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

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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