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

Baokun Li
2023-12-12T02:16:07Z

On 2023/12/12 1:49, Jan Kara wrote:
> On Thu 07-12-23 22:15:55, Baokun Li wrote:
>> On 2023/12/7 3:37, Jan Kara wrote:
>>> On Tue 05-12-23 20:50:30, Baokun Li wrote:
>>>> On 2023/12/4 22:41, Jan Kara wrote:
>>>>> On Mon 04-12-23 21:50:18, Baokun Li wrote:
>>>>>> On 2023/12/4 20:11, Jan Kara wrote:
>>>>>> 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.
>>>>> Yeah, from what you write it seems that the replay code is using buffered
>>>>> reads on the journal file. I guess you could confirm that with a bit of
>>>>> kernel tracing but the symptoms look pretty convincing. Did you try talking
>>>>> to MYSQL guys about why they are doing this?
>>>> The operations performed on the relay log file are buffered reads and
>>>> writes, which I confirmed with the following bpftrace script:
>>>> ```
>>>> #include <linux/fs.h>
>>>> #include <linux/path.h>
>>>> #include <linux/dcache.h>
>>>>
>>>> kprobe:generic_file_buffered_read /!strncmp(str(((struct kiocb
>>>> *)arg0)->ki_filp->f_path.dentry->d_name.name), "relay", 5)/ {
>>>>     printf("read path: %s\n", str(((struct kiocb
>>>> *)arg0)->ki_filp->f_path.dentry->d_name.name));
>>>> }
>>>>
>>>> kprobe:ext4_buffered_write_iter /!strncmp(str(((struct kiocb
>>>> *)arg0)->ki_filp->f_path.dentry->d_name.name), "relay", 5)/ {
>>>>     printf("write path: %s\n", str(((struct kiocb
>>>> *)arg0)->ki_filp->f_path.dentry->d_name.name));
>>>> }
>>>> ```
>>>> I suspect there are DIO writes causing the problem, but I haven't caught
>>>> any DIO writes to such files via bpftrace.
>>> Interesting. Not sure how your partially zeroed-out buffers could happen
>>> with fully buffered IO.
>>>
>> After looking at the code again and again, the following concurrency
>> seems to bypass the memory barrier:
>>
>> ext4_buffered_write_iter
>>  generic_perform_write
>>   copy_page_from_iter_atomic
>>   ext4_da_write_end
>>    ext4_da_do_write_end
>>     block_write_end
>>      __block_commit_write
>>       folio_mark_uptodate
>>        smp_wmb()
>>        set_bit(PG_uptodate, folio_flags(folio, 0))
>>     i_size_write(inode, pos + copied)
>>     // write isize 2048
>>     unlock_page(page)
>>
>> ext4_file_read_iter
>>  generic_file_read_iter
>>   filemap_read
>>    filemap_get_pages
>>     filemap_get_read_batch
>>     folio_test_uptodate(folio)
>>      ret = test_bit(PG_uptodate, folio_flags(folio, 0));
>>      if (ret)
>>       smp_rmb();
>>       // The read barrier here ensures
>>       // that data 0-2048 in the page is synchronized.
>>                            ext4_buffered_write_iter
>>                             generic_perform_write
>>                              copy_page_from_iter_atomic
>>                              ext4_da_write_end
>>                               ext4_da_do_write_end
>>                                block_write_end
>>                                 __block_commit_write
>>                                  folio_mark_uptodate
>>                                   smp_wmb()
>>                                   set_bit(PG_uptodate, folio_flags(folio,
>> 0))
>>                                i_size_write(inode, pos + copied)
>>                                // write isize 4096
>>                                unlock_page(page)
>>    // read isize 4096
>>    isize = i_size_read(inode)
>>    // But there is no read barrier here,
>>    // so the data in the 2048-4096 range
>>    // may not be synchronized yet !!!
>>    copy_page_to_iter()
>>    // copyout 4096
>>
>> In the concurrency above, we read the updated i_size, but there is
>> no read barrier to ensure that the data in the page is the same as
>> the i_size at this point. Therefore, we may copy the unsynchronized
>> page out. Is it normal for us to read zero-filled data in this case?
> Indeed, I have checked and filemap_read() (but this dates back even to old
> do_generic_file_read() code) indeed does copy data only after checking
> uptodate flag and then sampling i_size so we may be copying state in the
> middle of the racing write and indeed there is nothing which would prevent
> prefetching page data before fetching inode size. I agree this is kind of
> nasty so I think adding a read barrier between i_size_read() and
> copy_page_to_iter() makes sense. Does it fix your issue with MYSQL?
>
> Honza
Thank you very much for confirming this issue!

Adding a read barrier between i_size_read() and copy_page_to_iter()
does seem to have solved the problem. The environment, which was
previously reproducing once every 2-3h, has been running for 60+h
without reproducing after adding the read barrier.

This is a Load-Load reordering issue, which only occurs on some
weak mem-ordering architectures (e.g. ARM64, ALPHA), but not
on strong mem-ordering architectures (e.g. X86).  This issue was
exactly discovered when migrating MYSQL services from X86 to
ARM64.

And theoretically the problem doesn't only happen on ext4,
filesystems that call filemap_read() but don't hold inode lock
(e.g. btrfs, f2fs, ubifs ...) will have this problem, while filesystems
with inode lock (e.g. xfs, nfs) won't have this problem.

Later I will send the patch that adds the memory barrier.

Thanks!
--
With Best Regards,
Baokun Li
.