From bd775c42ea5f7c766d03a287083837cf05e7e738 Mon Sep 17 00:00:00 2001 From: KOSAKI Motohiro Date: Tue, 31 Mar 2009 15:19:37 -0700 Subject: [PATCH] mm: add comment why mark_page_accessed() would be better than pte_mkyoung() in follow_page() At first look, mark_page_accessed() in follow_page() seems a bit strange. It seems pte_mkyoung() would be better consistent with other kernel code. However, it is intentional. The commit log said: ------------------------------------------------ commit 9e45f61d69be9024a2e6bef3831fb04d90fac7a8 Author: akpm Date: Fri Aug 15 07:24:59 2003 +0000 [PATCH] Use mark_page_accessed() in follow_page() Touching a page via follow_page() counts as a reference so we should be either setting the referenced bit in the pte or running mark_page_accessed(). Altering the pte is tricky because we haven't implemented an atomic pte_mkyoung(). And mark_page_accessed() is better anyway because it has more aging state: it can move the page onto the active list. BKrev: 3f3c8acbplT8FbwBVGtth7QmnqWkIw ------------------------------------------------ The atomic issue is still true nowadays. adding comment help to understand code intention and it would be better. [akpm@linux-foundation.org: clarify text] Signed-off-by: KOSAKI Motohiro Signed-off-by: Hugh Dickins Cc: Nick Piggin Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/memory.c | 5 +++++ 1 file changed, 5 insertions(+) diff --git a/mm/memory.c b/mm/memory.c index 0017111..5b4ad5e 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -1151,6 +1151,11 @@ struct page *follow_page(struct vm_area_struct *vma, unsigned long address, if ((flags & FOLL_WRITE) && !pte_dirty(pte) && !PageDirty(page)) set_page_dirty(page); + /* + * pte_mkyoung() would be more correct here, but atomic care + * is needed to avoid losing the dirty bit: it is easier to use + * mark_page_accessed(). + */ mark_page_accessed(page); } unlock: -- 1.8.2.3