linux-kernel - Re: [PATCH v5 21/23] KVM: S390: Remove PGSTE code from linux/s390 mm

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Message-ID: <20251126094704.184c4987@p-imbrenda>
Date: Wed, 26 Nov 2025 09:47:04 +0100
From: Claudio Imbrenda <imbrenda@...ux.ibm.com>
To: Heiko Carstens <hca@...ux.ibm.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-s390@...r.kernel.org, borntraeger@...ibm.com,
        frankja@...ux.ibm.com, nsg@...ux.ibm.com, nrb@...ux.ibm.com,
        seiden@...ux.ibm.com, gra@...ux.ibm.com, schlameuss@...ux.ibm.com,
        svens@...ux.ibm.com, agordeev@...ux.ibm.com, gor@...ux.ibm.com,
        david@...hat.com, gerald.schaefer@...ux.ibm.com
Subject: Re: [PATCH v5 21/23] KVM: S390: Remove PGSTE code from linux/s390
 mm

On Wed, 26 Nov 2025 09:38:18 +0100
Heiko Carstens <hca@...ux.ibm.com> wrote:

> On Tue, Nov 25, 2025 at 08:24:12PM +0100, Heiko Carstens wrote:
> > On Mon, Nov 24, 2025 at 12:55:52PM +0100, Claudio Imbrenda wrote:  
> > > Remove the PGSTE config option.
> > > Remove all code from linux/s390 mm that involves PGSTEs.
> > > 
> > > Signed-off-by: Claudio Imbrenda <imbrenda@...ux.ibm.com>
> > > ---
> > >  arch/s390/Kconfig               |   3 -
> > >  arch/s390/include/asm/mmu.h     |  13 -
> > >  arch/s390/include/asm/page.h    |   4 -
> > >  arch/s390/include/asm/pgalloc.h |   4 -
> > >  arch/s390/include/asm/pgtable.h | 121 +----
> > >  arch/s390/kvm/dat.h             |   1 +
> > >  arch/s390/mm/hugetlbpage.c      |  24 -
> > >  arch/s390/mm/pgalloc.c          |  24 -
> > >  arch/s390/mm/pgtable.c          | 829 +-------------------------------
> > >  mm/khugepaged.c                 |   9 -
> > >  10 files changed, 17 insertions(+), 1015 deletions(-)  
> > 
> > ...
> >   
> > >  pte_t ptep_modify_prot_start(struct vm_area_struct *vma, unsigned long addr,
> > >  			     pte_t *ptep)
> > >  {
> > > +	preempt_disable();
> > > +	return ptep_flush_lazy(vma->vm_mm, addr, ptep, 1);
> > >  }
> > >  
> > >  void ptep_modify_prot_commit(struct vm_area_struct *vma, unsigned long addr,
> > >  			     pte_t *ptep, pte_t old_pte, pte_t pte)
> > >  {  
> ...
> > > +	set_pte(ptep, pte);
> > > +	preempt_enable();
> > >  }  
> > 
> > Why did you add the preempt_disable()/preempt_enable() pair?
> > This causes preempt_count overflows.
> > 
> > See modify_prot_start_ptes() + modify_prot_commit_ptes()...  
> 
> Ah, I guess this is probably just a rebase error, which by accident
> re-introduced the code which was removed with commit 57834ce5a6a4
> ("s390/mm: Prevent possible preempt_count overflow").

that would indeed explain where that code comes from...

I'll fix it