Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754442AbYGQEPA (ORCPT ); Thu, 17 Jul 2008 00:15:00 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1750821AbYGQEOu (ORCPT ); Thu, 17 Jul 2008 00:14:50 -0400 Received: from sh.osrg.net ([192.16.179.4]:53615 "EHLO sh.osrg.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750749AbYGQEOt (ORCPT ); Thu, 17 Jul 2008 00:14:49 -0400 Date: Thu, 17 Jul 2008 13:14:18 +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: <20080716093456J.fujita.tomonori@lab.ntt.co.jp> Mime-Version: 1.0 Content-Type: Text/Plain; charset=us-ascii Content-Transfer-Encoding: 7bit Message-Id: <20080717131444K.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: 2036 Lines: 45 On Wed, 16 Jul 2008 14:02:27 -0400 (EDT) Mikulas Patocka wrote: > On Wed, 16 Jul 2008, FUJITA Tomonori wrote: > > > On Tue, 15 Jul 2008 11:46:46 -0400 (EDT) > > Mikulas Patocka wrote: > > > > > >> Even if we fix it now, the question is: how long it will stay fixed? Until > > > >> someone makes another change to struct device that restricts boundaries on > > > >> some wacky hardware. > > > > > > > > I'm not sure how the boundary restriction of a device can break > > > > the VMERGE accounting. > > > > > > Because block layer code doesn't know anything about the device, pci > > > access restrictions and so on. > > > > Not true, the block layer knows about the device restrictions like DMA > > boundary. > > > > But it's not the point here because the boundary restriction doesn't > > matter for the VMERGE accounting. An IOMMU just returns an error if it > > can't allocate an I/O space fit for the device restrictions. > > > > > > 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. What is your new restriction? All restrictions that IOMMUs need to know are dma_get_seg_boundary and dma_get_max_seg_size. -- 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/