Re: [PATCH 03/22] mm: introduce MMF_KERNEL flag and set it for init_mm

["Christophe Leroy (CS GROUP)" <chleroy@kernel.org>]

Le 14/07/2026 à 16:03, Kevin Brodsky a écrit :
> mm code often needs to know whether some mm represents a kernel or
> user address space. This is currently done by comparing the mm
> pointer with &init_mm; besides not being particularly elegant, this
> ignores the fact that other mm's (e.g. efi_mm) may also represent
> parts of the kernel address space.
> 
> Introduce a new mm flag MMF_KERNEL and set it for init_mm.
> Subsequent patches will use this flag to replace comparisons with
> &init_mm. No functional change is introduced for now.

Did you consider performance impact ? This test is usually done in quite 
critical memory handling functions.

init_mm is known at link time. Before your patch 08/22 there is just a 
comparison of mm (r3) with a constant (loaded in r10):

c0014048 <assert_pte_locked>:
c0014048:	3d 40 c1 09 	lis     r10,-16119
c001404c:	39 4a 03 98 	addi    r10,r10,920
c0014050:	7c 03 50 00 	cmpw    r3,r10
c0014054:	4d 82 00 20 	beqlr
...

After patch 08/22 we have, it first checks that mm is not 0, then it 
loads the word located at mm+528 then AND it with 0x1. This load might 
be costly.

c0014048 <assert_pte_locked>:
c0014048:	2c 03 00 00 	cmpwi   r3,0
c001404c:	7c 85 23 78 	mr      r5,r4
c0014050:	41 82 00 10 	beq     c0014060 <assert_pte_locked+0x18>
c0014054:	81 23 02 10 	lwz     r9,528(r3)
c0014058:	71 29 00 01 	andi.   r9,r9,1
c001405c:	4c 82 00 20 	bnelr
...

Christophe

> 
> Assisted-by: Codex:GPT-5.5
> Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
> ---
>   include/linux/mm_types.h | 10 ++++++++++
>   mm/init-mm.c             |  1 +
>   2 files changed, 11 insertions(+)
> 
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index a2e0dc5892ff..7838ea3aac00 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -1414,6 +1414,8 @@ struct mm_struct {
>   	char flexible_array[] __aligned(__alignof__(unsigned long));
>   };
>   
> +#define MM_FLAGS_INIT(flags)	{ .__mm_flags = { BITMAP_FROM_U64(flags) } }
> +
>   static inline bool mm_flags_test(int flag, const struct mm_struct *mm)
>   {
>   	return test_bit(flag, ACCESS_PRIVATE(&mm->flags, __mm_flags));
> @@ -2001,6 +2003,9 @@ enum {
>   #define MMF_TOPDOWN		31	/* mm searches top down by default */
>   #define MMF_TOPDOWN_MASK	BIT(MMF_TOPDOWN)
>   
> +#define MMF_KERNEL		32	/* mm belongs to the kernel */
> +#define MMF_KERNEL_MASK		BIT_ULL(MMF_KERNEL)
> +
>   #define MMF_INIT_LEGACY_MASK	(MMF_DUMP_FILTER_MASK |\
>   				 MMF_DISABLE_THP_MASK | MMF_HAS_MDWE_MASK |\
>   				 MMF_VM_MERGE_ANY_MASK | MMF_TOPDOWN_MASK)
> @@ -2008,6 +2013,11 @@ enum {
>   /* Legacy flags must fit within 32 bits. */
>   static_assert((u64)MMF_INIT_LEGACY_MASK <= (u64)UINT_MAX);
>   
> +static inline bool mm_is_kernel(const struct mm_struct *mm)
> +{
> +	return mm && mm_flags_test(MMF_KERNEL, mm);
> +}
> +
>   /*
>    * Initialise legacy flags according to masks, propagating selected flags on
>    * fork. Further flag manipulation can be performed by the caller.
> diff --git a/mm/init-mm.c b/mm/init-mm.c
> index 3e792aad7626..93773269bf87 100644
> --- a/mm/init-mm.c
> +++ b/mm/init-mm.c
> @@ -34,6 +34,7 @@ struct mm_struct init_mm = {
>   	.pgd		= swapper_pg_dir,
>   	.mm_users	= ATOMIC_INIT(2),
>   	.mm_count	= ATOMIC_INIT(1),
> +	.flags		= MM_FLAGS_INIT(MMF_KERNEL_MASK),
>   	.write_protect_seq = SEQCNT_ZERO(init_mm.write_protect_seq),
>   	MMAP_LOCK_INITIALIZER(init_mm)
>   	.page_table_lock =  __SPIN_LOCK_UNLOCKED(init_mm.page_table_lock),
>