xxxxxxxxxx>

Can we also get a stable tag for this for v4.5 please?  Thanks,

Alex

>  include/linux/mm.h   |  9 +++++++
>  include/linux/swap.h |  8 ++++---
>  mm/huge_memory.c     | 67 +++++++++++++++++++++++++++++++++++++++++++++-------
>  mm/memory.c          | 22 ++++++++++-------
>  mm/swapfile.c        | 13 +++++-----
>  5 files changed, 93 insertions(+), 26 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index a55e5be..263f229 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -500,11 +500,20 @@ static inline int page_mapcount(struct page *page)
>  
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>  int total_mapcount(struct page *page);
> +int page_trans_huge_mapcount(struct page *page, int *total_mapcount);
>  #else
>  static inline int total_mapcount(struct page *page)
>  {
>  	return page_mapcount(page);
>  }
> +static inline int page_trans_huge_mapcount(struct page *page,
> +					   int *total_mapcount)
> +{
> +	int mapcount = page_mapcount(page);
> +	if (total_mapcount)
> +		*total_mapcount = mapcount;
> +	return mapcount;
> +}
>  #endif
>  
>  static inline struct page *virt_to_head_page(const void *x)
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 2b83359..acef20d 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -418,7 +418,7 @@ extern sector_t swapdev_block(int, pgoff_t);
>  extern int page_swapcount(struct page *);
>  extern int swp_swapcount(swp_entry_t entry);
>  extern struct swap_info_struct *page_swap_info(struct page *);
> -extern int reuse_swap_page(struct page *);
> +extern bool reuse_swap_page(struct page *, int *);
>  extern int try_to_free_swap(struct page *);
>  struct backing_dev_info;
>  
> @@ -513,8 +513,10 @@ static inline int swp_swapcount(swp_entry_t entry)
>  	return 0;
>  }
>  
> -#define reuse_swap_page(page) \
> -	(!PageTransCompound(page) && page_mapcount(page) == 1)
> +static inline bool reuse_swap_page(struct page *page, int *total_mapcount)
> +{
> +	return page_trans_huge_mapcount(page, total_mapcount) == 1;
> +}
>  
>  static inline int try_to_free_swap(struct page *page)
>  {
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 86f9f8b..9086793 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1298,15 +1298,9 @@ int do_huge_pmd_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
>  	VM_BUG_ON_PAGE(!PageCompound(page) || !PageHead(page), page);
>  	/*
>  	 * We can only reuse the page if nobody else maps the huge page or it's
> -	 * part. We can do it by checking page_mapcount() on each sub-page, but
> -	 * it's expensive.
> -	 * The cheaper way is to check page_count() to be equal 1: every
> -	 * mapcount takes page reference reference, so this way we can
> -	 * guarantee, that the PMD is the only mapping.
> -	 * This can give false negative if somebody pinned the page, but that's
> -	 * fine.
> +	 * part.
>  	 */
> -	if (page_mapcount(page) == 1 && page_count(page) == 1) {
> +	if (page_trans_huge_mapcount(page, NULL) == 1) {
>  		pmd_t entry;
>  		entry = pmd_mkyoung(orig_pmd);
>  		entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);
> @@ -2080,7 +2074,8 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
>  		if (pte_write(pteval)) {
>  			writable = true;
>  		} else {
> -			if (PageSwapCache(page) && !reuse_swap_page(page)) {
> +			if (PageSwapCache(page) &&
> +			    !reuse_swap_page(page, NULL)) {
>  				unlock_page(page);
>  				result = SCAN_SWAP_CACHE_PAGE;
>  				goto out;
> @@ -3225,6 +3220,60 @@ int total_mapcount(struct page *page)
>  }
>  
>  /*
> + * This calculates accurately how many mappings a transparent hugepage
> + * has (unlike page_mapcount() which isn't fully accurate). This full
> + * accuracy is primarily needed to know if copy-on-write faults can
> + * reuse the page and change the mapping to read-write instead of
> + * copying them. At the same time this returns the total_mapcount too.
> + *
> + * The function returns the highest mapcount any one of the subpages
> + * has. If the return value is one, even if different processes are
> + * mapping different subpages of the transparent hugepage, they can
> + * all reuse it, because each process is reusing a different subpage.
> + *
> + * The total_mapcount is instead counting all virtual mappings of the
> + * subpages. If the total_mapcount is equal to "one", it tells the
> + * caller all mappings belong to the same "mm" and in turn the
