Re: [PATCH 6.6.y] arm64: mm: clear extra idmap level before use

[Ard Biesheuvel]

On Mon, 31 Aug 2026, at 07:54, Greg Kroah-Hartman wrote:
> On Mon, Aug 31, 2026 at 07:42:47AM +0200, Karl Mehltretter wrote:
>> The 6.6.y adaptation of commit 0e9df1c905d8 ("arm64: mm: Don't remap
>> pgtables for allocate vs populate") removes the clearing performed by
>> early_pgtable_alloc().  Its replacement clears allocations made by the
>> generic page-table walkers, but 6.6's create_idmap() still allocates an
>> extra root level directly when a sub-48-bit VA kernel is loaded
>> sufficiently high in physical memory.
>> 
>> memblock_phys_alloc_range() does not zero the returned memory.  The direct
>> caller therefore publishes an uncleared root page and passes it to
>> __create_pgd_mapping().  A stale nonzero entry can trip the bad-descriptor
>> BUG_ON or be followed as a page-table descriptor, preventing the kernel
>> from booting.
>> 
>> Clear the direct allocation through its linear alias before publishing it.
>> init_clear_pgtable() also supplies the barrier required before the table
>> descriptor becomes visible.
>> 
>> Mainline is not affected because commit e6128a8e523c ("arm64: mm: Use
>> 48-bit virtual addressing for the permanent ID map") removed the dynamic
>> extra level before page-table initialization moved out of the allocator.
>
> Why can't we take that change instead?
>

Nope. That relies on a massive refactoring of the arm64 startup code.

Can we just revert 0e9df1c905d8 (aka 54322d95309d) instead?