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

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!