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

Kirill A. Shutemov
2022-06-19T23:38:35Z

On Thu, Jun 16, 2022 at 08:05:10PM +0300, Kirill A. Shutemov wrote:
> 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

Below is what I meant. Maybe it's not that bad. I donno.

Any opinions?

diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 56822d313b96..69e6b11efa62 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -752,6 +752,16 @@ static bool lam_u48_allowed(void)
return find_vma(mm, DEFAULT_MAP_WINDOW) == NULL;
}

+static void enable_lam_func(void *mm)
+{
+ struct mm_struct *loaded_mm = this_cpu_read(cpu_tlbstate.loaded_mm);
+
+ if (loaded_mm != mm)
+ return;
+
+ switch_mm(loaded_mm, loaded_mm, current);
+}
+
static int prctl_enable_tagged_addr(unsigned long nr_bits)
{
struct mm_struct *mm = current->mm;
@@ -760,10 +770,6 @@ static int prctl_enable_tagged_addr(unsigned long nr_bits)
if (mm->context.lam_cr3_mask)
return -EBUSY;

- /* LAM has to be enabled before spawning threads */
- if (get_nr_threads(current) > 1)
- return -EBUSY;
-
if (!nr_bits) {
return -EINVAL;
} else if (nr_bits <= 6) {
@@ -785,8 +791,8 @@ static int prctl_enable_tagged_addr(unsigned long nr_bits)
return -EINVAL;
}

- /* Update CR3 to get LAM active */
- switch_mm(current->mm, current->mm, current);
+ on_each_cpu_mask(mm_cpumask(mm), enable_lam_func, mm, true);
+
return 0;
}

--
Kirill A. Shutemov