[Patch v2] mm/rmap.c: split huge pmd when it really is

[Wei Yang]

When page is not NULL, function is called by try_to_unmap_one() with
TTU_SPLIT_HUGE_PMD set. 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 work.

Signed-off-by: Wei Yang <richardw.yang@xxxxxxxxxxxxxxx>

---
v2: move the check into split_huge_pmd_address().
---
 mm/huge_memory.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 893fecd5daa4..2b9c2f412b32 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2342,6 +2342,22 @@ void split_huge_pmd_address(struct vm_area_struct *vma, unsigned long address,
 	pud_t *pud;
 	pmd_t *pmd;
 
+	/*
+	 * When page is not NULL, function is called by try_to_unmap_one()
+	 * with TTU_SPLIT_HUGE_PMD set. There are two places set
+	 * TTU_SPLIT_HUGE_PMD
+	 *
+	 *     unmap_page()
+	 *     shrink_page_list()
+	 *
+	 * In both cases, the "page" here is the PageHead() of a THP.
+	 *
+	 * If the page is not a PMD mapped huge page, e.g. after mremap(), it
+	 * is not necessary to split it.
+	 */
+	if (page && !IS_ALIGNED(address, HPAGE_PMD_SIZE))
+		return;
+
 	pgd = pgd_offset(vma->vm_mm, address);
 	if (!pgd_present(*pgd))
 		return;
-- 
2.17.1