linux-kernel - Re: [PATCH 15/17] writeback: Add for_each_writeback

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Date: Thu, 21 Dec 2023 13:29:10 +0100
From: Christoph Hellwig <hch@....de>
To: Jan Kara <jack@...e.cz>
Cc: Christoph Hellwig <hch@....de>, linux-mm@...ck.org,
	"Matthew Wilcox (Oracle)" <willy@...radead.org>,
	Jan Kara <jack@...e.com>, David Howells <dhowells@...hat.com>,
	Brian Foster <bfoster@...hat.com>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 15/17] writeback: Add for_each_writeback_folio()

On Thu, Dec 21, 2023 at 12:51:49PM +0100, Jan Kara wrote:
> On Mon 18-12-23 16:35:51, Christoph Hellwig wrote:
> > From: "Matthew Wilcox (Oracle)" <willy@...radead.org>
> > 
> > Wrap up the iterator with a nice bit of syntactic sugar.  Now the
> > caller doesn't need to know about wbc->err and can just return error,
> > not knowing that the iterator took care of storing errors correctly.
> > 
> > Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
> > Signed-off-by: Christoph Hellwig <hch@....de>
> 
> Not sure if the trick with 'error' variable isn't a bit too clever for us
> ;) We'll see how many bugs it will cause in the future...

It's a bit too much syntactic sugar for my taste, but if we want a magic
for macro I can't really see a good way around it.  I personally wouldn't
mind a version where the writeback_get_folio moves out of
writeback_iter_init and the pattern would look more like:

	writeback_iter_init(mapping, wbc);
	while ((folio = writeback_iter_next(mapping, wbc, folio))) {
		wbc->err = <do something>
	}

	return wbc->err;