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 lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id CB172C433EF for ; Wed, 6 Jul 2022 08:08:55 +0000 (UTC) Received: from localhost ([::1]:43450 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1o905d-0008Ja-Qv for qemu-devel@archiver.kernel.org; Wed, 06 Jul 2022 04:08:53 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:44522) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1o900t-0006hG-73 for qemu-devel@nongnu.org; Wed, 06 Jul 2022 04:03:59 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:50724) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1o900n-0002r4-P8 for qemu-devel@nongnu.org; Wed, 06 Jul 2022 04:03:56 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1657094632; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding; bh=TiJLSdzPijaVhizwwu8WwjSu+VxysH4l9qlTlQKZA6Y=; b=d1pFQANdQNQGlCGOh8cGXpxHLg8eTstZgoNhs5YZGfyLopsmLdqdh6zj1PfC7vH8ExjpJN J1ku5JKN0hPhGS7lwl8cQpcBqEN1g27voUnLg9KXqRpDKGQicvpX4TIOtMJlF0VVgv8olO x596LUM+VymMrl8kjXM36IgY2Fywg/E= 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-168-NCVag2uaOtu_OIR-W1HRUA-1; Wed, 06 Jul 2022 04:03:43 -0400 X-MC-Unique: NCVag2uaOtu_OIR-W1HRUA-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.rdu2.redhat.com [10.11.54.1]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 0379A1019C8A; Wed, 6 Jul 2022 08:03:43 +0000 (UTC) Received: from localhost (unknown [10.39.194.11]) by smtp.corp.redhat.com (Postfix) with ESMTP id 9D0DA40CFD0B; Wed, 6 Jul 2022 08:03:42 +0000 (UTC) From: Stefan Hajnoczi To: qemu-devel@nongnu.org Cc: Stefan Hajnoczi , Kevin Wolf , Julia Suvorova , qemu-block@nongnu.org, Hanna Reitz , Aarushi Mehta , Stefano Garzarella , Dominique Martinet Subject: [PATCH] block/io_uring: clarify that short reads can happen Date: Wed, 6 Jul 2022 09:03:41 +0100 Message-Id: <20220706080341.1206476-1-stefanha@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 2.84 on 10.11.54.1 Received-SPF: pass client-ip=170.10.133.124; envelope-from=stefanha@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -21 X-Spam_score: -2.2 X-Spam_bar: -- X-Spam_report: (-2.2 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.082, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Jens Axboe has confirmed that short reads are rare but can happen: https://lore.kernel.org/io-uring/YsU%2FCGkl9ZXUI+Tj@stefanha-x1.localdomain/T/#m729963dc577d709b709c191922e98ec79d7eef54 The luring_resubmit_short_read() comment claimed they were only due to a specific io_uring bug that was fixed in Linux commit 9d93a3f5a0c ("io_uring: punt short reads to async context"), which is wrong. Dominique Martinet found that a btrfs bug also causes short reads. There may be more kernel code paths that result in short reads. Let's consider short reads fair game. Cc: Dominique Martinet Based-on: <20220630010137.2518851-1-dominique.martinet@atmark-techno.com> Signed-off-by: Stefan Hajnoczi --- block/io_uring.c | 8 ++------ 1 file changed, 2 insertions(+), 6 deletions(-) diff --git a/block/io_uring.c b/block/io_uring.c index b238661740..f8a19fd97f 100644 --- a/block/io_uring.c +++ b/block/io_uring.c @@ -73,12 +73,8 @@ static void luring_resubmit(LuringState *s, LuringAIOCB *luringcb) /** * luring_resubmit_short_read: * - * Before Linux commit 9d93a3f5a0c ("io_uring: punt short reads to async - * context") a buffered I/O request with the start of the file range in the - * page cache could result in a short read. Applications need to resubmit the - * remaining read request. - * - * This is a slow path but recent kernels never take it. + * Short reads are rare but may occur. The remaining read request needs to be + * resubmitted. */ static void luring_resubmit_short_read(LuringState *s, LuringAIOCB *luringcb, int nread) -- 2.36.1