Message-ID: <908502d6-cb0c-44ae-8c03-9a22c8c7fbf2@163.com>
Date: Wed, 16 Oct 2024 21:38:03 +0800
From: liubaolin <liubaolin12138@....com>
To: Jan Kara <jack@...e.cz>
Cc: tytso@....edu, adilger.kernel@...ger.ca, zhangshida@...inos.cn,
 longzhi@...gfor.com.cn, linux-ext4@...r.kernel.org,
 linux-kernel@...r.kernel.org, Baolin Liu <liubaolin@...inos.cn>
Subject: Re: [PATCH v1] ext4: fix a assertion failure due to ungranted bh
 dirting

> Hello,
> I reviewed the patch attached in your email. The issue you mentioned about clearing buffer_new(bh) in write_end_fn() is indeed a bug.
> However, this patch does not resolve the crash issue we encountered.
> 
> Let me explain my analysis in detail below.
> The crash occurs in the function jbd2_journal_dirty_metadata().
> 
> ext4_block_write_begin() -> ext4_journalled_zero_new_buffers() -> write_end_fn()
>  -> ext4_dirty_journalled_data() -> ext4_handle_dirty_metadata() -> __ext4_handle_dirty_metadata()
>  -> jbd2_journal_dirty_metadata()
> 
> In the function jbd2_journal_dirty_metadata(), there is the following condition:
> —---------------------------------------------------------------------------------------------------
>         if (data_race(jh->b_transaction != transaction &&
>             jh->b_next_transaction != transaction)) {
>                 spin_lock(&jh->b_state_lock);
>                 J_ASSERT_JH(jh, jh->b_transaction == transaction ||
>                                 jh->b_next_transaction == transaction);
>                 spin_unlock(&jh->b_state_lock);
>         }
> ----------------------------------------------------------------------------------------------------
> By analyzing the vmcore, I found that both jh->b_transaction and jh->b_next_transaction are NULL.
> Through code analysis, I discovered that the __jbd2_journal_file_buffer() function adds the corresponding transaction of bh to jh->b_transaction.
> Normally, this is accessed through do_journal_get_write_access(), which can call __jbd2_journal_file_buffer().
> The detailed function call process is as follows:
> do_journal_get_write_access() -> ext4_journal_get_write_access() -> __ext4_journal_get_write_access()
>  -> jbd2_journal_get_write_access() -> do_get_write_access() -> __jbd2_journal_file_buffer()
>  
> 
> Therefore, resolving the crash issue requires obtaining write access before calling the jbd2_journal_dirty_metadata() function.
> The comment at the definition of the jbd2_journal_dirty_metadata() function also states: 
> 	'The buffer must have previously had jbd2_journal_get_write_access().'
> 
> In the ext4_block_write_begin() function, if get_block() encounters an error, then neither bh->b_this_page nor the subsequent bh calls do_journal_get_write_access().
> If bh->b_this_page and the subsequent bh are in the new state, it will lead to a crash when reaching the jbd2_journal_dirty_metadata() function.
> 	
> So, there are two ways to resolve this crash issue:
> 1、Call do_journal_get_write_access() on bh that is not handled due to get_block() error.
> 	The patch modification is in the attachment 0001-ext4-fix-a-assertion-failure-due-to-ungranted-bh-dir.patch.
> 
> 2、Call clear_buffer_new() on bh that is not handled due to get_block() error.
> 	The patch modification is in the attachment 0001-ext4-fix-a-assertion-failure-due-to-bh-not-clear-new.patch.
> 
> Additionally, I have found a method to quickly reproduce this crash issue.
> For details, please refer to the email I previously sent you: “https://lore.kernel.org/all/bd41c24b-7325-4584-a965-392a32e32c74@163.com/”.
> I have verified that this quick reproduction method works for both solutions to resolve the issue.
> 
> Please continue to consider which method is better to resolve this issue. 
> If you think that using clear_buffer_new() is a better solution, I can resend the patch via git send-mail.



在 2024/10/16 18:33, Jan Kara 写道:
> Hello,
> 
> On Fri 11-10-24 12:08:58, Baolin Liu wrote:
>> Greetings，
>>
>> This problem is reproduced by our customer using their own testing tool
>> “run_bug”. When I consulted with a client, the testing tool “run_bug”
>> used a variety of background programs to benchmark (including memory
>> pressure, cpu pressure, file cycle manipulation, fsstress Stress testing
>> tool, postmark program，and so on).
>>
>> The recurrence probability is relatively low.
> 
> OK, thanks for asking!
> 
>> In response to your query, in ext4_block_write_begin, the new state will
>> be clear before get block, and the bh that failed get_block will not be
>> set to new. However, when the page size is greater than the block size, a
>> page will contain multiple bh.
> 
> True. I wanted to argue that the buffer_new bit should be either cleared in
> ext4_block_write_begin() (in case of error) or in
> ext4_journalled_write_end() (in case of success) but actually
> ext4_journalled_write_end() misses the clearing. So I think the better
> solution is like the attached patch. I'll submit it once testing finishes
> but it would be great if you could test that it fixes your problems as
> well. Thanks!
> 
> 								Honza

View attachment "0001-ext4-fix-a-assertion-failure-due-to-bh-not-clear-new.patch" of type "text/plain" (2616 bytes)

View attachment "0001-ext4-fix-a-assertion-failure-due-to-ungranted-bh-dir.patch" of type "text/plain" (2676 bytes)

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