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

David Hildenbrand
2025-08-26T13:12:16Z

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? :)

static_assert((u32)MMF_INIT_LEGACY_MASK != MMF_INIT_LEGACY_MASK);

> +
> +/*
> + * 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

Besides the assert simplification

Acked-by: David Hildenbrand <david@redhat.com>

--
Cheers

David / dhildenb