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Subject[RFC 09/11] mm, THP: Add can_split_huge_page()
Date
From: Huang Ying <ying.huang@intel.com>

Separates checking whether we can split the huge page from
split_huge_page_to_list() into a function. This will help to check that
before splitting the THP (Transparent Huge Page) really.

This will be used for delaying splitting THP during swapping out. Where
for a THP, we will allocate a swap cluster, add the THP into swap cache,
then split the THP. To avoid unnecessary operations for un-splittable
THP, we will check that firstly.

There is no functionality change in this patch.

Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Ebru Akagunduz <ebru.akagunduz@gmail.com>
Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
---
include/linux/huge_mm.h | 6 ++++++
mm/huge_memory.c | 13 ++++++++++++-
2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index 6f14de4..95ccbb4 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -90,6 +90,7 @@ extern unsigned long transparent_hugepage_flags;
extern void prep_transhuge_page(struct page *page);
extern void free_transhuge_page(struct page *page);

+bool can_split_huge_page(struct page *page);
int split_huge_page_to_list(struct page *page, struct list_head *list);
static inline int split_huge_page(struct page *page)
{
@@ -169,6 +170,11 @@ void put_huge_zero_page(void);
static inline void prep_transhuge_page(struct page *page) {}

#define transparent_hugepage_flags 0UL
+static inline bool
+can_split_huge_page(struct page *page)
+{
+ return false;
+}
static inline int
split_huge_page_to_list(struct page *page, struct list_head *list)
{
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 2373f0a..af65413 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1954,6 +1954,17 @@ int page_trans_huge_mapcount(struct page *page, int *total_mapcount)
return ret;
}

+/* Racy check whether the huge page can be split */
+bool can_split_huge_page(struct page *page)
+{
+ int extra_pins = 0;
+
+ /* Additional pins from radix tree */
+ if (!PageAnon(page))
+ extra_pins = HPAGE_PMD_NR;
+ return total_mapcount(page) == page_count(page) - extra_pins - 1;
+}
+
/*
* This function splits huge page into normal pages. @page can point to any
* subpage of huge page to split. Split doesn't change the position of @page.
@@ -2024,7 +2035,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
* Racy check if we can split the page, before freeze_page() will
* split PMDs
*/
- if (total_mapcount(head) != page_count(head) - extra_pins - 1) {
+ if (!can_split_huge_page(head)) {
ret = -EBUSY;
goto out_unlock;
}
--
2.8.1
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