Re: [PATCH -RFC 0/2] mm/ext4: avoid data corruption when extending DIO write race with buffered read

[Baokun Li <libaokun1@huawei.com>]

On 2023/12/4 20:11, Jan Kara wrote:
> Hello!
Thank you for your reply!
>
> On Sat 02-12-23 17:14:30, Baokun Li wrote:
>> Recently, while running some pressure tests on MYSQL, noticed that
>> occasionally a "corrupted data in log event" error would be reported.
>> After analyzing the error, I found that extending DIO write and buffered
>> read were competing, resulting in some zero-filled page end being read.
>> Since ext4 buffered read doesn't hold an inode lock, and there is no
>> field in the page to indicate the valid data size, it seems to me that
>> it is impossible to solve this problem perfectly without changing these
>> two things.
> Yes, combining buffered reads with direct IO writes is a recipe for
> problems and pretty much in the "don't do it" territory. So honestly I'd
> consider this a MYSQL bug. Were you able to identify why does MYSQL use
> buffered read in this case? It is just something specific to the test
> you're doing?
The problem is with a one-master-twoslave MYSQL database with three
physical machines, and using sysbench pressure testing on each of the
three machines, the problem occurs about once every two to three hours.

The problem is with the relay log file, and when the problem occurs, the
middle dozens of bytes of the file are read as all zeros, while the data on
disk is not. This is a journal-like file where a write process gets the 
data from
the master node and writes it locally, and another replay process reads the
file and performs the replay operation accordingly (some SQL statements).
The problem is that when replaying, it finds that the data read is 
corrupted,
not valid SQL data, while the data on disk is normal.

It's not confirmed that buffered reads vs direct IO writes is actually 
causing
this issue, but this is the only scenario that we can reproduce with our 
local
simplified scripts. Also, after merging in patch 1, the MYSQL pressure test
scenario has now been tested for 5 days and has not been reproduced.

I'll double-check the problem scenario, although buffered reads with 
buffered
writes doesn't seem to have this problem.
>> In this series, the first patch reads the inode size twice, and takes the
>> smaller of the two values as the copyout limit to avoid copying data that
>> was not actually read (0-padding) into the user buffer and causing data
>> corruption. This greatly reduces the probability of problems under 4k
>> page. However, the problem is still easily triggered under 64k page.
>>
>> The second patch waits for the existing dio write to complete and
>> invalidate the stale page cache before performing a new buffered read
>> in ext4, avoiding data corruption by copying the stale page cache to
>> the user buffer. This makes it much less likely that the problem will
>> be triggered in a 64k page.
>>
>> Do we have a plan to add a lock to the ext4 buffered read or a field in
>> the page that indicates the size of the valid data in the page? Or does
>> anyone have a better idea?
> No, there are no plans to address this AFAIK. Because such locking will
> slow down all the well behaved applications to fix a corner case for
> application doing unsupported things. Sure we must not crash the kernel,
> corrupt the filesystem or leak sensitive (e.g. uninitialized) data if app
> combines buffered and direct IO but returning zeros instead of valid data
> is in my opinion fully within the range of acceptable behavior for such
> case.
>
> 								Honza
I also feel that a scenario like buffered reads + DIO writes is strange. But
theoretically when read doesn't return an error, the data read shouldn't 
be wrong.
And I tested that xfs guarantees data consistency in this scenario, 
which is why I
thought it might be buggy.

Thanks!
-- 
With Best Regards,
Baokun Li
.