Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932287AbXHaIMW (ORCPT ); Fri, 31 Aug 2007 04:12:22 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1755113AbXHaIMI (ORCPT ); Fri, 31 Aug 2007 04:12:08 -0400 Received: from brick.kernel.dk ([87.55.233.238]:3395 "EHLO kernel.dk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750951AbXHaIME (ORCPT ); Fri, 31 Aug 2007 04:12:04 -0400 Date: Fri, 31 Aug 2007 10:12:01 +0200 From: Jens Axboe To: Christoph Lameter Cc: Dmitry Monakhov , torvalds@linux-foundation.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, Christoph Hellwig , Mel Gorman , William Lee Irwin III , David Chinner , Badari Pulavarty , Maxim Levitsky , Fengguang Wu , swin wang , totty.lu@gmail.com, "H. Peter Anvin" , joern@lazybastard.org, "Eric W. Biederman" Subject: Re: [11/36] Use page_cache_xxx in fs/buffer.c Message-ID: <20070831081201.GO29452@kernel.dk> References: <20070831065613.GH29452@kernel.dk> <20070831071142.GI29452@kernel.dk> <20070831072611.GK29452@kernel.dk> <20070831074338.GL29452@kernel.dk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2114 Lines: 46 On Fri, Aug 31 2007, Christoph Lameter wrote: > On Fri, 31 Aug 2007, Jens Axboe wrote: > > > They have nothing to do with each other, you are mixing things up. It > > has nothing to do with the device being able to dma into that memory or > > not, we have fine existing infrastructure to handle that. But different > > hardware have different characteristics on what a single segment is. You > > can say "a single segment cannot cross a 32kb boundary". So from the > > example above, your single 64k page may need to be split into two > > segments. Or it could have a maximum segment size of 32k, in which case > > it would have to be split as well. > > > > Do you see what I mean now? > > Ok. So another solution maybe to limit the blocksizes that can be used > with a device? That'd work for creation, but not for moving things around. > > > How do we split that up today? We could add processing to submit_bio > > > to check for the boundary and create two bios. > > > > But we do not split them up today - see what I wrote! Today we impose > > the restriction that a device must be able to handle a single "normal" > > page, and if it can't do that, it has to split it up itself. > > > > But yes, you would have to create some out-of-line function to use > > bio_split() until you have chopped things down enough. It's not a good > > thing for performance naturally, but if we consider this a "just make it > > work" fallback, I don't think it's too bad. You want to make a note of > > that it is happening though, so people realize that it is happening. > > Hmmmm.. We could keep the existing scheme too and check that device > drivers split things up if they are too large? Isnt it possible today > to create a huge bio of 2M for huge pages and send it to a device? Not sure, aren't the constituents of compound pages the basis for IO? -- Jens Axboe - 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/