[PATCH] mm/rmap.c: split huge pmd when it really is

[Wei Yang]

There are two cases to call try_to_unmap_one() with TTU_SPLIT_HUGE_PMD
set:

  * unmap_page()
  * shrink_page_list()

In both case, the page passed to try_to_unmap_one() is PageHead() of the
THP. If this page's mapping address in process is not HPAGE_PMD_SIZE
aligned, this means the THP is not mapped as PMD THP in this process.
This could happen when we do mremap() a PMD size range to an un-aligned
address.

Currently, this case is handled by following check in __split_huge_pmd()
luckily.

  page != pmd_page(*pmd)

This patch checks the address to skip some hard work.

Signed-off-by: Wei Yang <richardw.yang@xxxxxxxxxxxxxxx>
---
 mm/rmap.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index b3e381919835..79b9239f00e3 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1386,7 +1386,19 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
 	    is_zone_device_page(page) && !is_device_private_page(page))
 		return true;
 
-	if (flags & TTU_SPLIT_HUGE_PMD) {
+	/*
+	 * There are two places set TTU_SPLIT_HUGE_PMD
+	 *
+	 *     unmap_page()
+	 *     shrink_page_list()
+	 *
+	 * In both cases, the "page" here is the PageHead() of the THP.
+	 *
+	 * If the page is not a real PMD huge page, e.g. after mremap(), it is
+	 * not necessary to split.
+	 */
+	if ((flags & TTU_SPLIT_HUGE_PMD) &&
+		IS_ALIGNED(address, HPAGE_PMD_SIZE)) {
 		split_huge_pmd_address(vma, address,
 				flags & TTU_SPLIT_FREEZE, page);
 	}
-- 
2.17.1