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 47BB0C77B73 for ; Tue, 30 May 2023 09:39:23 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230169AbjE3JjW (ORCPT ); Tue, 30 May 2023 05:39:22 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:51644 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229550AbjE3JjV (ORCPT ); Tue, 30 May 2023 05:39:21 -0400 Received: from szxga02-in.huawei.com (szxga02-in.huawei.com [45.249.212.188]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 2E639A3; Tue, 30 May 2023 02:39:19 -0700 (PDT) Received: from dggpeml500021.china.huawei.com (unknown [172.30.72.54]) by szxga02-in.huawei.com (SkyGuard) with ESMTP id 4QVnMk1sQQzLqBN; Tue, 30 May 2023 17:36:18 +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 15.1.2507.23; Tue, 30 May 2023 17:39:16 +0800 Message-ID: <78f61c71-be47-cd4d-36c8-161c7a86c9c0@huawei.com> Date: Tue, 30 May 2023 17:39:16 +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] ext4: fix race condition between buffer write and page_mkwrite To: Jan Kara CC: , , , , , , , , , , , , Baokun Li References: <20230529080148.3810143-1-libaokun1@huawei.com> <20230529144435.bj65ltbww5jbh2uc@quack3> <7f6ab488-9eef-fb94-b007-839eb1c1f487@huawei.com> <20230530074225.ly6vnolykqu5teos@quack3> Content-Language: en-US From: Baokun Li In-Reply-To: <20230530074225.ly6vnolykqu5teos@quack3> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit X-Originating-IP: [10.174.177.174] X-ClientProxiedBy: dggems704-chm.china.huawei.com (10.3.19.181) 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 2023/5/30 15:42, Jan Kara wrote: > On Tue 30-05-23 10:00:44, Baokun Li wrote: >> On 2023/5/29 22:44, Jan Kara wrote: >>> On Mon 29-05-23 16:01:48, Baokun Li wrote: >>>> Syzbot reported a BUG_ON: >>>> ================================================================== >>>> EXT4-fs (loop0): mounted filesystem without journal. Quota mode: none. >>>> EXT4-fs error (device loop0): ext4_mb_generate_buddy:1098: group 0, block >>>> bitmap and bg descriptor inconsistent: 25 vs 150994969 free clusters >>>> ------------[ cut here ]------------ >>>> kernel BUG at fs/ext4/ext4_jbd2.c:53! >>>> invalid opcode: 0000 [#1] PREEMPT SMP KASAN >>>> CPU: 1 PID: 494 Comm: syz-executor.0 6.1.0-rc7-syzkaller-ga4412fdd49dc #0 >>>> RIP: 0010:__ext4_journal_stop+0x1b3/0x1c0 >>>> [...] >>>> Call Trace: >>>> ext4_write_inline_data_end+0xa39/0xdf0 >>>> ext4_da_write_end+0x1e2/0x950 >>>> generic_perform_write+0x401/0x5f0 >>>> ext4_buffered_write_iter+0x35f/0x640 >>>> ext4_file_write_iter+0x198/0x1cd0 >>>> vfs_write+0x8b5/0xef0 >>>> [...] >>>> ================================================================== >>>> >>>> The above BUG_ON is triggered by the following race: >>>> >>>> cpu1 cpu2 >>>> ________________________|________________________ >>>> ksys_write >>>> vfs_write >>>> new_sync_write >>>> ext4_file_write_iter >>>> ext4_buffered_write_iter >>>> generic_perform_write >>>> ext4_da_write_begin >>>> do_fault >>>> do_page_mkwrite >>>> ext4_page_mkwrite >>>> ext4_convert_inline_data >>>> ext4_convert_inline_data_nolock >>>> ext4_destroy_inline_data_nolock >>>> //clear EXT4_STATE_MAY_INLINE_DATA >>>> ext4_map_blocks --> return error >>>> ext4_test_inode_state(inode, EXT4_STATE_MAY_INLINE_DATA) >>>> ext4_block_write_begin >>>> ext4_restore_inline_data >>>> // set EXT4_STATE_MAY_INLINE_DATA >>>> ext4_da_write_end >>>> ext4_test_inode_state(inode, EXT4_STATE_MAY_INLINE_DATA) >>>> ext4_write_inline_data_end >>>> handle=NULL >>>> ext4_journal_stop(handle) >>>> __ext4_journal_stop >>>> ext4_put_nojournal(handle) >>>> ref_cnt = (unsigned long)handle >>>> BUG_ON(ref_cnt == 0) ---> BUG_ON >>>> >>>> The root cause of this problem is that the ext4_convert_inline_data() in >>>> ext4_page_mkwrite() does not grab i_rwsem, so it may race with >>>> ext4_buffered_write_iter() and cause the write_begin() and write_end() >>>> functions to be inconsistent and trigger BUG_ON. >>>> >>>> To solve the above issue, we cannot add inode_lock directly to >>>> ext4_page_mkwrite(), because this function is a hot path and frequent calls >>>> to inode_lock will cause performance degradation for multi-threaded reads >>>> and writes. Hence, we move ext4_convert_inline_data() to ext4_file_mmap(), >>>> and only when inline_data is enabled and mmap a file in shared write mode, >>>> we hold the lock to convert, which can reduce the impact on performance. >>>> >>>> Reported-by: Jun Nie >>>> Closes: https://lore.kernel.org/lkml/63903521.5040307@huawei.com/t/ >>>> Reported-by: syzbot+a158d886ca08a3fecca4@syzkaller.appspotmail.com >>>> Closes: https://syzkaller.appspot.com/bug?id=899b37f20ce4072bcdfecfe1647b39602e956e36 >>>> Fixes: 7b4cc9787fe3 ("ext4: evict inline data when writing to memory map") >>>> CC: stable@vger.kernel.org # 4.12+ >>>> Signed-off-by: Baokun Li >>> Thanks for the patch! The problem with i_rwsem in ext4_page_mkwrite() is >>> not so much about performance as about lock ordering. In >>> ext4_page_mkwrite() we are called with mmap_sem held and so we cannot >>> acquire i_rwsem because it ranks about it. >> Thank you for your review! >> >> I'm sorry I didn't make myself clear here. >> >> Yes, we can't get i_rwsem after holding mmap_sem at any time, otherwise >> ABBA deadlock may occur. The "add inode_lock directly" in my patch >> description actually looks like this: >> ``` >> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c >> index b98d2d58b900..c9318dc2a613 100644 >> --- a/fs/ext4/inode.c >> +++ b/fs/ext4/inode.c >> @@ -6025,12 +6025,14 @@ vm_fault_t ext4_page_mkwrite(struct vm_fault *vmf) >>         sb_start_pagefault(inode->i_sb); >>         file_update_time(vma->vm_file); >> >> -       filemap_invalidate_lock_shared(mapping); >> - >> +       inode_lock(inode); >>         err = ext4_convert_inline_data(inode); >> +       inode_unlock(inode); >>         if (err) >>                 goto out_ret; >> >> +       filemap_invalidate_lock_shared(mapping); >> + >>         /* >>          * On data journalling we skip straight to the transaction handle: >>          * there's no delalloc; page truncated will be checked later; the >> ``` > Yes, but even this could deadlock. The ABBA deadlock I'm speaking about > would not be created with mapping->invalidate_lock but rather with > task->mm->mmap_lock which is acquired at the beginning of page fault in the > arch code. Thanks for the explanation! I thought the mmap_sem said was "&EXT4_I(inode)->i_mmap_sem". But here are some more questions: 1) In the arch code page fault is handled by mmap_read_lock(mm) to get the    shared lock, why would this lead to ABBA deadlock? 2) Why would page fault be triggered in the write process? Could you explain it in more detail? > And when you do write(2), you hold inode_lock() and then you > copy data from the use provided buffer to the pagecache pages and that can > cause a page fault on the user provided buffer which will try to grab > task->mm->mmap_lock. > > This lock inversion is the main reason why inode lock cannot be used > anywhere in the page fault path. > > Honza > Looking forward to hearing from you! 🤔 -- With Best Regards, Baokun Li .