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

Matthew Wilcox
2023-06-05T14:55:54Z

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.