linux-ext4 - Re: [PATCH 2/2] jbd2: do not clear the BH_Mapped flag when forgetting a metadata buffer

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Date:   Wed, 12 Feb 2020 21:14:56 +0800
From:   "zhangyi (F)" <yi.zhang@...wei.com>
To:     Jan Kara <jack@...e.cz>
CC:     <tytso@....edu>, <linux-ext4@...r.kernel.org>,
        <luoshijie1@...wei.com>, <zhangxiaoxu5@...wei.com>
Subject: Re: [PATCH 2/2] jbd2: do not clear the BH_Mapped flag when forgetting
 a metadata buffer

Hi,

On 2020/2/12 16:47, Jan Kara wrote:
> On Tue 11-02-20 14:51:10, zhangyi (F) wrote:
>> On 2020/2/6 19:46, Jan Kara wrote:
>>> On Mon 03-02-20 22:04:58, zhangyi (F) wrote:
>>>> Commit 904cdbd41d74 ("jbd2: clear dirty flag when revoking a buffer from
>>>> an older transaction") set the BH_Freed flag when forgetting a metadata
>>>> buffer which belongs to the committing transaction, it indicate the
>>>> committing process clear dirty bits when it is done with the buffer. But
>>>> it also clear the BH_Mapped flag at the same time, which may trigger
>>>> below NULL pointer oops when block_size < PAGE_SIZE.
>>>>
>>>> rmdir 1             kjournald2                 mkdir 2
>>>>                     jbd2_journal_commit_transaction
>>>> 		    commit transaction N
>>>> jbd2_journal_forget
>>>> set_buffer_freed(bh1)
>>>>                     jbd2_journal_commit_transaction
>>>>                      commit transaction N+1
>>>>                      ...
>>>>                      clear_buffer_mapped(bh1)
>>>>                                                ext4_getblk(bh2 ummapped)
>>>>                                                ...
>>>>                                                grow_dev_page
>>>>                                                 init_page_buffers
>>>>                                                  bh1->b_private=NULL
>>>>                                                  bh2->b_private=NULL
>>>>                      jbd2_journal_put_journal_head(jh1)
>>>>                       __journal_remove_journal_head(hb1)
>>>> 		       jh1 is NULL and trigger oops
>>>>
>>>> *) Dir entry block bh1 and bh2 belongs to one page, and the bh2 has
>>>>    already been unmapped.
>>>>
>>>> For the metadata buffer we forgetting, clear the dirty flags is enough,
>>>> so this patch add BH_Unmap flag for the journal_unmap_buffer() case and
>>>> keep the mapped flag for the metadata buffer.
>>>>
>>>> Fixes: 904cdbd41d74 ("jbd2: clear dirty flag when revoking a buffer from an older transaction")
>>>> Signed-off-by: zhangyi (F) <yi.zhang@...wei.com>
>> [..]
>>>
>>> Also rather than introducing this new buffer_unmap bit, I'd use the fact
>>> this special treatment is needed only for buffers coming from the block device
>>> mapping. And we can check for that like:
>>>
>>> 		/*
>>> 		 * We can (and need to) unmap buffer only for normal mappings.
>>> 		 * Block device buffers need to stay mapped all the time.
>>> 		 * We need to be careful about the check because the page
>>> 		 * mapping can get cleared under our hands.
>>> 		 */
>>> 		mapping = READ_ONCE(bh->b_page->mapping);
>>> 		if (mapping && !sb_is_blkdev_sb(mapping->host->i_sb)) {
>>> 			...
>>> 		}
>>
>> Think about it again, it may missing clearing of mapped flag if 'mapping'
>> of journalled data page was cleared, and finally trigger exception if
>> we reuse the buffer again. So I think it should be:
>>
>> 		if (!(mapping && sb_is_blkdev_sb(mapping->host->i_sb))) {
>> 			...
>> 		}
> 
> Well, if b_page->mapping got cleared, it means the page got fully truncated
> and in such case buffers can never be reused - the page and buffers will be
> freed once we are done with them. So what you are concerned about cannot
> happen. But you're right it is good to explain this in the comment.
> 
Yes, you are right, the page and buffer will be freed in release_buffer_page()
and it seems there is no exception, I will send V3 to back to use the judgement
condition as you suggested and add comments after tests.

Thanks,
Yi.