Re: [PATCH v2] ext4: fix race condition between buffer write and page_mkwrite

Baokun Li
2023-06-05T02:17:38Z

On 2023/6/4 11:04, Theodore Ts'o wrote:
> I tried testing to see if this fixed [1], and it appears to be
> triggering a lockdep warning[2] at this line in the patch:
>
> [1] https://syzkaller.appspot.com/bug?extid=f4582777a19ec422b517
> [2] https://syzkaller.appspot.com/x/report.txt?x=17260843280000
>
>> diff --git a/fs/ext4/file.c b/fs/ext4/file.c
>> index d101b3b0c7da..9df82d72eb90 100644
>> --- a/fs/ext4/file.c
>> +++ b/fs/ext4/file.c
>> @@ -808,6 +809,27 @@ static int ext4_file_mmap(struct file *file, struct vm_area_struct *vma)
>> if (!daxdev_mapping_supported(vma, dax_dev))
>> return -EOPNOTSUPP;
>>
>> + /*
>> + * Writing via mmap has no logic to handle inline data, so we
>> + * need to call ext4_convert_inline_data() to convert the inode
>> + * to normal format before doing so, otherwise a BUG_ON will be
>> + * triggered in ext4_writepages() due to the
>> + * EXT4_STATE_MAY_INLINE_DATA flag. Moreover, we need to grab
>> + * i_rwsem during conversion, since clearing and setting the
>> + * inline data flag may race with ext4_buffered_write_iter()
>> + * to trigger a BUG_ON.
>> + */
>> + if (ext4_has_feature_inline_data(sb) &&
>> + vma->vm_flags & VM_SHARED && vma->vm_flags & VM_MAYWRITE) {
>> + int err;
>> +
>> + inode_lock(inode); <=================== LOCKDEP warning
>> + err = ext4_convert_inline_data(inode);
>> + inode_unlock(inode);
>> + if (err)
>> + return err;
>> + }
>
> The details of the lockdep warning from [2], which appears to be a
> mmap(2) racing with a buffered write(2) are below. Could you take a
> look?
>
> Thanks!
>
> - Ted

Sorry for the late reply!

Had a look at this question which is similar to the one Honza mentioned
earlier.
Concurrency between write and mmap as follows can lead to ABBA deadlocks:

CPU0                   CPU1
write(2)                mmap(2)
ext4_file_write_iter
ext4_buffered_write_iter
inode_lock(inode)  ---> LOCK A
generic_perform_write
ksys_mmap_pgoff
vm_mmap_pgoff
mmap_write_lock_killable(mm) ---> LOCK B
do_mmap
mmap_region
call_mmap
ext4_file_mmap
inode_lock(inode)  ---> try LOCK A again
fault_in_iov_iter_readable              |
fault_in_readable                      |
asm_exc_page_fault                ABBA deadlock
handle_page_fault                    |
do_user_addr_fault                  |
mmap_read_lock(mm) ---> try LOCK B again

Thanks!
--
With Best Regards,
Baokun Li
.