Re: [PATCH v2] udf: Fix race between file type conversion and writeback

[Christoph Hellwig <hch@infradead.org>]

On Wed, Mar 25, 2026 at 05:22:10PM +0100, Jan Kara wrote:
> udf_setsize() can race with udf_writepages() as follows:
> 
> udf_setsize()			udf_writepages()
> 				  if (iinfo->i_alloc_type ==
> 						ICBTAG_FLAG_AD_IN_ICB)
>   err = udf_expand_file_adinicb(inode);
>   err = udf_extend_file(inode, newsize);
> 				    udf_adinicb_writepages()
> 				      memcpy_from_file_folio() - crash
> 					because inode size is too big.
> 
> Fix the problem by rechecking file type under folio lock in
> udf_writepages() which properly serializes with
> udf_expand_file_adinicb(). Since it is quite difficult to implement this
> locking with current writeback_iter() logic, let's just opencode the
> logic necessary to prepare (the only) folio the inode can have for
> writeback.

Still not a fan of open coding this logic, which is even worse than
the previous export.  Inline data is a relatively common case in
various file systems, and we should be able to handle it in common
code.  So having a callback in mpage_writepages seems much better,
but if you detest that we could just export mpage_write_folio and
mpage_bio_submit_write (and make mpage_data public) instead.  I think
that's significantly worse than just adding the callback, but still
much better than open coding more writeback internals, something I
want to get away from.