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

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Message-ID: <d39e029a-8c12-42fb-8046-8e4a568134dc@redhat.com>
Date: Tue, 26 Aug 2025 16:28:20 +0200
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
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Subject: Re: [PATCH 08/10] mm: update fork mm->flags initialisation to use
 bitmap

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@...cle.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