Messages in this thread | | | Date | Thu, 19 Mar 2009 21:40:29 -0700 | From | "Paul E. McKenney" <> | Subject | Re: Question about x86/mm/gup.c's use of disabled interrupts |
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On Thu, Mar 19, 2009 at 10:31:55AM -0700, Jeremy Fitzhardinge wrote: > Nick Piggin wrote: >>> Also, assuming that disabling the interrupt is enough to get the >>> guarantees we need here, there's a Xen problem because we don't use IPIs >>> for cross-cpu tlb flushes (well, it happens within Xen). I'll have to >>> think a bit about how to deal with that, but I'm thinking that we could >>> add a per-cpu "tlb flushes blocked" flag, and maintain some kind of >>> per-cpu deferred tlb flush count so we can get around to doing the flush >>> eventually. >>> >>> But I want to make sure I understand the exact algorithm here. >> >> FWIW, powerpc actually can flush tlbs without IPIs, and it also has >> a gup_fast. powerpc RCU frees its page _tables_ so we can walk them, >> and then I use speculative page references in order to be able to >> take a reference on the page without having it pinned. > > Ah, interesting. So disabling interrupts prevents the RCU free from > happening, and non-atomic pte fetching is a non-issue. So it doesn't > address the PAE side of the problem.
This would be rcu_sched, correct?
Thanx, Paul
>> Turning gup_get_pte into a pvop would be a bit nasty because on !PAE >> it is just a single load, and even on PAE it is pretty cheap. >> > > Well, it wouldn't be too bad; for !PAE it would turn into something we > could inline, so there'd be little to no cost. For PAE it would be out of > line, but a direct function call, which would be nicely cached and very > predictable once we've gone through the the loop once (and for Xen I think > I'd just make it a cmpxchg8b-based implementation, assuming that the tlb > flush hypercall would offset the cost of making gup_fast a bit slower). > > But it would be better if we can address it at a higher level. > > J > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/
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