[PATCH 02/13] mm: extract ensure_on_mmlist() helper

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

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 ensure_on_mmlist() 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.

Signed-off-by: Usama Arif <usama.arif@linux.dev>
---
 mm/internal.h | 13 +++++++++++++
 mm/memory.c   |  9 +--------
 mm/rmap.c     |  7 +------
 3 files changed, 15 insertions(+), 14 deletions(-)

diff --git a/mm/internal.h b/mm/internal.h
index 5a2ddcf68e0b..7de489689f54 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -1952,4 +1952,17 @@ 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);
 
+/*
+ * Ensure @mm is on the init_mm.mmlist so swapoff can find it.
+ */
+static inline void ensure_on_mmlist(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 ea6568571131..33d7cc274e23 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -937,14 +937,7 @@ 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);
-		}
+		ensure_on_mmlist(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 78b7fb5f367c..057e18cb80b0 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -2302,12 +2302,7 @@ 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);
-			}
+			ensure_on_mmlist(mm);
 			dec_mm_counter(mm, MM_ANONPAGES);
 			inc_mm_counter(mm, MM_SWAPENTS);
 			swp_pte = swp_entry_to_pte(entry);
-- 
2.52.0