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 EF336C433EF for ; Fri, 22 Jul 2022 08:15:58 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230429AbiGVIP5 (ORCPT ); Fri, 22 Jul 2022 04:15:57 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:41234 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233544AbiGVIP4 (ORCPT ); Fri, 22 Jul 2022 04:15:56 -0400 Received: from out30-130.freemail.mail.aliyun.com (out30-130.freemail.mail.aliyun.com [115.124.30.130]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id E9EF05007C; Fri, 22 Jul 2022 01:15:54 -0700 (PDT) X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R881e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=ay29a033018046050;MF=liusong@linux.alibaba.com;NM=1;PH=DS;RN=4;SR=0;TI=SMTPD_---0VK4HbEZ_1658477750; Received: from 30.178.80.135(mailfrom:liusong@linux.alibaba.com fp:SMTPD_---0VK4HbEZ_1658477750) by smtp.aliyun-inc.com; Fri, 22 Jul 2022 16:15:51 +0800 Message-ID: Date: Fri, 22 Jul 2022 16:15:49 +0800 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:102.0) Gecko/20100101 Thunderbird/102.0.3 Subject: Re: [PATCH] blk-mq: avoid potential infinite loop in __blk_mq_alloc_request To: Christoph Hellwig Cc: axboe@kernel.dk, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org References: <1658467343-55843-1-git-send-email-liusong@linux.alibaba.com> From: Liu Song In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org > On Fri, Jul 22, 2022 at 01:22:23PM +0800, Liu Song wrote: >> From: Liu Song >> >> If "blk_mq_get_tag" returns BLK_MQ_NO_TAG because the value of >> "tags->nr_reserved_tags" is 0, it will fall into an infinite loop in >> "__blk_mq_alloc_requests", so borrow BLK_MQ_REQ_NOWAIT to exit the loop. > That means the driver calling blk_mq_alloc_request has a bug, and > we should not work round that in the low level tag allocation path. > > If we want to be nice we can add a WARN_ON before going all the way > down into the tag allocator, something like: > > diff --git a/block/blk-mq.c b/block/blk-mq.c > index 92aae03103b74..d6c7e2ece025f 100644 > --- a/block/blk-mq.c > +++ b/block/blk-mq.c > @@ -520,6 +520,10 @@ struct request *blk_mq_alloc_request(struct request_queue *q, unsigned int op, > struct request *rq; > int ret; > > + if (WARN_ON_ONCE((flags & BLK_MQ_REQ_RESERVED) && > + !q->tag_set->reserved_tags)) > + return ERR_PTR(-EINVAL); > + Hi， It is a reasonable approach to prevent abnormal alloc from going down, but this is a very rare exception after all, and above modification is checked every alloc request, which seems to be a bit excessive overhead. After the rare exception occurs and fix it, the impact on the alloc request will be lower. Thanks > ret = blk_queue_enter(q, flags); > if (ret) > return ERR_PTR(ret);