# Re: \[PATCH 03/22\] mm: introduce MMF\_KERNEL flag and set it for init\_mm

- From: Lorenzo Stoakes (ARM) \<ljs@kernel.org\>
- Date: 2026-07-14 15:05:14 
- List: linuxppc-dev
- Page: https://ratatoskr.run/linuxppc-dev/2026/07/17258162
- Thread: https://ratatoskr.run/linux-mm/2026/07/17257816/t.md

```text
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.
>
> 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);
	}

:)

Cheers, Lorenzo

```