On Sat, 4 Mar 2023 14:35:00 -0800 Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Sat, Mar 4, 2023 at 1:15 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > 17 hotfixes. Eight are for MM and seven are for other parts of the
> > kernel. Seven are cc:stable and eight address post-6.3 issues or were
> > judged unsuitable for -stable backporting.
>
> Hmm. Since this pull didn't fix the gcc note about playing pointer
> games that I get for my allmodconfig test build, and since I _really_
> don't want to have an rc1 release tomorrow with that (valid) warning,
> I fixed it up myself.
Ah. Ying did it this way:
From: Huang Ying <ying.huang@xxxxxxxxx>
Subject: migrate_pages: silence gcc notes for mis-casting
Date: Thu, 2 Mar 2023 09:26:10 +0800
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.
Link: https://lkml.kernel.org/r/20230302012610.17055-1-ying.huang@xxxxxxxxx
Fixes: 64c8902ed441 ("migrate_pages: split unmap_and_move() to _unmap() and _move()")
Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
Cc: Xin Hao <xhao@xxxxxxxxxxxxxxxxx>
Cc: Zi Yan <ziy@xxxxxxxxxx>
Cc: Yang Shi <shy828301@xxxxxxxxx>
Cc: Oscar Salvador <osalvador@xxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Bharata B Rao <bharata@xxxxxxx>
Cc: Alistair Popple <apopple@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
--- a/mm/migrate.c~migrate_pages-silence-gcc-notes-for-mis-casting
+++ a/mm/migrate.c
@@ -1047,7 +1047,16 @@ static void __migrate_folio_extract(stru
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;
_
