On Fri, Mar 08, 2024 at 07:23:58AM -0800, Dave Hansen wrote:
> On 3/7/24 17:34, Andy Lutomirski wrote:
> >> Fix this by making sure we write a new CR3 if LAM is not
> >> up-to-date. No problems were observed in practice, this was found
> >> by code inspection.
> > I think it should be fixed with a much bigger hammer: explicit IPIs.
> > Just don't ever let it get out of date, like install_ldt().
> I guess it matters whether the thing that matters is having a persistent
> inconsistency or a temporary one. IPIs will definitely turn a permanent
> one into a temporary one.
>
> But this is all easier to reason about if we can get rid of even the
> temporary inconsistency.
>
> Wouldn't this be even simpler than IPIs?
>
> static inline unsigned long set_tlbstate_lam_mode(struct mm_struct *mm)
> {
> unsigned long lam = READ_ONCE(mm->context.lam_cr3_mask);
>
> + /* LAM is for userspace only. Ignore it for kernel threads: */
> + if (tsk->flags & PF_KTHREAD)
> + return 0;
I like this approach. kthread_use_mm() WARNs if it called for
non-PF_KTHREAD task, so it should be okay.
I was worried that it would also exclude io_uring, but worker threads
don't have the flag set.
--
Kiryl Shutsemau / Kirill A. Shutemov
