[PATCH v2] mm/migrate_device: document folio_get requirement before frozen PMD split

[Usama Arif]

split_huge_pmd_address() with freeze=true splits a PMD migration entry
into PTE migration entries, consuming one folio reference in the
process.  The folio_get() before it provides this reference.

Add a comment explaining this relationship. The expected folio refcount
at the start of migrate_vma_split_unmapped_folio() is 1.

Suggested-by: Zi Yan <ziy@xxxxxxxxxx>
Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>
Reviewed-by: Nico Pache <npache@xxxxxxxxxx>
Signed-off-by: Usama Arif <usama.arif@xxxxxxxxx>
---
v1 -> v2:
- Remove warning if folio refcount !=1 at the start of
  migrate_vma_split_unmapped_folio() (David)
---
 mm/migrate_device.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/mm/migrate_device.c b/mm/migrate_device.c
index 78c7acf02461..fbfe5715f635 100644
--- a/mm/migrate_device.c
+++ b/mm/migrate_device.c
@@ -908,6 +908,10 @@ static int migrate_vma_split_unmapped_folio(struct migrate_vma *migrate,
 	unsigned long flags;
 	int ret = 0;
 
+	/*
+	 * take a reference, since split_huge_pmd_address() with freeze = true
+	 * drops a reference at the end.
+	 */
 	folio_get(folio);
 	split_huge_pmd_address(migrate->vma, addr, true);
 	ret = folio_split_unmapped(folio, 0);
-- 
2.47.3