Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751078AbVJQRw4 (ORCPT ); Mon, 17 Oct 2005 13:52:56 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751044AbVJQRw4 (ORCPT ); Mon, 17 Oct 2005 13:52:56 -0400 Received: from ns.virtualhost.dk ([195.184.98.160]:13333 "EHLO virtualhost.dk") by vger.kernel.org with ESMTP id S1751078AbVJQRwz (ORCPT ); Mon, 17 Oct 2005 13:52:55 -0400 Date: Mon, 17 Oct 2005 19:53:27 +0200 From: Jens Axboe To: Badari Pulavarty Cc: li nux , Grzegorz Kulewski , Erik Mouw , colin , lkml Subject: Re: A problem about DIRECT IO on ext3 Message-ID: <20051017175326.GX2811@suse.de> References: <20051017091710.GT2811@suse.de> <20051017094140.14685.qmail@web33301.mail.mud.yahoo.com> <20051017095133.GU2811@suse.de> <1129566970.23632.8.camel@localhost.localdomain> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1129566970.23632.8.camel@localhost.localdomain> Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2711 Lines: 80 On Mon, Oct 17 2005, Badari Pulavarty wrote: > On Mon, 2005-10-17 at 11:51 +0200, Jens Axboe wrote: > > On Mon, Oct 17 2005, li nux wrote: > > > > > > > > > --- Jens Axboe wrote: > > > > > > > On Mon, Oct 17 2005, Grzegorz Kulewski wrote: > > > > > On Mon, 17 Oct 2005, Jens Axboe wrote: > > > > > >>how to correct this problem ? > > > > > > > > > > > >See your buffer address, it's not aligned. You > > > > need to align that as > > > > > >well. This is needed because the hardware will > > > > dma directly to the user > > > > > >buffer, and to be on the safe side we require the > > > > same alignment as the > > > > > >block layer will normally generate for file > > > > system io. > > > > > > > > > > > >So in short, just align your read buffer to the > > > > same as your block size > > > > > >and you will be fine. Example: > > > > > > > > > > > >#define BS (4096) > > > > > >#define MASK (BS - 1) > > > > > >#define ALIGN(buf) (((unsigned long) (buf) + > > > > MASK) & ~(MASK)) > > > > > > > > > > > >char *ptr = malloc(BS + MASK); > > > > > >char *buf = (char *) ALIGN(ptr); > > > > > > > > > > > >read(fd, buf, BS); > > > > > > > > > > Shouldn't one use posix_memalign(3) for that? > > > > > > > > Dunno if one 'should', one 'can' if one wants to. I > > > > prefer to do it > > > > manually so I don't have to jump through #define > > > > hoops to get at it > > > > (which, btw, still doesn't expose it on this > > > > machine). > > > > > > > > -- > > > > Jens Axboe > > > > > > Thanx a lot Jens :-) > > > Its working now. > > > I did not have to make these adjustments on 2.6 > > > Is looks to be having more relaxation. > > > > 2.6 does have the option of checking the hardware dma requirement > > seperately, but for this path you should run into the same restrictions. > > Perhaps you just got lucky when testing 2.6? > > 2.6 also has the same restriction. But, if the "filesystem > blocksize alignment" (soft block size) fails, we try to see > if its aligned with hard sector size (512). If so, we can do the IO. > > 2.4 fails if the offset or buffer is NOT filesystem blocksize > aligned. Period. I'm aware of that, however this particular case was about the buffer alignment (which was 32-bytes in the strace). And that should not work for 2.6 either. The block-size alignment is really a separate property of correctness. > BTW, posix_memalign() or valloc() should be safe. Certainly. -- 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/