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

David Hildenbrand
2025-08-26T15:24:29Z

On 26.08.25 16:32, Lorenzo Stoakes wrote:
> On Tue, Aug 26, 2025 at 04:28:20PM +0200, David Hildenbrand wrote:
>> 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.
>
> Yeah, I don't love that sorry. Firstly it's a warning, so you may well miss it
> (I just tried),

Upstream bots usually complain at you for warnings :P

> and secondly you're making the static assert not have any
> meaning except that you expect to trigger a compiler warning, it's a bit
> bizarre.

On 64 bit where BIT(32) *makes any sense* it triggers as expected, no?

>
> My solution works (unless you can see a reason it shouldn't) and I don't find
> this approach any simpler.

Please explain to me like I am a 5 yo how your approach works with
BIT(32) on 32bit when the behavior on 32bit is undefined. :P

--
Cheers

David / dhildenb