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 1A691C7619A for ; Mon, 27 Mar 2023 11:12:04 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232241AbjC0LMC (ORCPT ); Mon, 27 Mar 2023 07:12:02 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:59560 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229662AbjC0LMA (ORCPT ); Mon, 27 Mar 2023 07:12:00 -0400 Received: from szxga08-in.huawei.com (szxga08-in.huawei.com [45.249.212.255]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 98A1010D0; Mon, 27 Mar 2023 04:11:58 -0700 (PDT) Received: from dggpeml500021.china.huawei.com (unknown [172.30.72.54]) by szxga08-in.huawei.com (SkyGuard) with ESMTP id 4PlVRv3jqJz17KJF; Mon, 27 Mar 2023 19:08:43 +0800 (CST) Received: from [10.174.177.174] (10.174.177.174) by dggpeml500021.china.huawei.com (7.185.36.21) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2507.21; Mon, 27 Mar 2023 19:11:55 +0800 Message-ID: <7144edc4-b771-7c92-5ec3-ac78a123d37c@huawei.com> Date: Mon, 27 Mar 2023 19:11:55 +0800 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:102.0) Gecko/20100101 Thunderbird/102.1.2 Subject: Re: [PATCH 3/3] ext4: fix race between writepages and remount Content-Language: en-US To: Jan Kara CC: , , , , , , , , , Baokun Li References: <20230316112832.2711783-1-libaokun1@huawei.com> <20230316112832.2711783-4-libaokun1@huawei.com> <20230323114407.xenntblzv4ewfqkk@quack3> <269d37fd-d3f2-d059-b71f-acaea2e7ce4b@huawei.com> <20230327093553.up7dhoyqe4ecpn7y@quack3> From: Baokun Li In-Reply-To: <20230327093553.up7dhoyqe4ecpn7y@quack3> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit X-Originating-IP: [10.174.177.174] X-ClientProxiedBy: dggems703-chm.china.huawei.com (10.3.19.180) To dggpeml500021.china.huawei.com (7.185.36.21) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-ext4@vger.kernel.org On 2023/3/27 17:35, Jan Kara wrote: > On Thu 23-03-23 22:18:53, Baokun Li wrote: >> On 2023/3/23 19:44, Jan Kara wrote: >>>> --- >>>> fs/ext4/ext4.h | 3 ++- >>>> fs/ext4/ext4_jbd2.h | 9 +++++---- >>>> fs/ext4/super.c | 14 ++++++++++++++ >>>> 3 files changed, 21 insertions(+), 5 deletions(-) >>>> >>>> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h >>>> index 08b29c289da4..f60967fa648f 100644 >>>> --- a/fs/ext4/ext4.h >>>> +++ b/fs/ext4/ext4.h >>>> @@ -1703,7 +1703,8 @@ struct ext4_sb_info { >>>> /* >>>> * Barrier between writepages ops and changing any inode's JOURNAL_DATA >>>> - * or EXTENTS flag. >>>> + * or EXTENTS flag or between changing SHOULD_DIOREAD_NOLOCK flag on >>>> + * remount and writepages ops. >>>> */ >>>> struct percpu_rw_semaphore s_writepages_rwsem; >>>> struct dax_device *s_daxdev; >>>> diff --git a/fs/ext4/ext4_jbd2.h b/fs/ext4/ext4_jbd2.h >>>> index 0c77697d5e90..d82bfcdd56e5 100644 >>>> --- a/fs/ext4/ext4_jbd2.h >>>> +++ b/fs/ext4/ext4_jbd2.h >>>> @@ -488,6 +488,9 @@ static inline int ext4_free_data_revoke_credits(struct inode *inode, int blocks) >>>> return blocks + 2*(EXT4_SB(inode->i_sb)->s_cluster_ratio - 1); >>>> } >>>> +/* delalloc is a temporary fix to prevent generic/422 test failures*/ >>>> +#define EXT4_MOUNT_SHOULD_DIOREAD_NOLOCK (EXT4_MOUNT_DIOREAD_NOLOCK | \ >>>> + EXT4_MOUNT_DELALLOC) >>>> /* >>>> * This function controls whether or not we should try to go down the >>>> * dioread_nolock code paths, which makes it safe to avoid taking >>>> @@ -499,7 +502,8 @@ static inline int ext4_free_data_revoke_credits(struct inode *inode, int blocks) >>>> */ >>>> static inline int ext4_should_dioread_nolock(struct inode *inode) >>>> { >>>> - if (!test_opt(inode->i_sb, DIOREAD_NOLOCK)) >>>> + if (test_opt(inode->i_sb, SHOULD_DIOREAD_NOLOCK) != >>>> + EXT4_MOUNT_SHOULD_DIOREAD_NOLOCK) >>>> return 0; >>>> if (!S_ISREG(inode->i_mode)) >>>> return 0; >>>> @@ -507,9 +511,6 @@ static inline int ext4_should_dioread_nolock(struct inode *inode) >>>> return 0; >>>> if (ext4_should_journal_data(inode)) >>>> return 0; >>>> - /* temporary fix to prevent generic/422 test failures */ >>>> - if (!test_opt(inode->i_sb, DELALLOC)) >>>> - return 0; >>>> return 1; >>>> } >>> Is there a need for this SHOULD_DIOREAD_NOLOCK? When called from writeback >>> we will be protected by s_writepages_rwsem anyway. When called from other >>> places, we either decide to do dioread_nolock or don't but the situation >>> can change at any instant so I don't see how unifying this check would >>> help. And the new SHOULD_DIOREAD_NOLOCK somewhat obfuscates what's going >>> on. >> We're thinking that the mount-related flags in >> ext4_should_dioread_nolock() might be modified, such as DELALLOC being >> removed because generic/422 test failures were fixed in some other way, >> resulting in some unnecessary locking during remount, or for whatever >> reason a mount-related flag was added to ext4_should_dioread_nolock(), >> and we didn't make a synchronization change in __ext4_remount() that >> would cause the problem to recur.  So we added this flag to this function >> (instead of in ext4.h), so that when we change the mount option in >> ext4_should_dioread_nolock(), we directly change this flag, and we don't >> have to consider making synchronization changes in __ext4_remount(). >> >> We have checked where this function is called and there are two types of >> calls to this function: >> 1. One category is ext4_do_writepages() and mpage_map_one_extent(), which >> are protected by s_writepages_rwsem, the location of the problem; >> 2. The other type is in ext4_page_mkwrite(), >> ext4_convert_inline_data_to_extent(), ext4_write_begin() to determine >> whether to get the block using ext4_get_block_unwritten() or >> ext4_get_block(). >> >>     1) If we just started fetching written blocks, it looks like there is no >> problem; >>     2) If we start getting unwritten blocks, when DIOREAD_NOLOCK is cleared >> by remount, >>         we will convert the blocks to written in ext4_map_blocks(). The >> data=ordered mode ensures that we don't see stale data. > Yes. So do you agree that EXT4_MOUNT_SHOULD_DIOREAD_NOLOCK is not really > needed? > > Honza Yes, I totally agree! If we unconditionally grabbed s_writepages_rwsem when remounting, there would be no mount option synchronization problem, and the flag would be completely unnecessary. I will send a patch V2 with the changes suggested by you. -- With Best Regards, Baokun Li .