From d2ee40eae98d8a41ff27dcdd13b1b656c4c1ad00 Mon Sep 17 00:00:00 2001 From: Jianyu Zhan Date: Wed, 4 Jun 2014 16:08:02 -0700 Subject: [PATCH] mm: introdule compound_head_by_tail() MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit Currently, in put_compound_page(), we have ====== if (likely(!PageTail(page))) { <------ (1) if (put_page_testzero(page)) { /* ¦* By the time all refcounts have been released ¦* split_huge_page cannot run anymore from under us. ¦*/ if (PageHead(page)) __put_compound_page(page); else __put_single_page(page); } return; } /* __split_huge_page_refcount can run under us */ page_head = compound_head(page); <------------ (2) ====== if at (1) , we fail the check, this means page is *likely* a tail page. Then at (2), as compoud_head(page) is inlined, it is : ====== static inline struct page *compound_head(struct page *page) { if (unlikely(PageTail(page))) { <----------- (3) struct page *head = page->first_page; smp_rmb(); if (likely(PageTail(page))) return head; } return page; } ====== here, the (3) unlikely in the case is a negative hint, because it is *likely* a tail page. So the check (3) in this case is not good, so I introduce a helper for this case. So this patch introduces compound_head_by_tail() which deals with a possible tail page(though it could be spilt by a racy thread), and make compound_head() a wrapper on it. This patch has no functional change, and it reduces the object size slightly: text data bss dec hex filename 11003 1328 16 12347 303b mm/swap.o.orig 10971 1328 16 12315 301b mm/swap.o.patched I've ran "perf top -e branch-miss" to observe branch-miss in this case. As Michael points out, it's a slow path, so only very few times this case happens. But I grep'ed the code base, and found there still are some other call sites could be benifited from this helper. And given that it only bloating up the source by only 5 lines, but with a reduced object size. I still believe this helper deserves to exsit. Signed-off-by: Jianyu Zhan Cc: Kirill A. Shutemov Cc: Rik van Riel Cc: Jiang Liu Cc: Peter Zijlstra Cc: Johannes Weiner Cc: Mel Gorman Cc: Andrea Arcangeli Cc: Sasha Levin Cc: Wanpeng Li Cc: Hugh Dickins Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- include/linux/mm.h | 29 +++++++++++++++++------------ mm/swap.c | 2 +- 2 files changed, 18 insertions(+), 13 deletions(-) diff --git a/include/linux/mm.h b/include/linux/mm.h index d677706..3686006 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -407,20 +407,25 @@ static inline void compound_unlock_irqrestore(struct page *page, #endif } +static inline struct page *compound_head_by_tail(struct page *tail) +{ + struct page *head = tail->first_page; + + /* + * page->first_page may be a dangling pointer to an old + * compound page, so recheck that it is still a tail + * page before returning. + */ + smp_rmb(); + if (likely(PageTail(tail))) + return head; + return tail; +} + static inline struct page *compound_head(struct page *page) { - if (unlikely(PageTail(page))) { - struct page *head = page->first_page; - - /* - * page->first_page may be a dangling pointer to an old - * compound page, so recheck that it is still a tail - * page before returning. - */ - smp_rmb(); - if (likely(PageTail(page))) - return head; - } + if (unlikely(PageTail(page))) + return compound_head_by_tail(page); return page; } diff --git a/mm/swap.c b/mm/swap.c index d089c5a..c8d6df5 100644 --- a/mm/swap.c +++ b/mm/swap.c @@ -253,7 +253,7 @@ static void put_compound_page(struct page *page) * Case 3 is possible, as we may race with * __split_huge_page_refcount tearing down a THP page. */ - page_head = compound_head(page); + page_head = compound_head_by_tail(page); if (!__compound_tail_refcounted(page_head)) put_unrefcounted_compound_page(page_head, page); else -- 2.7.4