Re: [PATCH 00/11] Always call constructor for kernel page tables

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

On 17/03/2025 16:30, Ryan Roberts wrote:
> On 17/03/2025 14:16, Kevin Brodsky wrote:
>> The complications in those special pgtable allocators beg the question:
>> does it really make sense to treat efi_mm and init_mm differently in
>> e.g. apply_to_pte_range()? Maybe what we really need is a way to tell if
>> an mm corresponds to user memory or not, and never use split locks for
>> non-user mm's. Feedback and suggestions welcome!
> The difference in treatment is whether or not the ptl is taken, right? So the
> real question is when calling apply_to_pte_range() for efi_mm, is there already
> a higher level serialization mechanism that prevents racy accesses? For init_mm,
> I think this is handled implicitly because there is no way for user space to
> cause apply_to_pte_range() for an arbitrary piece of kernel memory. Although I
> can't even see where apply_to_page_range() is called for efi_mm.

The commit I mentioned above, 61444cde9170 ("ARM: 8591/1: mm: use fully
constructed struct pages for EFI pgd allocations"), shows that
apply_to_page_range() is called from efi_set_mapping_permissions(), and
this indeed hasn't changed. It is itself called from efi_virtmap_init().
I would expect that no locking at all is necessary here, since the
mapping has just been created and surely isn't used yet. Now the
question is where exactly init_mm is special-cased in this manner. I can
see that walk_page_range() does something similar, there may be more
cases. And the other question is whether those functions are ever used
on special mm's, aside from efi_set_mapping_permissions().
> FWIW, contpte.c has mm_is_user() which is used by arm64.

Interesting! But not pretty, that's basically checking that the mm is
not &init_mm or &efi_mm... which wouldn't work for a generic
implementation. It feels like adding some attribute to mm_struct
wouldn't hurt. It looks like we've run out of MMF_* flags though :/

- Kevin