> + * anon_vma of the transparent hugepage can become the vma->anon_vma
> + * local one as no other process may be mapping any of the subpages.
> + *
> + * It would be more accurate to replace page_mapcount() with
> + * page_trans_huge_mapcount(), however we only use
> + * page_trans_huge_mapcount() in the copy-on-write faults where we
> + * need full accuracy to avoid breaking page pinning, because
> + * page_trans_huge_mapcount() is slower than page_mapcount().
> + */
> +int page_trans_huge_mapcount(struct page *page, int *total_mapcount)
> +{
> +	int i, ret, _total_mapcount, mapcount;
> +
> +	/* hugetlbfs shouldn't call it */
> +	VM_BUG_ON_PAGE(PageHuge(page), page);
> +
> +	if (likely(!PageTransCompound(page)))
> +		return atomic_read(&page->_mapcount) + 1;
> +
> +	page = compound_head(page);
> +
> +	_total_mapcount = ret = 0;
> +	for (i = 0; i < HPAGE_PMD_NR; i++) {
> +		mapcount = atomic_read(&page[i]._mapcount) + 1;
> +		ret = max(ret, mapcount);
> +		_total_mapcount += mapcount;
> +	}
> +	if (PageDoubleMap(page)) {
> +		ret -= 1;
> +		_total_mapcount -= HPAGE_PMD_NR;
> +	}
> +	mapcount = compound_mapcount(page);
> +	ret += mapcount;
> +	_total_mapcount += mapcount;
> +	if (total_mapcount)
> +		*total_mapcount = _total_mapcount;
> +	return ret;
> +}
> +
> +/*
>   * This function splits huge page into normal pages. @page can point to any
>   * subpage of huge page to split. Split doesn't change the position of @page.
>   *
> diff --git a/mm/memory.c b/mm/memory.c
> index 93897f2..d7a2af3 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2340,6 +2340,7 @@ static int do_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
>  	 * not dirty accountable.
>  	 */
>  	if (PageAnon(old_page) && !PageKsm(old_page)) {
> +		int total_mapcount;
>  		if (!trylock_page(old_page)) {
>  			get_page(old_page);
>  			pte_unmap_unlock(page_table, ptl);
> @@ -2354,13 +2355,18 @@ static int do_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
>  			}
>  			put_page(old_page);
>  		}
> -		if (reuse_swap_page(old_page)) {
> -			/*
> -			 * The page is all ours.  Move it to our anon_vma so
> -			 * the rmap code will not search our parent or siblings.
> -			 * Protected against the rmap code by the page lock.
> -			 */
> -			page_move_anon_rmap(old_page, vma, address);
> +		if (reuse_swap_page(old_page, &total_mapcount)) {
> +			if (total_mapcount == 1) {
> +				/*
> +				 * The page is all ours. Move it to
> +				 * our anon_vma so the rmap code will
> +				 * not search our parent or siblings.
> +				 * Protected against the rmap code by
> +				 * the page lock.
> +				 */
> +				page_move_anon_rmap(compound_head(old_page),
> +						    vma, address);
> +			}
>  			unlock_page(old_page);
>  			return wp_page_reuse(mm, vma, address, page_table, ptl,
>  					     orig_pte, old_page, 0, 0);
> @@ -2584,7 +2590,7 @@ static int do_swap_page(struct mm_struct *mm, struct vm_area_struct *vma,
>  	inc_mm_counter_fast(mm, MM_ANONPAGES);
>  	dec_mm_counter_fast(mm, MM_SWAPENTS);
>  	pte = mk_pte(page, vma->vm_page_prot);
> -	if ((flags & FAULT_FLAG_WRITE) && reuse_swap_page(page)) {
> +	if ((flags & FAULT_FLAG_WRITE) && reuse_swap_page(page, NULL)) {
>  		pte = maybe_mkwrite(pte_mkdirty(pte), vma);
>  		flags &= ~FAULT_FLAG_WRITE;
>  		ret |= VM_FAULT_WRITE;
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 83874ec..031713ab 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -922,18 +922,19 @@ out:
>   * to it.  And as a side-effect, free up its swap: because the old content
>   * on disk will never be read, and seeking back there to write new content
>   * later would only waste time away from clustering.
> + *
> + * NOTE: total_mapcount should not be relied upon by the caller if
> + * reuse_swap_page() returns false, but it may be always overwritten
> + * (see the other implementation for CONFIG_SWAP=n).
>   */
> -int reuse_swap_page(struct page *page)
> +bool reuse_swap_page(struct page *page, int *total_mapcount)
>  {
>  	int count;
>  
>  	VM_BUG_ON_PAGE(!PageLocked(page), page);
>  	if (unlikely(PageKsm(page)))
> -		return 0;
> -	/* The page is part of THP and cannot be reused */
> -	if (PageTransCompound(page))
> -		return 0;
> -	count = page_mapcount(page);
> +		return false;
> +	count = page_trans_huge_mapcount(page, total_mapcount);
>  	if (count <= 1 && PageSwapCache(page)) {
>  		count += page_swapcount(page);
>  		if (count == 1 && !PageWriteback(page)) {