From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Tue, 22 Apr 2008 15:48:47 +0200 From: Andrea Arcangeli Subject: Re: [PATCH 0 of 9] mmu notifier #v12 Message-ID: <20080422134847.GT12709@duo.random> References: <20080409131709.GR11364@sgi.com> <20080409144401.GT10133@duo.random> <20080409185500.GT11364@sgi.com> <20080422072026.GM12709@duo.random> <20080422120056.GR12709@duo.random> <20080422130120.GR22493@sgi.com> <20080422132143.GS12709@duo.random> <20080422133604.GN30298@sgi.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20080422133604.GN30298@sgi.com> Sender: owner-linux-mm@kvack.org Return-Path: To: Robin Holt Cc: Christoph Lameter , akpm@linux-foundation.org, Nick Piggin , Steve Wise , Peter Zijlstra , linux-mm@kvack.org, Kanoj Sarcar , Roland Dreier , Jack Steiner , linux-kernel@vger.kernel.org, Avi Kivity , kvm-devel@lists.sourceforge.net, general@lists.openfabrics.org, Hugh Dickins List-ID: On Tue, Apr 22, 2008 at 08:36:04AM -0500, Robin Holt wrote: > I am a little confused about the value of the seq_lock versus a simple > atomic, but I assumed there is a reason and left it at that. There's no value for anything but get_user_pages (get_user_pages takes its own lock internally though). I preferred to explain it as a seqlock because it was simpler for reading, but I totally agree in the final implementation it shouldn't be a seqlock. My code was meant to be pseudo-code only. It doesn't even need to be atomic ;). > I don't know what you mean by "it'd" run slower and what you mean by > "armed and disarmed". 1) when armed the time-window where the kvm-page-fault would be blocked would be a bit larger without invalidate_page for no good reason 2) if you were to remove invalidate_page when disarmed the VM could would need two branches instead of one in various places I don't want to waste cycles if not wasting them improves performance both when armed and disarmed. > For the sake of this discussion, I will assume "it'd" means the kernel in > general and not KVM. With the two call sites for range_begin/range_end, I actually meant for both. > By disarmed, I will assume you mean no notifiers registered for a > particular mm. In that case, the cache will make the second call > effectively free. So, for the disarmed case, I see no measurable > difference. For rmap is sure effective free, for do_wp_page it costs one branch for no good reason. > For the case where there is a notifier registered, I certainly can see > a difference. I am not certain how to quantify the difference as it Agreed. > When I was discussing this difference with Jack, he reminded me that > the GRU, due to its hardware, does not have any race issues with the > invalidate_page callout simply doing the tlb shootdown and not modifying > any of its internal structures. He then put a caveat on the discussion > that _either_ method was acceptable as far as he was concerned. The real > issue is getting a patch in that satisfies all needs and not whether > there is a seperate invalidate_page callout. Sure, we have that patch now, I'll send it out in a minute, I was just trying to explain why it makes sense to have an invalidate_page too (which remains the only difference by now), removing it would be a regression on all sides, even if a minor one. -- 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