Re: [PATCH] mm: avoid tail page refcounting on non-THP compound pages

[Hugh Dickins <hughd@google.com>]

On Thu, 26 Mar 2015, Kirill A. Shutemov wrote:

> THP uses tail page refcounting to be able to split huge page at any
> time. Tail page refcounting is not needed for rest users of compound
> pages and it's harmful because of overhead.
> 
> We try to exclude non-THP pages from tail page refcounting using
> __compound_tail_refcounted() check. It excludes most common non-THP
> compound pages: SL*B and hugetlb, but it doesn't catch rest of
> __GFP_COMP users -- drivers.
> 
> And it's not only about overhead.
> 
> Drivers might want to use compound pages to get refcounting semantics
> suitable for mapping high-order pages to userspace. But tail page
> refcounting breaks it.
> 
> Tail page refcounting uses ->_mapcount in tail pages to store GUP pins
> on them. It means GUP pins would affect page_mapcount() for tail pages.
> It's not a problem for THP, because it never maps tail pages. But unlike
> THP, drivers map parts of compound pages with PTEs and it makes
> page_mapcount() be called for tail pages.
> 
> In particular, GUP pins would shift PSS up and affect /proc/kpagecount
> for such pages. But, I'm not aware about anything which can lead to
> crash or other serious misbehaviour.
> 
> Since currently all THP pages are anonymous and all drivers pages are
> not, we can fix the __compound_tail_refcounted() check by requiring
> PageAnon() to enable tail page refcounting.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

Acked-by: Hugh Dickins <hughd@google.com>

> ---
>  include/linux/mm.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 4a3a38522ab4..16fe322b66ea 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -456,7 +456,7 @@ static inline int page_count(struct page *page)
>  
>  static inline bool __compound_tail_refcounted(struct page *page)
>  {
> -	return !PageSlab(page) && !PageHeadHuge(page);
> +	return PageAnon(page) && !PageSlab(page) && !PageHeadHuge(page);
>  }
>  
>  /*
> -- 
> 2.1.4

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