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Subject[PATCH] mm: adding comment why mark_page_accessed() would be better than pte_mkyoung() in follow_page()
Date

I and hugh discussed about mark_page_accessed() in follow_page() should be
change or not. and we agreed it isn't needed without adding comment.

==
At first look, mark_page_accessed() in follow_page() seems a bit strange.
it seems pte_mkyoung() would be better and to consist other kernel code.

However, it is intentionally. past commitlog said,

------------------------------------------------
commit 9e45f61d69be9024a2e6bef3831fb04d90fac7a8
Author: akpm <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.


Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Cc: Nick Piggin <npiggin@suse.de>
---
mm/memory.c | 5 +++++
1 file changed, 5 insertions(+)

Index: b/mm/memory.c
===================================================================
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1151,6 +1151,11 @@ struct page *follow_page(struct vm_area_
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 dirty bit: easier to
+ * mark_page_accessed().
+ */
mark_page_accessed(page);
}
unlock:



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