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

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Mon, Aug 31, 2026 at 01:00:34PM +0200, Karl Mehltretter wrote:
> 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.

REvert would be great, thanks!

greg k-h