[PATCH -mmotm-2009-12-10-17-19] Prevent churning of zero page in LRU list.

[Minchan Kim <minchan.kim@gmail.com>]

VM doesn't add zero page to LRU list. 
It means zero page's churning in LRU list is pointless. 

As a matter of fact, zero page can't be promoted by mark_page_accessed
since it doesn't have PG_lru. 

This patch prevent unecessary mark_page_accessed call of zero page 
alghouth caller want FOLL_TOUCH. 

Signed-off-by: Minchan Kim <minchan.kim@gmail.com>
---
 mm/memory.c |    7 ++++---
 1 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 09e4b1b..485f727 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1152,6 +1152,7 @@ struct page *follow_page(struct vm_area_struct *vma, unsigned long address,
 	spinlock_t *ptl;
 	struct page *page;
 	struct mm_struct *mm = vma->vm_mm;
+	int zero_pfn = 0;
 
 	page = follow_huge_addr(mm, address, flags & FOLL_WRITE);
 	if (!IS_ERR(page)) {
@@ -1196,15 +1197,15 @@ struct page *follow_page(struct vm_area_struct *vma, unsigned long address,
 
 	page = vm_normal_page(vma, address, pte);
 	if (unlikely(!page)) {
-		if ((flags & FOLL_DUMP) ||
-		    !is_zero_pfn(pte_pfn(pte)))
+		zero_pfn = is_zero_pfn(pte_pfn(pte));
+		if ((flags & FOLL_DUMP) || !zero_pfn )
 			goto bad_page;
 		page = pte_page(pte);
 	}
 
 	if (flags & FOLL_GET)
 		get_page(page);
-	if (flags & FOLL_TOUCH) {
+	if (flags & FOLL_TOUCH && !zero_pfn) {
 		if ((flags & FOLL_WRITE) &&
 		    !pte_dirty(pte) && !PageDirty(page))
 			set_page_dirty(page);
-- 
1.5.6.3


-- 
Kind regards,
Minchan Kim

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