Re: [PATCH 16/22] efi: mark efi_mm as a kernel mm

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] Incomplete implementation: split page table locks are still initialized for efi_mm on ARM64 and RISC-V.
--

--- Patch [16]: [PATCH 16/22] efi: mark efi_mm as a kernel mm ---
commit ebd104815a1ccc8b9a1160dde0da2bdd7e88ce96
Author: Kevin Brodsky <kevin.brodsky@arm.com>

efi: mark efi_mm as a kernel mm

This commit marks efi_mm with the MMF_KERNEL flag, treating it as a
kernel-owned address space. This avoids unnecessary complications by skipping
split page table lock initialization and constructor/destructor calls.

> diff --git a/drivers/firmware/efi/efi.c b/drivers/firmware/efi/efi.c
> index 0327a39d31fa5..e5ca45bdc890c 100644
> --- a/drivers/firmware/efi/efi.c
> +++ b/drivers/firmware/efi/efi.c
> @@ -69,6 +69,7 @@ struct mm_struct efi_mm = {
>  	.mm_mt			= MTREE_INIT_EXT(mm_mt, MM_MT_FLAGS, efi_mm.mmap_lock),
>  	.mm_users		= ATOMIC_INIT(2),
>  	.mm_count		= ATOMIC_INIT(1),
> +	.flags			= MM_FLAGS_INIT(MMF_KERNEL_MASK),

[Severity: Medium]
The commit message notes that split page table locks are no longer initialized
for efi_mm's page tables. Is this accurate for all architectures?

On ARM64 and RISC-V, the EFI page table allocation paths pass NULL instead of
the mm_struct to the generic page table constructors:

arch/arm64/mm/mmu.c:pgd_pgtable_alloc_special_mm() {
    ...
    return __pgd_pgtable_alloc(NULL, GFP_PGTABLE_KERNEL, pgtable_level);
}

arch/riscv/mm/init.c:alloc_pte_late() {
    ...
    BUG_ON(!ptdesc || !pagetable_pte_ctor(NULL, ptdesc));
    ...
}

Because NULL is passed instead of &efi_mm, the constructors cannot check the
MMF_KERNEL flag, meaning the split page table locks will still be
unconditionally initialized.

Does this optimization require updating the architecture-specific EFI allocation
paths to pass the mm_struct instead of NULL?

>  	.write_protect_seq      = SEQCNT_ZERO(efi_mm.write_protect_seq),
>  	MMAP_LOCK_INITIALIZER(efi_mm)
>  	.page_table_lock	= __SPIN_LOCK_UNLOCKED(efi_mm.page_table_lock),

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260714-remove_pgtable_cdtor-v1-0-44be8a7685d7@arm.com?part=16