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

[Kevin Brodsky <kevin.brodsky@arm.com>]

On 14/07/2026 17:04, Lorenzo Stoakes (ARM) wrote:
> On Tue, Jul 14, 2026 at 07:47:43AM -0700, Dave Hansen wrote:
>> On 7/14/26 07:03, Kevin Brodsky wrote:
>>> +static inline bool mm_is_kernel(const struct mm_struct *mm)
>>> +{
>>> +	return mm && mm_flags_test(MMF_KERNEL, mm);
>>> +}
>> Could we give this some nice comments explaining what a kernel mm is,
>> please? Part of the problem with the init_mm checks is that they're
>> magic and it's not always clear what's special about init_mm.

Agreed, we need to define what this property means exactly, and your
comments on patch 14 show that extending it to efi_mm isn't necessarily
as benign as it appeared to me at first.

My motivation really is about how page tables are handled (in particular
whether ptlocks are used), so as you suggested on patch 19 maybe this
flag should be narrower in scope, and the naming should reflect it.
Possibly MMF_KERNEL_PGTABLES? That's at least one thing that should be
true of all of init_mm, efi_mm and tboot_mm: their page tables use
kernel permissions, not user, and should follow the same rules including
not using ptlocks.

>> Maybe start with this list?
>>
>> 1. There's only one of them.
>> 2. All kernel threads share it. tsk->mm is the same for all kernel
>>    threads.
>> 3. It holds the reference copy of the kernel page tables
>> 4. Userspace can't be entered when it is the current mm
>> 5. It has different TLB flushing rules than userspace mms
>>
>> I _think_ those are universal across all architectures.
> Well point 1 isn't true of efimm or tboot_mm so we possibly need a better
> name :)
>
> "Special" is overloaded too much already. I quite like "eternal" so:
>
> 	static inline bool mm_is_eternal(const struct mm_struct *mm)
> 	{
> 		return mm && mm_flags_test(MMF_ETERNAL, mm);
> 	}

Cheeky but why not! I do wonder whether this conveys the right idea
though. The main point of this flag is that page tables are
allocated/initialised/locked differently; in principle you could have a
temporary non-user mm.

- Kevin