From: Usama Arif <usama.arif@linux.dev>
Date: Mon, 9 Mar 2026 21:25:02 +0000 (-0700)
Subject: mm/migrate_device: document folio_get requirement before frozen PMD split
X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=5a14198ec63c130b7377911d3cf99c93547d5fb8;p=thirdparty%2Flinux.git

mm/migrate_device: document folio_get requirement before frozen PMD split

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.

Link: https://lkml.kernel.org/r/20260309212502.3922825-1-usama.arif@linux.dev
Signed-off-by: Usama Arif <usama.arif@linux.dev>
Suggested-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Nico Pache <npache@redhat.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Byungchul Park <byungchul@sk.com>
Cc: Gregory Price <gourry@gourry.net>
Cc: "Huang, Ying" <ying.huang@linux.alibaba.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Rakie Kim <rakie.kim@sk.com>
Cc: Ying Huang <ying.huang@linux.alibaba.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

diff --git a/mm/migrate_device.c b/mm/migrate_device.c
index 8079676c8f1f3..2912eba575d5e 100644
--- a/mm/migrate_device.c
+++ b/mm/migrate_device.c
@@ -914,6 +914,10 @@ 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);