[PATCH] migrate_pages: silence gcc notes for mis-casting

[Huang Ying <ying.huang@intel.com>]

The following GCC notes was reported for commit 64c8902ed441
("migrate_pages: split unmap_and_move() to _unmap() and _move()").

    mm/migrate.c: In function ‘__migrate_folio_extract’:
    mm/migrate.c:1050:20: note: randstruct: casting between randomized
structure pointer types (ssa): ‘struct anon_vma’ and ‘struct
address_space’

     1050 |         *anon_vmap = (void *)dst->mapping;
          |         ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~

The casting itself is safe.  Because we only use dst->mapping to store
the pointer itself temporarily and dst is a newly allocated folio and
not used by anyone else during that.  But the notes should be silenced
and some comments are deserved.  So, we do that in this patch.

Fixes: 64c8902ed441 ("migrate_pages: split unmap_and_move() to _unmap() and _move()")
Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Xin Hao <xhao@linux.alibaba.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Bharata B Rao <bharata@amd.com>
Cc: Alistair Popple <apopple@nvidia.com>
---
 mm/migrate.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index c17ce5ee8d92..53fcc23025c7 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1047,7 +1047,16 @@ static void __migrate_folio_extract(struct folio *dst,
 				   int *page_was_mappedp,
 				   struct anon_vma **anon_vmap)
 {
-	*anon_vmap = (void *)dst->mapping;
+	struct anon_vma *anon_vma;
+
+	/*
+	 * 2 steps assignment to silence gcc notes for mis-casting. The
+	 * casting is safe.  Because we only use dst->mapping to store
+	 * the pointer itself temporarily and dst is a newly allocated
+	 * folio and not used by anyone else during that.
+	 */
+	anon_vma = (void *)dst->mapping;
+	*anon_vmap = anon_vma;
 	*page_was_mappedp = (unsigned long)dst->private;
 	dst->mapping = NULL;
 	dst->private = NULL;
-- 
2.39.2