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

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Date: Thu, 21 Dec 2023 13:49:36 +0100
From: Jan Kara <jack@...e.cz>
To: Christoph Hellwig <hch@....de>
Cc: Jan Kara <jack@...e.cz>, 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 21-12-23 13:29:10, Christoph Hellwig wrote:
> 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

Agreed. The macro is kind of neat but a magic like this tends to bite us in
surprising ways. E.g. if someone breaks out of the loop, things will go
really wrong (missing writeback_finish() call). That would be actually a
good usecase for the cleanup handlers PeterZ has been promoting - we could
make sure writeback_finish() is called whenever we exit the loop block.

> 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;

That would work for me as well. But I don't feel to strongly about this
either way.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR