[PATCH V1 2/2] mm: document when rmap locks can be skipped when setting need_rmap_locks

[Harry Yoo <harry.yoo@oracle.com>]

While move_ptes() explains when rmap locks can be skipped, when reading
the code setting pmc.need_rmap_locks it is not immediately obvious when
need_rmap_locks can be false. Add a brief explanation in copy_vma() and
relocate_vma_down(), and add a pointer to the comment in move_ptes().

Meanwhile, fix and improve the comment in move_ptes().

Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
---
 mm/mremap.c   | 4 +++-
 mm/vma.c      | 7 +++++++
 mm/vma_exec.c | 5 +++++
 3 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/mm/mremap.c b/mm/mremap.c
index e618a706aff5..86adb872bea0 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -218,8 +218,10 @@ static int move_ptes(struct pagetable_move_control *pmc,
 	 * When need_rmap_locks is false, we use other ways to avoid
 	 * such races:
 	 *
-	 * - During exec() shift_arg_pages(), we use a specially tagged vma
+	 * - During exec() relocate_vma_down(), we use a specially tagged vma
 	 *   which rmap call sites look for using vma_is_temporary_stack().
+	 *   Folios mapped in the temporary stack vma cannot be migrated until
+	 *   the relocation is complete.
 	 *
 	 * - During mremap(), new_vma is often known to be placed after vma
 	 *   in rmap traversal order. This ensures rmap will always observe
diff --git a/mm/vma.c b/mm/vma.c
index 3b12c7579831..3da49f79e9ba 100644
--- a/mm/vma.c
+++ b/mm/vma.c
@@ -1842,6 +1842,11 @@ struct vm_area_struct *copy_vma(struct vm_area_struct **vmap,
 	vmg.next = vma_iter_next_rewind(&vmi, NULL);
 	new_vma = vma_merge_new_range(&vmg);
 
+	/*
+	 * rmap locks can be skipped as long as new_vma is traversed
+	 * after vma during rmap walk (new_vma->vm_pgoff >= vma->vm_pgoff).
+	 * See the comment in move_ptes().
+	 */
 	if (new_vma) {
 		/*
 		 * Source vma may have been merged into new_vma
@@ -1879,6 +1884,8 @@ struct vm_area_struct *copy_vma(struct vm_area_struct **vmap,
 			new_vma->vm_ops->open(new_vma);
 		if (vma_link(mm, new_vma))
 			goto out_vma_link;
+
+		/* new_vma->pg_off is always >= vma->pg_off if not merged */
 		*need_rmap_locks = false;
 	}
 	return new_vma;
diff --git a/mm/vma_exec.c b/mm/vma_exec.c
index 922ee51747a6..a895dd39ac46 100644
--- a/mm/vma_exec.c
+++ b/mm/vma_exec.c
@@ -63,6 +63,11 @@ int relocate_vma_down(struct vm_area_struct *vma, unsigned long shift)
 	 * process cleanup to remove whatever mess we made.
 	 */
 	pmc.for_stack = true;
+	/*
+	 * pmc.need_rmap_locks is false since rmap locks can be safely skipped
+	 * because migration is disabled for this vma during relocation.
+	 * See the comment in move_ptes().
+	 */
 	if (length != move_page_tables(&pmc))
 		return -ENOMEM;
 
-- 
2.43.0