[PATCH -mm -v6 7/9] mm, THP: Add can_split_huge_page()

[Huang, Ying]

From: Huang Ying <ying.hu...@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 the swap
cache, then split the THP.  To avoid the unnecessary operations for the
un-splittable THP, we will check that firstly.

There is no functionality change in this patch.

Cc: Andrea Arcangeli <aarca...@redhat.com>
Cc: Ebru Akagunduz <ebru.akagun...@gmail.com>
Signed-off-by: "Huang, Ying" <ying.hu...@intel.com>
Acked-by: Kirill A. Shutemov <kirill.shute...@linux.intel.com>
---
 include/linux/huge_mm.h |  7 +++++++
 mm/huge_memory.c        | 17 ++++++++++++++---
 2 files changed, 21 insertions(+), 3 deletions(-)

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index a3762d49ba39..d3b3e8fcc717 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -113,6 +113,7 @@ extern unsigned long thp_get_unmapped_area(struct file 
*filp,
 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 *pextra_pins);
 int split_huge_page_to_list(struct page *page, struct list_head *list);
 static inline int split_huge_page(struct page *page)
 {
@@ -231,6 +232,12 @@ static inline void prep_transhuge_page(struct page *page) 
{}
 
 #define thp_get_unmapped_area  NULL
 
+static inline bool
+can_split_huge_page(struct page *page, int *pextra_pins)
+{
+       BUILD_BUG();
+       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 2b4120f6930c..45f944db43b0 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2362,6 +2362,19 @@ 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 *pextra_pins)
+{
+       int extra_pins = 0;
+
+       /* Additional pins from radix tree */
+       if (!PageAnon(page))
+               extra_pins = HPAGE_PMD_NR;
+       if (pextra_pins)
+               *pextra_pins = extra_pins;
+       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.
@@ -2421,8 +2434,6 @@ int split_huge_page_to_list(struct page *page, struct 
list_head *list)
                        goto out;
                }
 
-               /* Addidional pins from radix tree */
-               extra_pins = HPAGE_PMD_NR;
                anon_vma = NULL;
                i_mmap_lock_read(mapping);
        }
@@ -2431,7 +2442,7 @@ 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, &extra_pins)) {
                ret = -EBUSY;
                goto out_unlock;
        }
-- 
2.11.0