Re: [QUESTION] question for commit 2d01ddc86606 ("ext4: save error info to sb through journal if available")

[yangerkun <yangerkun@huawei.com>]

在 2021/8/25 18:25, Jan Kara 写道:
> 
> Hello Kun!
> 
> On Wed 25-08-21 10:13:03, yangerkun wrote:
>> There is a question about 2d01ddc86606 ("ext4: save error info to sb through
>> journal if available"). This commit describe that we can have checksum
>> failure with follow case:
>>
>> 1. ext4_handle_error will call ext4_commit_super which write directly to the
>> superblock
>> 2. At the same time, jounalled update of the superblock is ongoing
>>
>> However, after commit 05c2c00f3769 ("ext4: protect superblock modifications
>> with a buffer lock"), all the update for superblock and the csum will be
>> protected with buffer lock. It seems we won't get a csum error after that
>> commit and the journal logic in flush_stashed_error_work seems useless.
>>
>> Maybe there is something missing... Can you help to explain more for that...
> 
> You are correct that after commit 05c2c00f3769 the checksum will be
> correct. However there are also other problems that 2d01ddc86606 addresses
> and that are mentioned in the commit description like "writing inconsistent
> information". The fundamental problem is that you cannot mix journalled and
> non-journalled updates to any block. Because e.g. the unjournalled update
> could store to disk information that was changed only as part of the
> currently running transaction and if the machine crashes before the
> transaction commits, we have too new information in the block and thus
> inconsistent filesystem. Or in the other direction, journal replay can
> overwrite unjournalled modifications to the superblock if we crash at the
> right moment.

Got it! Thanks for your explain!

> 
> 								Honza
>