From b707057443702993350c7a7740ef6135fee9befe Mon Sep 17 00:00:00 2001
From: Usama Arif <usama.arif@linux.dev>
Date: Mon, 6 Jul 2026 04:42:04 -0700
Subject: [PATCH] mm: extract mm_prepare_for_swap_entries() helper

When a swap entry is installed in a page table, the mm must be added to
init_mm.mmlist so that swapoff can find and unuse its swap entries.  This
double-checked locking pattern is currently open-coded in
try_to_unmap_one() and copy_nonpresent_pte().

Move it into mm_prepare_for_swap_entries() in mm/internal.h and convert
both callers so it can be reused by upcoming PMD-level swap entry code
paths that also need to register the mm with swapoff.

copy_nonpresent_pte() previously inserted into &src_mm->mmlist rather than
&init_mm.mmlist, but the insertion point is irrelevant, mmlist is a
circular list and swapoff walks it entirely from init_mm.mmlist, so only
membership matters, not position.

Link: https://lore.kernel.org/20260706114320.1643046-3-usama.arif@linux.dev
Signed-off-by: Usama Arif <usama.arif@linux.dev>
Reviewed-by: Dev Jain <dev.jain@arm.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Alexandre Ghiti <alex@ghiti.fr>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Baoquan He <baoquan.he@linux.dev>
Cc: Barry Song <baohua@kernel.org>
Cc: Chris Li <chrisl@kernel.org>
Cc: "Huang, Ying" <ying.huang@linux.alibaba.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kairui Song <kasong@tencent.com>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Kiryl Shutsemau <kas@kernel.org>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Nico Pache <npache@redhat.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Vlastimil Babka <vbabka@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---
 mm/internal.h | 10 ++++++++++
 mm/memory.c   |  9 +--------
 mm/rmap.c     |  7 +------
 3 files changed, 12 insertions(+), 14 deletions(-)

diff --git a/mm/internal.h b/mm/internal.h
index 430aa72a4575..0360648e5726 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -1955,4 +1955,14 @@ static inline int get_sysctl_max_map_count(void)
 bool may_expand_vm(struct mm_struct *mm, const vma_flags_t *vma_flags,
 		   unsigned long npages);
 
+static inline void mm_prepare_for_swap_entries(struct mm_struct *mm)
+{
+	if (list_empty(&mm->mmlist)) {
+		spin_lock(&mmlist_lock);
+		if (list_empty(&mm->mmlist))
+			list_add(&mm->mmlist, &init_mm.mmlist);
+		spin_unlock(&mmlist_lock);
+	}
+}
+
 #endif	/* __MM_INTERNAL_H */
diff --git a/mm/memory.c b/mm/memory.c
index a3fcaf8cfe8f..6637c5b13c9b 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -953,14 +953,7 @@ copy_nonpresent_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
 		if (swap_dup_entry_direct(entry) < 0)
 			return -EIO;
 
-		/* make sure dst_mm is on swapoff's mmlist. */
-		if (unlikely(list_empty(&dst_mm->mmlist))) {
-			spin_lock(&mmlist_lock);
-			if (list_empty(&dst_mm->mmlist))
-				list_add(&dst_mm->mmlist,
-						&src_mm->mmlist);
-			spin_unlock(&mmlist_lock);
-		}
+		mm_prepare_for_swap_entries(dst_mm);
 		/* Mark the swap entry as shared. */
 		if (pte_swp_exclusive(orig_pte)) {
 			pte = pte_swp_clear_exclusive(orig_pte);
diff --git a/mm/rmap.c b/mm/rmap.c
index 1c77d5dc06e9..b93caabd186f 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -2304,12 +2304,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
 				set_pte_at(mm, address, pvmw.pte, pteval);
 				goto walk_abort;
 			}
-			if (list_empty(&mm->mmlist)) {
-				spin_lock(&mmlist_lock);
-				if (list_empty(&mm->mmlist))
-					list_add(&mm->mmlist, &init_mm.mmlist);
-				spin_unlock(&mmlist_lock);
-			}
+			mm_prepare_for_swap_entries(mm);
 			dec_mm_counter(mm, MM_ANONPAGES);
 			inc_mm_counter(mm, MM_SWAPENTS);
 			swp_pte = swp_entry_to_pte(entry);