On Tue, Aug 26, 2025 at 03:12:08PM +0200, David Hildenbrand wrote:

On 12.08.25 17:44, Lorenzo Stoakes wrote:

quoted

We now need to account for flag initialisation on fork. We retain the
existing logic as much as we can, but dub the existing flag mask legacy.

These flags are therefore required to fit in the first 32-bits of the flags
field.

However, further flag propagation upon fork can be implemented in mm_init()
on a per-flag basis.

We ensure we clear the entire bitmap prior to setting it, and use
__mm_flags_get_word() and __mm_flags_set_word() to manipulate these legacy
fields efficiently.

Signed-off-by: Lorenzo Stoakes <redacted>
---
  include/linux/mm_types.h | 13 ++++++++++---
  kernel/fork.c            |  7 +++++--
  2 files changed, 15 insertions(+), 5 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 38b3fa927997..25577ab39094 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h

@@ -1820,16 +1820,23 @@ enum {
  #define MMF_TOPDOWN		31	/* mm searches top down by default */
  #define MMF_TOPDOWN_MASK	_BITUL(MMF_TOPDOWN)
-#define MMF_INIT_MASK		(MMF_DUMPABLE_MASK | MMF_DUMP_FILTER_MASK |\
+#define MMF_INIT_LEGACY_MASK	(MMF_DUMPABLE_MASK | MMF_DUMP_FILTER_MASK |\
  				 MMF_DISABLE_THP_MASK | MMF_HAS_MDWE_MASK |\
  				 MMF_VM_MERGE_ANY_MASK | MMF_TOPDOWN_MASK)
-static inline unsigned long mmf_init_flags(unsigned long flags)
+/* Legacy flags must fit within 32 bits. */
+static_assert((u64)MMF_INIT_LEGACY_MASK <= (u64)UINT_MAX);

Why not use the magic number 32 you are mentioning in the comment? :)

Meh I mean UINT_MAX works as a good 'any bit' mask and this will work on
both 32-bit and 64-bit systems.

static_assert((u32)MMF_INIT_LEGACY_MASK != MMF_INIT_LEGACY_MASK);

On 32-bit that'd not work would it?

Kinda the point here is that the issue would get picked up on either.

I think with the comment above it's fine as-is! :)

quoted

+
+/*
+ * Initialise legacy flags according to masks, propagating selected flags on
+ * fork. Further flag manipulation can be performed by the caller.

It's weird not reading "initialize", but I am afraid the kernel is already
tainted :P

t14s: ~/git/linux nth_page $ git grep "initialise" | wc -l
1778
t14s: ~/git/linux nth_page $ git grep "initialize" | wc -l
22043

British English in the kernel will survive if I have anything to do with it
;)

Besides the assert simplification

Acked-by: David Hildenbrand <redacted>

Thanks!

--
Cheers

David / dhildenb