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

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Message-ID: <73a39f45-e10c-42f8-819b-7289733eae03@redhat.com>
Date: Tue, 26 Aug 2025 15:12:08 +0200
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: Alexander Gordeev <agordeev@...ux.ibm.com>,
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Subject: Re: [PATCH 08/10] mm: update fork mm->flags initialisation to use
 bitmap

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? :)

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@...hat.com>

-- 
Cheers

David / dhildenb