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

[Usama Arif <usama.arif@linux.dev>]

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 and a VM_WARN_ON_ONCE to
catch an unexpected refcount != 1 entry state.

Suggested-by: Zi Yan <ziy@nvidia.com>
Signed-off-by: Usama Arif <usama.arif@linux.dev>
---
 mm/migrate_device.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/mm/migrate_device.c b/mm/migrate_device.c
index 78c7acf024615..6fa2878848a7e 100644
--- a/mm/migrate_device.c
+++ b/mm/migrate_device.c
@@ -908,6 +908,11 @@ static int migrate_vma_split_unmapped_folio(struct migrate_vma *migrate,
 	unsigned long flags;
 	int ret = 0;
 
+	VM_WARN_ON_ONCE(folio_ref_count(folio) != 1);
+	/*
+	 * 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