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

Liam R. Howlett
2025-08-12T17:32:47Z

* Lorenzo Stoakes <lorenzo.stoakes@oracle.com> [250812 11:47]:
> 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>

Reviewed-by: Liam R. Howlett <Liam.Howlett@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);
> +
> +/*
> + * 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
>