Re: [PATCH v4 5/8] s390/mm: Remove folio handling for kernel faults in do_secure_storage_access()

[Claudio Imbrenda <imbrenda@linux.ibm.com>]

On Mon, 20 Jul 2026 12:53:29 +0200
Christian Borntraeger <borntraeger@linux.ibm.com> wrote:

> Am 20.07.26 um 10:58 schrieb Heiko Carstens:
> > Remove folio handling for secure storage access exceptions that happen
> > via the kernel mapping. Using folio_try_get() for exceptions via the
> > kernel mapping is racy: for example such an exception may happen on one
> > CPU, while a different CPU frees the corresponding page. Subsequently the
> > page can be part of a folio of different size, before the faulting CPU
> > executes folio_try_get(). This may lead to data corruption.
> > 
> > As of now this scenario does not seem to be possible, however with the
> > upcoming load_unaligned_zeropad() this is possible.
> > 
> > As a side effect PG_arch_1 is not cleared anymore for such faults via the
> > kernel mapping. However given that this bit is over-indicating it
> > shouldn't matter.
> > 
> > Tested-by: Christian Borntraeger <borntraeger@linux.ibm.com>
> > Signed-off-by: Heiko Carstens <hca@linux.ibm.com>  
> 
> Ideally Claudio should confirm that PG_arch_1 is over-indicating.

PG_arch_1 is over-indicating.

Acked-by: Claudio Imbrenda <imbrenda@linux.ibm.com>

> 
> Reviewed-by: Christian Borntraeger <borntraeger@linux.ibm.com>
> 
> > ---
> >   arch/s390/mm/fault.c | 8 +-------
> >   1 file changed, 1 insertion(+), 7 deletions(-)
> > 
> > diff --git a/arch/s390/mm/fault.c b/arch/s390/mm/fault.c
> > index e650cdfa2523..776549d55c94 100644
> > --- a/arch/s390/mm/fault.c
> > +++ b/arch/s390/mm/fault.c
> > @@ -440,13 +440,7 @@ void do_secure_storage_access(struct pt_regs *regs)
> >   	if (is_kernel_fault(regs)) {
> >   		if (is_vmalloc_addr((void *)addr))
> >   			return handle_fault_error_nolock(regs, 0);
> > -		folio = virt_to_folio((void *)addr);
> > -		if (unlikely(!folio_try_get(folio)))
> > -			return;
> > -		rc = uv_convert_from_secure(folio_to_phys(folio));
> > -		if (!rc)
> > -			clear_bit(PG_arch_1, &folio->flags.f);
> > -		folio_put(folio);
> > +		rc = uv_convert_from_secure(__pa(addr));
> >   		/*
> >   		 * There are some valid fixup types for kernel
> >   		 * accesses to donated secure memory. zeropad is one  
>