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

[Baokun Li]

On 2023/6/5 23:08, Jan Kara wrote:
> On Mon 05-06-23 15:55:35, Matthew Wilcox wrote:
> > On Mon, Jun 05, 2023 at 02:21:41PM +0200, Jan Kara wrote:
> > > On Mon 05-06-23 11:16:55, Jan Kara wrote:
> > > > Yeah, I agree, that is also the conclusion I have arrived at when thinking
> > > > about this problem now. We should be able to just remove the conversion
> > > > from ext4_page_mkwrite() and rely on write(2) or truncate(2) doing it when
> > > > growing i_size.
> > > OK, thinking more about this and searching through the history, I've
> > > realized why the conversion is originally in ext4_page_mkwrite(). The
> > > problem is described in commit 7b4cc9787fe35b ("ext4: evict inline data
> > > when writing to memory map") but essentially it boils down to the fact that
> > > ext4 writeback code does not expect dirty page for a file with inline data
> > > because ext4_write_inline_data_end() should have copied the data into the
> > > inode and cleared the folio's dirty flag.
> > >
> > > Indeed messing with xattrs from the writeback path to copy page contents
> > > into inline data xattr would be ... interesting. Hum, out of good ideas for
> > > now :-|.
> > Is it so bad?  Now that we don't have writepage in ext4, only
> > writepages, it seems like we have a considerably more benign locking
> > environment to work in.
> Well, yes, without ->writepage() it might be *possible*. But still rather
> ugly. The problem is that in ->writepages() i_size is not stable. Thus also
> whether the inode data is inline or not is not stable. We'd need inode_lock
> for that but that is not easily doable in the writeback path - inode lock
> would then become fs_reclaim unsafe...
>
> 								Honza

If we try to add inode_lock to ext4_writepages to complete the
conversion, there may be a deadlock as follows:

      CPU0             CPU1
writeback_single_inode
 spin_lock(&inode->i_lock) ---> LOCK B
                   enable_verity
                    inode_lock(inode)  ---> LOCK A
                    vops->begin_enable_verity
                    ext4_begin_enable_verity
                     ext4_inode_attach_jinode
                      spin_lock(&inode->i_lock)   ---> try LOCK B
 __writeback_single_inode          |
  do_writepages                ABBA deadlock
   ext4_writepages                 |
    inode_lock(inode)  ---> try LOCK A

If we add inode_lock to the write back process to complete the inline 
conversion,
it seems that we still have to add an ops ...

I've been going over this problem for a long time, but I can't think of 
a good way
to solve it.

--
With Best Regards,
Baokun Li
.