+ mm-migrate_device-document-folio_get-requirement-before-frozen-pmd-split.patch added to mm-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: mm/migrate_device: document folio_get requirement before frozen PMD split
has been added to the -mm mm-unstable branch.  Its filename is
     mm-migrate_device-document-folio_get-requirement-before-frozen-pmd-split.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-migrate_device-document-folio_get-requirement-before-frozen-pmd-split.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

Before you just go and hit "reply", please:
   a) Consider who else should be cc'ed
   b) Prefer to cc a suitable mailing list as well
   c) Ideally: find the original patch on the mailing list and do a
      reply-to-all to that, adding suitable additional cc's

*** Remember to use Documentation/process/submit-checklist.rst when testing your code ***

The -mm tree is included into linux-next via various
branches at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
and is updated there most days

------------------------------------------------------
From: Usama Arif <usama.arif@linux.dev>
Subject: mm/migrate_device: document folio_get requirement before frozen PMD split
Date: Mon, 9 Mar 2026 14:25:02 -0700

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>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Byungchul Park <byungchul@sk.com>
Cc: David Hildenbrand <david@kernel.org>
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>
---

 mm/migrate_device.c |    4 ++++
 1 file changed, 4 insertions(+)

--- a/mm/migrate_device.c~mm-migrate_device-document-folio_get-requirement-before-frozen-pmd-split
+++ a/mm/migrate_device.c
@@ -908,6 +908,10 @@ static int migrate_vma_split_unmapped_fo
 	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);
_

Patches currently in -mm which might be from usama.arif@linux.dev are

mm-migrate_device-document-folio_get-requirement-before-frozen-pmd-split.patch