Re: [PATCH 12/13] writeback: add a writeback iterator

[Brian Foster <bfoster@redhat.com>]

On Wed, Feb 07, 2024 at 09:42:24AM +0100, Jan Kara wrote:
> On Tue 06-02-24 09:54:01, Brian Foster wrote:
> > On Mon, Feb 05, 2024 at 10:33:52AM -0500, Brian Foster wrote:
> > > On Sat, Feb 03, 2024 at 08:11:46AM +0100, Christoph Hellwig wrote:
> > > > Refactor the code left in write_cache_pages into an iterator that the
> > > > file system can call to get the next folio for a writeback operation:
> > > > 
> > > > 	struct folio *folio = NULL;
> > > > 
> > > > 	while ((folio = writeback_iter(mapping, wbc, folio, &error))) {
> > > > 		error = <do per-foli writeback>;
> > > > 	}
> > > > 
> > > > The twist here is that the error value is passed by reference, so that
> > > > the iterator can restore it when breaking out of the loop.
> > > > 
> > > > Handling of the magic AOP_WRITEPAGE_ACTIVATE value stays outside the
> > > > iterator and needs is just kept in the write_cache_pages legacy wrapper.
> > > > in preparation for eventually killing it off.
> > > > 
> > > > Heavily based on a for_each* based iterator from Matthew Wilcox.
> > > > 
> > > > Signed-off-by: Christoph Hellwig <hch@lst.de>
> > > > ---
> > > >  include/linux/writeback.h |   4 +
> > > >  mm/page-writeback.c       | 192 ++++++++++++++++++++++----------------
> > > >  2 files changed, 118 insertions(+), 78 deletions(-)
> > > > 
> > > ...
> > > > diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> > > > index 3abb053e70580e..5fe4cdb7dbd61a 100644
> > > > --- a/mm/page-writeback.c
> > > > +++ b/mm/page-writeback.c
> > > ...
> > > > @@ -2434,69 +2434,68 @@ static struct folio *writeback_get_folio(struct address_space *mapping,
> > > >  }
> > > >  
> > > >  /**
> > > ...
> > > >   */
> > > > -int write_cache_pages(struct address_space *mapping,
> > > > -		      struct writeback_control *wbc, writepage_t writepage,
> > > > -		      void *data)
> > > > +struct folio *writeback_iter(struct address_space *mapping,
> > > > +		struct writeback_control *wbc, struct folio *folio, int *error)
> > > >  {
> > > ...
> > > > +	} else {
> > > >  		wbc->nr_to_write -= folio_nr_pages(folio);
> > > >  
> > > > -		if (error == AOP_WRITEPAGE_ACTIVATE) {
> > > > -			folio_unlock(folio);
> > > > -			error = 0;
> > > > -		}
> > > > +		WARN_ON_ONCE(*error > 0);
> > > 
> > > Why the warning on writeback error here? It looks like new behavior, but
> > > maybe I missed something. Otherwise the factoring LGTM.
> > 
> > Err, sorry.. I glossed over the > 0 check and read it as < 0.
> > Disregard, this seems reasonable to me as long as we no longer expect
> > those AOP returns (which I'm not really clear on either, but anyways..):
> > 
> > Reviewed-by: Brian Foster <bfoster@redhat.com>
> 
> So my understanding is that AOP_WRITEPAGE_ACTIVATE should be now handled
> directly by the caller of ->writepage hook and not by writeback_iter()
> which is the reason why the warning is here.
> 

Yeah, I wasn't really familiar with the AOP error codes, saw that
multiple existed and just assumed they might be arbitrarily relevant
across different aop callbacks (and so then filtered the check/clear for
WRITEPAGE_ACTIVATE out of my brain ;). On taking a closer look, it seems
like the only other one (AOP_TRUNCATED_PAGE) doesn't have any relevance
to ->writepage(), so this all makes more sense to me now. Thanks.

Brian

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