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

[Lorenzo Stoakes]

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@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);
>
> 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?

Kinda the point here is that the issue would get picked up on either.

I think with the comment above it's fine as-is! :)

>
> > +
> > +/*
> > + * 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

British English in the kernel will survive if I have anything to do with it
;)

>
> Besides the assert simplification
>
> Acked-by: David Hildenbrand <david@xxxxxxxxxx>

Thanks!

>
> --
> Cheers
>
> David / dhildenb
>