Re: [PATCH] x86/mm: fix _pgd_alloc() for Xen PV mode

[Petr Vaněk]

On Thu, Apr 17, 2025 at 04:48:08PM +0200, Juergen Gross wrote:
> Recently _pgd_alloc() was switched from using __get_free_pages() to
> pagetable_alloc_noprof(), which might return a compound page in case
> the allocation order is larger than 0.
> 
> On x86 this will be the case if CONFIG_MITIGATION_PAGE_TABLE_ISOLATION
> is set, even if PTI has been disabled at runtime.
> 
> When running as a Xen PV guest (this will always disable PTI), using
> a compound page for a PGD will result in VM_BUG_ON_PGFLAGS being
> triggered when the Xen code tries to pin the PGD.

> Fix the Xen issue together with the not needed 8k allocation for a
> PGD with PTI disabled by using a variable holding the PGD allocation
> order in case CONFIG_MITIGATION_PAGE_TABLE_ISOLATION is set.
> 
> Reported-by: Petr Vaněk <arkamar@xxxxxxxx>
> Fixes: a9b3c355c2e6 ("asm-generic: pgalloc: provide generic __pgd_{alloc,free}")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>

I have runtime tested this patch, and it fixes the reported issue. The
following trailers can be appended to the commit message (as per [1]):

Closes: https://lore.kernel.org/lkml/202541612720-Z_-deOZTOztMXHBh-arkamar@xxxxxxxx/
Tested-by: Petr Vaněk <arkamar@xxxxxxxx>

Cheers,
Petr

[1] https://docs.kernel.org/process/5.Posting.html#patch-formatting-and-changelogs