Re: [PATCHv3 6/8] x86/mm: Provide ARCH_GET_UNTAG_MASK and ARCH_ENABLE_TAGGED_ADDR

[Kirill A. Shutemov]

On Mon, Jun 13, 2022 at 04:42:57PM +0200, Michal Hocko wrote:
> On Fri 10-06-22 17:35:25, Kirill A. Shutemov wrote:
> [...]
> > diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
> > index 1962008fe743..93c8eba1a66d 100644
> > --- a/arch/x86/kernel/process_64.c
> > +++ b/arch/x86/kernel/process_64.c
> > @@ -742,6 +742,32 @@ static long prctl_map_vdso(const struct vdso_image *image, unsigned long addr)
> >  }
> >  #endif
> >  
> > +static int prctl_enable_tagged_addr(unsigned long nr_bits)
> > +{
> > +	struct mm_struct *mm = current->mm;
> > +
> > +	/* Already enabled? */
> > +	if (mm->context.lam_cr3_mask)
> > +		return -EBUSY;
> > +
> > +	/* LAM has to be enabled before spawning threads */
> > +	if (get_nr_threads(current) > 1)
> > +		return -EBUSY;
> 
> This will not be sufficient in general. You can have mm shared with a
> process without CLONE_THREAD. So you would also need to check also
> MMF_MULTIPROCESS. But I do remember that general get_nr_threads is quite
> tricky to use properly. Make sure to CC Oleg Nesterov for more details.
> 
> Also how does this work when the mm is shared with a kernel thread?

It seems we need to check mm_count to exclude kernel threads that use the
mm. But I expect it to produce bunch of false-positives.

Or we can make all CPUs to do

	switch_mm(current->mm, current->mm, current);

and get LAM bits updated regardless what mm it runs. It would also remove
limitation that LAM can only be enabled when there's no threads.

But I feel that is a bad idea, but I have no clue why. :P

-- 
 Kirill A. Shutemov