Re: [PATCH 08/10] mm: update fork mm->flags initialisation to use bitmap

David Hildenbrand
2025-08-26T14:28:29Z

On 26.08.25 16:21, Lorenzo Stoakes wrote:
> On Tue, Aug 26, 2025 at 03:12:08PM +0200, David Hildenbrand wrote:
>> On 12.08.25 17:44, Lorenzo Stoakes wrote:
>>> 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 <lorenzo.stoakes@oracle.com>
>>> ---
>>> 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?

On 32bit, BIT(32) would exceed the shift width of unsigned long ->
undefined behavior.

The compiler should naturally complain.

--
Cheers

David / dhildenb