Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756069AbYGQNUk (ORCPT ); Thu, 17 Jul 2008 09:20:40 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1755969AbYGQNUa (ORCPT ); Thu, 17 Jul 2008 09:20:30 -0400 Received: from sh.osrg.net ([192.16.179.4]:54376 "EHLO sh.osrg.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753899AbYGQNU3 (ORCPT ); Thu, 17 Jul 2008 09:20:29 -0400 Date: Thu, 17 Jul 2008 22:18:44 +0900 To: mpatocka@redhat.com Cc: fujita.tomonori@lab.ntt.co.jp, jens.axboe@oracle.com, linux-kernel@vger.kernel.org, linux-scsi@vger.kernel.org, davem@davemloft.net, linux-parisc@vger.kernel.org Subject: Re: [PATCH] block: fix q->max_segment_size checking in blk_recalc_rq_segments about VMERGE From: FUJITA Tomonori In-Reply-To: References: <20080717131444K.fujita.tomonori@lab.ntt.co.jp> Mime-Version: 1.0 Content-Type: Text/Plain; charset=us-ascii Content-Transfer-Encoding: 7bit Message-Id: <20080717221908D.fujita.tomonori@lab.ntt.co.jp> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2073 Lines: 47 On Thu, 17 Jul 2008 07:50:24 -0400 (EDT) Mikulas Patocka wrote: > > > > Please give me an example how the boundary restriction of a device can > > > > break the VMERGE accounting and an IOMMU if you aren't still sure. > > > > > > You have dma_get_seg_boundary and dma_get_max_seg_size. On sparc64, adding > > > one of these broken VMERGE accounting (the VMERGE didn't happen past 64-kb > > > boundary and bio layer thought that VMERGE would be possible). > > > > If the device has 64KB boundary restriction, the device also has > > max_seg_size restriction of 64KB or under. So the vmerge acounting > > works (though we need to fix it to handle max_seg_size, as discussed). > > > > > And if you fix this case, someone will break it again, sooner or later, by > > > adding new restriction. > > > > All restrictions that IOMMUs need to know are dma_get_seg_boundary and > > dma_get_max_seg_size. > > > > What is your new restriction? > > We don't know what happens in the future. It's very unlikely to add new restrictions. > And that is the problem that we > don't know --- but we have two pieces of code (blk-merge and iommu) that > try to calculate the same number (number of hw segments) and if they get > different result, it will crash. If the calculations were done at one > place, there would be no problem with that. I don't think that your argument, 'the problem that we don't know', is true. With the vmerge accounting, we calculate at two places. So if we add a new restriction, we need to handle it at two places. It's a logical result. Of course, it's easier to calculate at one place rather than two places. But 'we don't know what restriction we will need' isn't a problem. BTW, as I've already said, I'm not against removing the vmerge accounting from the block layer. -- 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/