On Fri, Jun 23, 2017 at 1:42 AM, Borislav Petkov <bp@xxxxxxxxx> wrote: > On Thu, Jun 22, 2017 at 11:08:38AM -0700, Andy Lutomirski wrote: >> Yes, I agree it's confusing. There really are three numbers. Those >> numbers are: the latest generation, the generation that this CPU has >> caught up to, and the generation that the requester of the flush we're >> currently handling has asked us to catch up to. I don't see a way to >> reduce the complexity. > > Yeah, can you pls put that clarification what what is, over it. It > explains it nicely what the check is supposed to do. Done. I've tried to improve a bunch of the comments in this function. > >> >> The flush IPI hits after a switch_mm_irqs_off() call notices the >> >> change from 1 to 2. switch_mm_irqs_off() will do a full flush and >> >> increment the local tlb_gen to 2, and the IPI handler for the partial >> >> flush will see local_tlb_gen == mm_tlb_gen - 1 (because local_tlb_gen >> >> == 2 and mm_tlb_gen == 3) and do a partial flush. >> > >> > Why, the 2->3 flush has f->end == TLB_FLUSH_ALL. >> > >> > That's why you have this thing in addition to the tlb_gen. >> >> Yes. The idea is that we only do remote partial flushes when it's >> 100% obvious that it's safe. > > So why wouldn't my simplified suggestion work then? > > if (f->end != TLB_FLUSH_ALL && > mm_tlb_gen == local_tlb_gen + 1) > > 1->2 is a partial flush - gets promoted to a full one > 2->3 is a full flush - it will get executed as one due to the f->end setting to > TLB_FLUSH_ALL. This could still fail in some cases, I think. Suppose 1->2 is a partial flush and 2->3 is a full flush. We could have this order of events: - CPU 1: Partial flush. Increase context.tlb_gen to 2 and send IPI. - CPU 0: switch_mm(), observe mm_tlb_gen == 2, set local_tlb_gen to 2. - CPU 2: Full flush. Increase context.tlb_gen to 3 and send IPI. - CPU 0: Receive partial flush IPI. mm_tlb_gen == 2 and local_tlb_gen == 3. Do __flush_tlb_single() and set local_tlb_gen to 3. Our invariant is now broken: CPU 0's percpu tlb_gen is now ahead of its actual TLB state. - CPU 0: Receive full flush IPI and skip the flush. Oops. I think my condition makes it clear that the invariants we need hold no matter it. > >> It could be converted to two full flushes or to just one, I think, >> depending on what order everything happens in. > > Right. One flush at the right time would be optimal. > >> But this approach of using three separate tlb_gen values seems to >> cover all the bases, and I don't think it's *that* bad. > > Sure. > > As I said in IRC, let's document that complexity then so that when we > stumble over it in the future, we at least know why it was done this > way. I've given it a try. Hopefully v4 is more clear. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
