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

[Mike Rapoport]

On Tue, Aug 12, 2025 at 04:44:17PM +0100, 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@xxxxxxxxxx>

Reviewed-by: Mike Rapoport (Microsoft) <rppt@xxxxxxxxxx>

> ---
>  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);
> +
> +/*
> + * Initialise legacy flags according to masks, propagating selected flags on
> + * fork. Further flag manipulation can be performed by the caller.
> + */
> +static inline unsigned long mmf_init_legacy_flags(unsigned long flags)
>  {
>  	if (flags & (1UL << MMF_HAS_MDWE_NO_INHERIT))
>  		flags &= ~((1UL << MMF_HAS_MDWE) |
>  			   (1UL << MMF_HAS_MDWE_NO_INHERIT));
> -	return flags & MMF_INIT_MASK;
> +	return flags & MMF_INIT_LEGACY_MASK;
>  }
>  
>  #endif /* _LINUX_MM_TYPES_H */
> diff --git a/kernel/fork.c b/kernel/fork.c
> index c4ada32598bd..b311caec6419 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1056,11 +1056,14 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
>  	mm_init_uprobes_state(mm);
>  	hugetlb_count_init(mm);
>  
> +	mm_flags_clear_all(mm);
>  	if (current->mm) {
> -		mm->flags = mmf_init_flags(current->mm->flags);
> +		unsigned long flags = __mm_flags_get_word(current->mm);
> +
> +		__mm_flags_set_word(mm, mmf_init_legacy_flags(flags));
>  		mm->def_flags = current->mm->def_flags & VM_INIT_DEF_MASK;
>  	} else {
> -		mm->flags = default_dump_filter;
> +		__mm_flags_set_word(mm, default_dump_filter);
>  		mm->def_flags = 0;
>  	}
>  
> -- 
> 2.50.1
> 

-- 
Sincerely yours,
Mike.