On Thu, Jan 25, 2018 at 11:32:46AM -0800, Tim Chen wrote: > > This patch is not ideal as it comes with the caveats that > patch 2 tries to close. I put it out here to see if it can prompt > people to come up with a better solution. Keeping active_mm around would > have been cleaner but it looks like there are issues that Andy mentioned. > > The "A -> idle -> A" case would not trigger IBPB if > tlb_defer_switch_to_init_mm() > is true (non pcid) as we does not change the mm. > > This patch tries to address the case when we do switch to init_mm and back. > Do you still have objections to the approach in this patch > to save the last active mm before switching to init_mm?
I still think the existing active_mm is sufficient. Something like: switch_mm() { ... if (prev && next != prev) ibpb(); ... } should work. Because while the idle crud does leave_mm() and PCID does enter_lazy_tlb() and both end up doing: switch_mm(NULL, &init_mm, NULL), nothing there affects tsk->active_mm. So over the "A -> idle -> A" transition, active_mm should actually track what you want.