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

[Baokun Li]

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
.