From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id CDB41C433F5 for ; Mon, 3 Oct 2022 07:12:11 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229638AbiJCHMJ (ORCPT ); Mon, 3 Oct 2022 03:12:09 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:46816 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229528AbiJCHMF (ORCPT ); Mon, 3 Oct 2022 03:12:05 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 4DF3A10573 for ; Mon, 3 Oct 2022 00:12:02 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1664781121; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=I0LXfsSyKXGjmai9f263mAbHu12AmzCqqawR+zfFGBM=; b=AjzRq3FBS8LXWiDQdr+/Qs4l5fKDfko7lbmWgC38pRWn4zeOnSP6rcjVvcnY1T4PWrJ0Mo NlJIdhvtmNJmqJ8DbWWTeMsPrxxd8YS6YPl64X4RpAWSy8ZWzqA9wTt6XmIsyB8vgxwOnU d1+BCVVFb7ee0FLawTDNN6UhTBOZ2m0= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-477-GNJWiLMGNs6-wrhEm2uRtw-1; Mon, 03 Oct 2022 03:11:59 -0400 X-MC-Unique: GNJWiLMGNs6-wrhEm2uRtw-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.rdu2.redhat.com [10.11.54.2]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 2F28D85A59D; Mon, 3 Oct 2022 07:11:59 +0000 (UTC) Received: from starship (unknown [10.40.193.232]) by smtp.corp.redhat.com (Postfix) with ESMTP id ED50440C6EC2; Mon, 3 Oct 2022 07:11:56 +0000 (UTC) Message-ID: <2a9fe4f9759b9971e76f719f4c1295eed41ed50c.camel@redhat.com> Subject: Re: Commit 'iomap: add support for dma aligned direct-io' causes qemu/KVM boot failures From: Maxim Levitsky To: Keith Busch Cc: Paolo Bonzini , Christoph Hellwig , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, qemu-devel@nongnu.org, kvm@vger.kernel.org, Kevin Wolf , Michael Roth In-Reply-To: References: <20220929163931.GA10232@lst.de> <32db4f89-a83f-aac4-5d27-0801bdca60bf@redhat.com> <28ce86c01271c1b9b8f96a7783b55a8d458325d2.camel@redhat.com> Content-Type: text/plain; charset="UTF-8" MIME-Version: 1.0 Date: Mon, 03 Oct 2022 10:06:48 +0300 User-Agent: Evolution 3.36.5 (3.36.5-2.fc32) Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 3.1 on 10.11.54.2 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, 2022-10-02 at 07:56 -0600, Keith Busch wrote: > On Sun, Oct 02, 2022 at 11:59:42AM +0300, Maxim Levitsky wrote: > > On Thu, 2022-09-29 at 19:35 +0200, Paolo Bonzini wrote: > > > On 9/29/22 18:39, Christoph Hellwig wrote: > > > > On Thu, Sep 29, 2022 at 10:37:22AM -0600, Keith Busch wrote: > > > > > > I am aware, and I've submitted the fix to qemu here: > > > > > > > > > > > > https://lists.nongnu.org/archive/html/qemu-block/2022-09/msg00398.html > > > > > > > > > > I don't think so. Memory alignment and length granularity are two completely > > > > > different concepts. If anything, the kernel's ABI had been that the length > > > > > requirement was also required for the memory alignment, not the other way > > > > > around. That usage will continue working with this kernel patch. > > > > Yes, this is how I also understand it - for example for O_DIRECT on a file which > > resides on 4K block device, you have to use page aligned buffers. > > > > But here after the patch, 512 aligned buffer starts working as well - If I > > understand you correctly the ABI didn't guarantee that such usage would fail, > > but rather that it might fail. > > The kernel patch will allow buffer alignment to work with whatever the hardware > reports it can support. It could even as low as byte aligned if that's the > hardware can use that. > > The patch aligns direct-io with the same criteria blk_rq_map_user() has always > used to know if the user space buffer is compatible with the hardware's dma > requirements. Prior to this patch, the direct-io memory alignment was an > artificial software constraint, and that constraint creates a lot of > unnecessary memory pressure. > > As has always been the case, each segment needs to be a logical block length > granularity. QEMU assumed a buffer's page offset also defined the logical block > size instead of using the actual logical block size that it had previously > discovered directly. > > > If I understand that correctly, after the patch in question, > > qemu is able to use just 512 bytes aligned buffer to read a single 4K block from the disk, > > which supposed to fail but wasn't guarnteed to fail. > > > > Later qemu it submits iovec which also reads a 4K block but in two parts, > > and if I understand that correctly, each part (iov) is considered > > to be a separate IO operation, and thus each has to be in my case 4K in size, > > and its memory buffer *should* also be 4K aligned. > > > > (but it can work with smaller alignement as well). > > Right. The iov length needs to match the logical block size. The iov's memory > offset needs to align to the queue's dma_alignment attribute. The memory > alignment may be smaller than a block size. > > > Assuming that I understand all of this correctly, I agree with Paolo that this is qemu > > bug, but I do fear that it can cause quite some problems for users, > > especially for users that use outdated qemu version. > > > > It might be too much to ask, but maybe add a Kconfig option to keep legacy behavier > > for those that need it? > > Kconfig doesn't sound right. > > The block layer exports all the attributes user space needs to know about for > direct io. > > iov length: /sys/block//queue/logical_block_size > iov mem align: /sys/block//queue/dma_alignment > > If you really want to change the behavior, I think maybe we could make the > dma_alignment attribute writeable (or perhaps add a new attribute specifically > for dio_alignment) so the user can request something larger. > All makes sense now. New attribute could make sense I guess, and can be set by an udev rule or something. Anyway I won't worry about this for now, and if there are issues I'll see how we could work around them. Thanks for everything, Best regards, Maxim Levitsky