linux-kernel - Re: [RFC PATCH] f2fs: Convert f2fs_write_cache_pages() to use filemap_get_folios

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Date:   Tue, 3 Jan 2023 20:53:47 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Jaegeuk Kim <jaegeuk@...nel.org>
Cc:     "Vishal Moola (Oracle)" <vishal.moola@...il.com>, chao@...nel.org,
        linux-kernel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, linux-mm@...ck.org,
        fengnanchang@...il.com, linux-fsdevel@...r.kernel.org
Subject: Re: [RFC PATCH] f2fs: Convert f2fs_write_cache_pages() to use
 filemap_get_folios_tag()

On Thu, Dec 15, 2022 at 11:02:24AM -0800, Jaegeuk Kim wrote:
> On 12/12, Vishal Moola (Oracle) wrote:
> > @@ -2994,13 +2998,38 @@ static int f2fs_write_cache_pages(struct address_space *mapping,
> >  		tag_pages_for_writeback(mapping, index, end);
> >  	done_index = index;
> >  	while (!done && !retry && (index <= end)) {
> > -		nr_pages = find_get_pages_range_tag(mapping, &index, end,
> > -				tag, F2FS_ONSTACK_PAGES, pages);
> > -		if (nr_pages == 0)
> > +		nr_pages = 0;
> > +again:
> > +		nr_folios = filemap_get_folios_tag(mapping, &index, end,
> > +				tag, &fbatch);
> 
> Can't folio handle this internally with F2FS_ONSTACK_PAGES and pages?

I really want to discourage filesystems from doing this kind of thing.
The folio_batch is the natural size for doing batches of work, and
having the consistency across all these APIs of passing in a folio_batch
is quite valuable.  I understand f2fs wants to get more memory in a
single batch, but the right way to do that is to use larger folios.