[PATCH 16/16] mm/huge_memory: comment the subtle logic in __split_huge_pmd

[Miaohe Lin <linmiaohe@huawei.com>]

It's dangerous and wrong to call page_folio(pmd_page(*pmd)) when pmd isn't
present. But the caller guarantees pmd is present when folio is set. So we
should be safe here. Add comment to make it clear.

Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
---
 mm/huge_memory.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 8bd937cc1f74..b98b97592bd3 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2234,6 +2234,10 @@ void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
 
 	if (pmd_trans_huge(*pmd) || pmd_devmap(*pmd) ||
 	    is_pmd_migration_entry(*pmd)) {
+		/*
+		 * It's safe to call pmd_page when folio is set because it's
+		 * guaranteed that pmd is present.
+		 */
 		if (folio && folio != page_folio(pmd_page(*pmd)))
 			goto out;
 		__split_huge_pmd_locked(vma, pmd, range.start, freeze);
-- 
2.23.0