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

[Karl Mehltretter <kmehltretter@gmail.com>]

On Mon, Aug 31, 2026 at 08:15:52AM +0100, Ard Biesheuvel wrote:
> 
> 
> 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?
> 

Yes, that fixes the issue too.

The commit was patch 3/3 of the following speedup series:

https://lore.kernel.org/r/20240412131908.433043-1-ryan.roberts@arm.com

Patches 1 and 2 accounted for most of the speedup.

I'll wait a bit to give the others a chance to comment.

I can send the revert as v2, or my original patch as v2 with the
missing "Assisted-by: LLM" and "Signed-off-by:" trailers.

Thanks,
Karl