From 44518d2b32646e37b4b7a0813bbbe98dc21c7f8f Mon Sep 17 00:00:00 2001 From: Andrea Arcangeli Date: Tue, 21 Jan 2014 15:48:54 -0800 Subject: mm: tail page refcounting optimization for slab and hugetlbfs This skips the _mapcount mangling for slab and hugetlbfs pages. The main trouble in doing this is to guarantee that PageSlab and PageHeadHuge remains constant for all get_page/put_page run on the tail of slab or hugetlbfs compound pages. Otherwise if they're set during get_page but not set during put_page, the _mapcount of the tail page would underflow. PageHeadHuge will remain true until the compound page is released and enters the buddy allocator so it won't risk to change even if the tail page is the last reference left on the page. PG_slab instead is cleared before the slab frees the head page with put_page, so if the tail pin is released after the slab freed the page, we would have a problem. But in the slab case the tail pin cannot be the last reference left on the page. This is because the slab code is free to reuse the compound page after a kfree/kmem_cache_free without having to check if there's any tail pin left. In turn all tail pins must be always released while the head is still pinned by the slab code and so we know PG_slab will be still set too. Signed-off-by: Andrea Arcangeli Reviewed-by: Khalid Aziz Cc: Pravin Shelar Cc: Greg Kroah-Hartman Cc: Ben Hutchings Cc: Christoph Lameter Cc: Johannes Weiner Cc: Mel Gorman Cc: Rik van Riel Cc: Andi Kleen Cc: Minchan Kim Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/internal.h | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) (limited to 'mm/internal.h') diff --git a/mm/internal.h b/mm/internal.h index 684f7aa9692a..a85a3ab1f7ef 100644 --- a/mm/internal.h +++ b/mm/internal.h @@ -51,7 +51,8 @@ static inline void __get_page_tail_foll(struct page *page, VM_BUG_ON(page_mapcount(page) < 0); if (get_page_head) atomic_inc(&page->first_page->_count); - atomic_inc(&page->_mapcount); + if (compound_tail_refcounted(page->first_page)) + atomic_inc(&page->_mapcount); } /* -- cgit