linux-ext4 - Re: [PATCH v3 2/2] jbd2: do not clear the BH_Mapped flag when forgetting a metadata buffer

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Date:   Mon, 17 Feb 2020 18:38:23 +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 v3 2/2] jbd2: do not clear the BH_Mapped flag when
 forgetting a metadata buffer

Hi,

On 2020/2/17 17:36, Jan Kara wrote:
> On Thu 13-02-20 14:38:21, 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, we should always keep the mapped
>> flag and clear the dirty flags is enough, so this patch pick out the
>> these buffers and keep their BH_Mapped flag.
>>
>> Fixes: 904cdbd41d74 ("jbd2: clear dirty flag when revoking a buffer from an older transaction")
>> Signed-off-by: zhangyi (F) <yi.zhang@...wei.com>
>> ---
>>  fs/jbd2/commit.c | 25 +++++++++++++++++++++----
>>  1 file changed, 21 insertions(+), 4 deletions(-)
> 
> Thanks! The patch looks good. I have just some comment reformulation
> suggestion below, otherwise feel free to add:
> 
> Reviewed-by: Jan Kara <jack@...e.cz>
> 
>> diff --git a/fs/jbd2/commit.c b/fs/jbd2/commit.c
>> index 6396fe70085b..27373f5792a4 100644
>> --- a/fs/jbd2/commit.c
>> +++ b/fs/jbd2/commit.c
>> @@ -985,12 +985,29 @@ void jbd2_journal_commit_transaction(journal_t *journal)
>>  		 * pagesize and it is attached to the last partial page.
>>  		 */
>>  		if (buffer_freed(bh) && !jh->b_next_transaction) {
>> +			struct address_space *mapping;
>> +
>>  			clear_buffer_freed(bh);
>>  			clear_buffer_jbddirty(bh);
>> -			clear_buffer_mapped(bh);
>> -			clear_buffer_new(bh);
>> -			clear_buffer_req(bh);
>> -			bh->b_bdev = NULL;
>> +
>> +			/*
>> +			 * Block device buffers need to stay mapped all the
>> +			 * time, so it is enough to clear buffer_jbddirty and
>> +			 * buffer_freed bits. For the file mapping buffers (i.e.
>> +			 * journalled data) we need to unmap buffer and clear
>> +			 * more bits. We also need to be careful about the check
>> +			 * because the data page mapping can get cleared under
>> +			 * out hands, which alse need not to clear more bits
> 			   ^^^ our    ^^^^ Maybe I'd rephrase this like:
> 
> ... under our hands. Note that if mapping == NULL, we don't need to make
> buffer unmapped because the page is already detached from the mapping and
> buffers cannot get reused.
> 
Thanks for your suggestion, Ted has already pushed this patch to upstream,
I could write an appending patch to fix this.

Thanks,
Yi.