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 mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 9B398C433FE for ; Tue, 12 Oct 2021 11:46:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 759D260EFE for ; Tue, 12 Oct 2021 11:46:28 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236197AbhJLLs3 (ORCPT ); Tue, 12 Oct 2021 07:48:29 -0400 Received: from szxga01-in.huawei.com ([45.249.212.187]:13726 "EHLO szxga01-in.huawei.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232665AbhJLLs2 (ORCPT ); Tue, 12 Oct 2021 07:48:28 -0400 Received: from dggeme754-chm.china.huawei.com (unknown [172.30.72.57]) by szxga01-in.huawei.com (SkyGuard) with ESMTP id 4HTDNc6jYCzVflZ; Tue, 12 Oct 2021 19:44:48 +0800 (CST) Received: from [10.174.178.185] (10.174.178.185) by dggeme754-chm.china.huawei.com (10.3.19.100) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256_P256) id 15.1.2308.8; Tue, 12 Oct 2021 19:46:24 +0800 Subject: Re: [PATCH -next v2 2/6] ext4: introduce last_check_time record previous check time To: Jan Kara References: <20210911090059.1876456-1-yebin10@huawei.com> <20210911090059.1876456-3-yebin10@huawei.com> <20211007123100.GG12712@quack2.suse.cz> <615FA55B.5070404@huawei.com> <615FAF27.8070000@huawei.com> <20211012084727.GF9697@quack2.suse.cz> CC: , , , From: yebin Message-ID: <61657590.2050407@huawei.com> Date: Tue, 12 Oct 2021 19:46:24 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:38.0) Gecko/20100101 Thunderbird/38.1.0 MIME-Version: 1.0 In-Reply-To: <20211012084727.GF9697@quack2.suse.cz> Content-Type: text/plain; charset="utf-8"; format=flowed Content-Transfer-Encoding: 8bit X-Originating-IP: [10.174.178.185] X-ClientProxiedBy: dggems706-chm.china.huawei.com (10.3.19.183) To dggeme754-chm.china.huawei.com (10.3.19.100) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-ext4@vger.kernel.org On 2021/10/12 16:47, Jan Kara wrote: > On Fri 08-10-21 10:38:31, yebin wrote: >> On 2021/10/8 9:56, yebin wrote: >>> On 2021/10/7 20:31, Jan Kara wrote: >>>> On Sat 11-09-21 17:00:55, Ye Bin wrote: >>>>> kmmpd: >>>>> ... >>>>> diff = jiffies - last_update_time; >>>>> if (diff > mmp_check_interval * HZ) { >>>>> ... >>>>> As "mmp_check_interval = 2 * mmp_update_interval", 'diff' always little >>>>> than 'mmp_update_interval', so there will never trigger detection. >>>>> Introduce last_check_time record previous check time. >>>>> >>>>> Signed-off-by: Ye Bin >>>> I think the check is there only for the case where write_mmp_block() + >>>> sleep took longer than mmp_check_interval. I agree that should rarely >>>> happen but on a really busy system it is possible and in that case >>>> we would >>>> miss updating mmp block for too long and so another node could have >>>> started >>>> using the filesystem. I actually don't see a reason why kmmpd should be >>>> checking the block each mmp_check_interval as you do - >>>> mmp_check_interval >>>> is just for ext4_multi_mount_protect() to know how long it should wait >>>> before considering mmp block stale... Am I missing something? >>>> >>>> Honza >>> I'm sorry, I didn't understand the detection mechanism here before. Now >>> I understand >>> the detection mechanism here. >>> As you said, it's just an abnormal protection. There's really no problem. >>> >> Yeah, i did test as following steps >> hostA hostB >> mount >> ext4_multi_mount_protect -> seq == EXT4_MMP_SEQ_CLEAN >> delay 5s after label "skip" so hostB will see seq is >> EXT4_MMP_SEQ_CLEAN >> mount >> ext4_multi_mount_protect -> seq == EXT4_MMP_SEQ_CLEAN >> run kmmpd >> run kmmpd >> >> Actually，in this situation kmmpd will not detect confliction. >> In ext4_multi_mount_protect function we write mmp data first and wait >> 'wait_time * HZ' seconds, >> read mmp data do check. Most of the time, If 'wait_time' is zero, it can pass >> check. > But how can be wait_time zero? As far as I'm reading the code, wait_time > must be at least EXT4_MMP_MIN_CHECK_INTERVAL... > > Honza int ext4_multi_mount_protect(struct super_block *sb, ext4_fsblk_t mmp_block) { struct ext4_super_block *es = EXT4_SB(sb)->s_es; struct buffer_head *bh = NULL; struct mmp_struct *mmp = NULL; u32 seq; unsigned int mmp_check_interval = le16_to_cpu(es->s_mmp_update_interval); unsigned int wait_time = 0; --> wait_time is equal with zero int retval; if (mmp_block < le32_to_cpu(es->s_first_data_block) || mmp_block >= ext4_blocks_count(es)) { ext4_warning(sb, "Invalid MMP block in superblock"); goto failed; } retval = read_mmp_block(sb, &bh, mmp_block); if (retval) goto failed; mmp = (struct mmp_struct *)(bh->b_data); if (mmp_check_interval < EXT4_MMP_MIN_CHECK_INTERVAL) mmp_check_interval = EXT4_MMP_MIN_CHECK_INTERVAL; /* * If check_interval in MMP block is larger, use that instead of * update_interval from the superblock. */ if (le16_to_cpu(mmp->mmp_check_interval) > mmp_check_interval) mmp_check_interval = le16_to_cpu(mmp->mmp_check_interval); seq = le32_to_cpu(mmp->mmp_seq); if (seq == EXT4_MMP_SEQ_CLEAN) --> If hostA and hostB mount the same block device at the same time, --> HostA and hostB maybe get 'seq' with the same value EXT4_MMP_SEQ_CLEAN. goto skip; ... skip: /* * write a new random sequence number. */ seq = mmp_new_seq(); mmp->mmp_seq = cpu_to_le32(seq); retval = write_mmp_block(sb, bh); if (retval) goto failed; /* * wait for MMP interval and check mmp_seq. */ if (schedule_timeout_interruptible(HZ * wait_time) != 0) { --> If seq is equal with EXT4_MMP_SEQ_CLEAN, wait_time is zero. ext4_warning(sb, "MMP startup interrupted, failing mount"); goto failed; } retval = read_mmp_block(sb, &bh, mmp_block); -->We may get the same data with which we wrote, so we can't detect conflict at here. if (retval) goto failed; mmp = (struct mmp_struct *)(bh->b_data); if (seq != le32_to_cpu(mmp->mmp_seq)) { dump_mmp_msg(sb, mmp, "Device is already active on another node."); goto failed; } ... }