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

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Date:   Fri,  4 Jun 2021 13:35:13 -0700
From:   Yang Shi <shy828301@...il.com>
To:     ziy@...dia.com, nao.horiguchi@...il.com, mhocko@...e.com,
        kirill.shutemov@...ux.intel.com, hughd@...gle.com,
        akpm@...ux-foundation.org
Cc:     shy828301@...il.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH] mm: mempolicy: don't have to split pmd for huge zero page

When trying to migrate pages to obey mempolicy, the huge zero page is
split then the page table walk at PTE level just skips zero page.  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.

Signed-off-by: Yang Shi <shy828301@...il.com>
---
 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