From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Mon, 18 Feb 2008 13:35:51 +0100 From: Andrea Arcangeli Subject: Re: [PATCH] KVM swapping with MMU Notifiers V7 Message-ID: <20080218123551.GS11732@v2.random> References: <20080215064859.384203497@sgi.com> <20080216104827.GI11732@v2.random> <20080216115138.GA11391@sgi.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20080216115138.GA11391@sgi.com> Sender: owner-linux-mm@kvack.org Return-Path: To: Robin Holt Cc: Christoph Lameter , akpm@linux-foundation.org, Avi Kivity , Izik Eidus , kvm-devel@lists.sourceforge.net, Peter Zijlstra , general@lists.openfabrics.org, Steve Wise , Roland Dreier , Kanoj Sarcar , steiner@sgi.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, daniel.blueman@quadrics.com List-ID: On Sat, Feb 16, 2008 at 05:51:38AM -0600, Robin Holt wrote: > I am doing this in xpmem with a stack-based structure in the function > calling get_user_pages. That structure describes the start and > end address of the range we are doing the get_user_pages on. If an > invalidate_range_begin comes in while we are off to the kernel doing > the get_user_pages, the invalidate_range_begin marks that structure > indicating an invalidate came in. When the get_user_pages gets the > structures relocked, it checks that flag (really a generation counter) > and if it is set, retries the get_user_pages. After 3 retries, it > returns -EAGAIN and the fault is started over from the remote side. A seqlock sounds a good optimization for the non-swapping fast path, a per-VM-guest seqlock number can allow us to know when we need to worry to call get_user_pages a second time, but won't be really a retry like in 99% of seqlock usages for the reader side, but just a second get_user_pages to trigger a minor fault. Then if the page is different in the second run, we'll really retry (so not in function of the seqlock but in function of the get_user_pages page array), and there's no risk of livelocks because get_user_pages returning a different page won't be the common case. The seqlock should be increased first before the invalidate and a second time once the invalidate is over. -- 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