Re: [PATCH 1/6] fs: Move clearing of mappedtodisk to buffer.c

[Matthew Wilcox <willy@infradead.org>]

On Thu, Oct 03, 2024 at 02:10:20PM +0200, Jan Kara wrote:
> On Wed 02-10-24 05:01:03, Matthew Wilcox (Oracle) wrote:
> > The mappedtodisk flag is only meaningful for buffer head based
> > filesystems.  It should not be cleared for other filesystems.  This allows
> > us to reuse the mappedtodisk flag to have other meanings in filesystems
> > that do not use buffer heads.
> > 
> > Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> 
> The patch looks good. But I'm bit confused about the changelog. There's no
> generic code checking for mappedtodisk. Only nilfs2 actually uses it for
> anything, all other filesystems just never look at it as far as my grepping
> shows. So speaking about "filesystems that do not use buffer heads" looks
> somewhat broad to me. Anyway feel free to add:

Hmm.  f2fs also uses it in page_mkwrite().  But it looks odd to me.
Perhaps we could get rid of mappedtodisk entirely ... I see ext4
used to use it until someone removed it in 9ea7df534ed2 ;-)

Anyway, what the changelog is trying to say is that only
buffer-head filesystems ever have the mappedtodisk flag set, eg by
block_read_full_folio() or do_mpage_readpage().  So it doesn't make
sense to clear it for non-buffer-head filesystems, and may inhibit their
ability to use it for unrelated purposes.