Re: [PATCH 11/24] huge tmpfs: shrinker to migrate and free underused holes

[Hugh Dickins]

On Tue, 24 Mar 2015, Kirill A. Shutemov wrote:
> On Sun, Mar 22, 2015 at 09:40:02PM -0700, Hugh Dickins wrote:
> > (I think Kirill has a problem of that kind in his page_remove_rmap scan).

(And this one I mentioned to you at the conference :)

> > 
> > It will be interesting to see what Kirill does to maintain the stats
> > for huge pagecache: but he will have no difficulty in finding fields
> > to store counts, because he's got lots of spare fields in those 511
> > tail pages - that's a useful benefit of the compound page, but does
> > prevent the tails from being used in ordinary ways.  (I did try using
> > team_head[1].team_usage for more, but atomicity needs prevented it.)
> 
> The patch below should address the race you pointed, if I've got all
> right. Not hugely happy with the change though.

Yes, without thinking too hard about it, something like what you have
below should do it.  Not very pretty; maybe a neater idea will come up.

Hugh

> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 435c90f59227..a3e6b35520f8 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -423,8 +423,17 @@ static inline void page_mapcount_reset(struct page *page)
>  
>  static inline int page_mapcount(struct page *page)
>  {
> +	int ret;
>  	VM_BUG_ON_PAGE(PageSlab(page), page);
> -	return atomic_read(&page->_mapcount) + compound_mapcount(page) + 1;
> +	ret = atomic_read(&page->_mapcount) + 1;
> +	if (compound_mapcount(page)) {
> +		/*
> +		 * positive compound_mapcount() offsets ->_mapcount by one --
> +		 * substract here.
> +		*/
> +	       ret += compound_mapcount(page) - 1;
> +	}
> +	return ret;
>  }
>  
>  static inline int page_count(struct page *page)
> diff --git a/mm/rmap.c b/mm/rmap.c
> index fc6eee4ed476..f4ab976276e7 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1066,9 +1066,17 @@ void do_page_add_anon_rmap(struct page *page,
>  		 * disabled.
>  		 */
>  		if (compound) {
> +			int i;
>  			VM_BUG_ON_PAGE(!PageTransHuge(page), page);
>  			__inc_zone_page_state(page,
>  					      NR_ANON_TRANSPARENT_HUGEPAGES);
> +			/*
> +			 * While compound_mapcount() is positive we keep *one*
> +			 * mapcount reference in all subpages. It's required
> +			 * for atomic removal from rmap.
> +			 */
> +			for (i = 0; i < nr; i++)
> +				atomic_set(&page[i]._mapcount, 0);
>  		}
>  		__mod_zone_page_state(page_zone(page), NR_ANON_PAGES, nr);
>  	}
> @@ -1103,10 +1111,19 @@ void page_add_new_anon_rmap(struct page *page,
>  	VM_BUG_ON_VMA(address < vma->vm_start || address >= vma->vm_end, vma);
>  	SetPageSwapBacked(page);
>  	if (compound) {
> +		int i;
> +
>  		VM_BUG_ON_PAGE(!PageTransHuge(page), page);
>  		/* increment count (starts at -1) */
>  		atomic_set(compound_mapcount_ptr(page), 0);
>  		__inc_zone_page_state(page, NR_ANON_TRANSPARENT_HUGEPAGES);
> +		/*
> +		 * While compound_mapcount() is positive we keep *one* mapcount
> +		 * reference in all subpages. It's required for atomic removal
> +		 * from rmap.
> +		 */
> +		for (i = 0; i < nr; i++)
> +			atomic_set(&page[i]._mapcount, 0);
>  	} else {
>  		/* Anon THP always mapped first with PMD */
>  		VM_BUG_ON_PAGE(PageTransCompound(page), page);
> @@ -1174,9 +1191,6 @@ out:
>   */
>  void page_remove_rmap(struct page *page, bool compound)
>  {
> -	int nr = compound ? hpage_nr_pages(page) : 1;
> -	bool partial_thp_unmap;
> -
>  	if (!PageAnon(page)) {
>  		VM_BUG_ON_PAGE(compound && !PageHuge(page), page);
>  		page_remove_file_rmap(page);
> @@ -1184,10 +1198,20 @@ void page_remove_rmap(struct page *page, bool compound)
>  	}
>  
>  	/* page still mapped by someone else? */
> -	if (!atomic_add_negative(-1, compound ?
> -			       compound_mapcount_ptr(page) :
> -			       &page->_mapcount))
> +	if (compound) {
> +		int i;
> +
> +		VM_BUG_ON_PAGE(!PageTransHuge(page), page);
> +		if (!atomic_add_negative(-1, compound_mapcount_ptr(page)))
> +			return;
> +		__dec_zone_page_state(page, NR_ANON_TRANSPARENT_HUGEPAGES);
> +		for (i = 0; i < hpage_nr_pages(page); i++)
> +			page_remove_rmap(page + i, false);
>  		return;
> +	} else {
> +		if (!atomic_add_negative(-1, &page->_mapcount))
> +			return;
> +	}
>  
>  	/* Hugepages are not counted in NR_ANON_PAGES for now. */
>  	if (unlikely(PageHuge(page)))
> @@ -1198,26 +1222,12 @@ void page_remove_rmap(struct page *page, bool compound)
>  	 * these counters are not modified in interrupt context, and
>  	 * pte lock(a spinlock) is held, which implies preemption disabled.
>  	 */
> -	if (compound) {
> -		int i;
> -		VM_BUG_ON_PAGE(!PageTransHuge(page), page);
> -		__dec_zone_page_state(page, NR_ANON_TRANSPARENT_HUGEPAGES);
> -		/* The page can be mapped with ptes */
> -		for (i = 0; i < hpage_nr_pages(page); i++)
> -			if (page_mapcount(page + i))
> -				nr--;
> -		partial_thp_unmap = nr != hpage_nr_pages(page);
> -	} else if (PageTransCompound(page)) {
> -		partial_thp_unmap = !compound_mapcount(page);
> -	} else
> -		partial_thp_unmap = false;
> -
> -	__mod_zone_page_state(page_zone(page), NR_ANON_PAGES, -nr);
> +	__dec_zone_page_state(page, NR_ANON_PAGES);
>  
>  	if (unlikely(PageMlocked(page)))
>  		clear_page_mlock(page);
>  
> -	if (partial_thp_unmap)
> +	if (PageTransCompound(page))
>  		deferred_split_huge_page(compound_head(page));
>  
>  	/*
> -- 
>  Kirill A. Shutemov
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