From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail172.messagelabs.com (mail172.messagelabs.com [216.82.254.3]) by kanga.kvack.org (Postfix) with ESMTP id 7E0896B003D for ; Fri, 20 Mar 2009 01:07:36 -0400 (EDT) Received: from d01relay04.pok.ibm.com (d01relay04.pok.ibm.com [9.56.227.236]) by e9.ny.us.ibm.com (8.13.1/8.13.1) with ESMTP id n2K4wZOt008522 for ; Fri, 20 Mar 2009 00:58:35 -0400 Received: from d01av04.pok.ibm.com (d01av04.pok.ibm.com [9.56.224.64]) by d01relay04.pok.ibm.com (8.13.8/8.13.8/NCO v9.2) with ESMTP id n2K57Y6Q151364 for ; Fri, 20 Mar 2009 01:07:34 -0400 Received: from d01av04.pok.ibm.com (loopback [127.0.0.1]) by d01av04.pok.ibm.com (8.12.11.20060308/8.13.3) with ESMTP id n2K52BLv001889 for ; Fri, 20 Mar 2009 01:07:34 -0400 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 Message-ID: <20090320044029.GD6807@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <49C148AF.5050601@goop.org> <200903191232.05459.nickpiggin@yahoo.com.au> <49C2818B.9060201@goop.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <49C2818B.9060201@goop.org> Sender: owner-linux-mm@kvack.org To: Jeremy Fitzhardinge Cc: Nick Piggin , Avi Kivity , Linux Kernel Mailing List , Linux Memory Management List , Xen-devel , Jan Beulich , Ingo Molnar List-ID: 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/ -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org