[v2 PATCH] mm: mempolicy: don't have to split pmd for huge zero page

[Yang Shi]

When trying to migrate pages to obey mempolicy, the huge zero page is
split by inserting base zero pfn to all PTEs, then the page table walk
fallback to PTE level and just skips zero page.  Skipping zero page for
mempolicy has been the behavior of kernel since v2.6.16 due to commit
f4598c8b3678 ("[PATCH] migration: make sure there is no attempt to migrate
reserved pages.").  So it seems pointless to split huge zero page, it could
be just skipped like base zero page.

Set ACTION_CONTINUE to prevent the walk_page_range() split the pmd for
this case.

Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Signed-off-by: Yang Shi <shy828301@xxxxxxxxx>
---
v2: Rephrased the commit log per Michal Hocko.
    Collected the review and ack tags.

 mm/mempolicy.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index b5f4f584009b..205c1a768775 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -436,7 +436,8 @@ static inline bool queue_pages_required(struct page *page,
 
 /*
  * queue_pages_pmd() has four possible return values:
- * 0 - pages are placed on the right node or queued successfully.
+ * 0 - pages are placed on the right node or queued successfully, or
+ *     special page is met, i.e. huge zero page.
  * 1 - there is unmovable page, and MPOL_MF_MOVE* & MPOL_MF_STRICT were
  *     specified.
  * 2 - THP was split.
@@ -460,8 +461,7 @@ static int queue_pages_pmd(pmd_t *pmd, spinlock_t *ptl, unsigned long addr,
 	page = pmd_page(*pmd);
 	if (is_huge_zero_page(page)) {
 		spin_unlock(ptl);
-		__split_huge_pmd(walk->vma, pmd, addr, false, NULL);
-		ret = 2;
+		walk->action = ACTION_CONTINUE;
 		goto out;
 	}
 	if (!queue_pages_required(page, qp))
@@ -488,7 +488,8 @@ static int queue_pages_pmd(pmd_t *pmd, spinlock_t *ptl, unsigned long addr,
  * and move them to the pagelist if they do.
  *
  * queue_pages_pte_range() has three possible return values:
- * 0 - pages are placed on the right node or queued successfully.
+ * 0 - pages are placed on the right node or queued successfully, or
+ *     special page is met, i.e. zero page.
  * 1 - there is unmovable page, and MPOL_MF_MOVE* & MPOL_MF_STRICT were
  *     specified.
  * -EIO - only MPOL_MF_STRICT was specified and an existing page was already
-- 
2.26.2