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Date: Tue, 27 Feb 2024 11:22:24 -0500
From: Kent Overstreet <kent.overstreet@linux.dev>
To: "Pankaj Raghav (Samsung)" <kernel@pankajraghav.com>
Cc: Matthew Wilcox <willy@infradead.org>, linux-xfs@vger.kernel.org, 
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, david@fromorbit.com, 
	chandan.babu@oracle.com, akpm@linux-foundation.org, mcgrof@kernel.org, ziy@nvidia.com, 
	hare@suse.de, djwong@kernel.org, gost.dev@samsung.com, linux-mm@kvack.org, 
	Pankaj Raghav <p.raghav@samsung.com>
Subject: Re: [PATCH 03/13] filemap: align the index to mapping_min_order in
 the page cache
Message-ID: <hjrsbb34ghop4qbb6owmg3wqkxu4l42yrekshwfleeqattscqp@z2epeibc67lt>
References: <20240226094936.2677493-1-kernel@pankajraghav.com>
 <20240226094936.2677493-4-kernel@pankajraghav.com>
 <Zdyi6lFDAHXi8GPz@casper.infradead.org>
 <37kubwweih4zwvxzvjbhnhxunrafawdqaqggzcw6xayd6vtrfl@dllnk6n53akf>
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In-Reply-To: <37kubwweih4zwvxzvjbhnhxunrafawdqaqggzcw6xayd6vtrfl@dllnk6n53akf>

On Tue, Feb 27, 2024 at 11:06:37AM +0100, Pankaj Raghav (Samsung) wrote:
> On Mon, Feb 26, 2024 at 02:40:42PM +0000, Matthew Wilcox wrote:
> > On Mon, Feb 26, 2024 at 10:49:26AM +0100, Pankaj Raghav (Samsung) wrote:
> > > From: Luis Chamberlain <mcgrof@kernel.org>
> > > 
> > > Supporting mapping_min_order implies that we guarantee each folio in the
> > > page cache has at least an order of mapping_min_order. So when adding new
> > > folios to the page cache we must ensure the index used is aligned to the
> > > mapping_min_order as the page cache requires the index to be aligned to
> > > the order of the folio.
> > 
> > This seems like a remarkably complicated way of achieving:
> > 
> > diff --git a/mm/filemap.c b/mm/filemap.c
> > index 5603ced05fb7..36105dad4440 100644
> > --- a/mm/filemap.c
> > +++ b/mm/filemap.c
> > @@ -2427,9 +2427,11 @@ static int filemap_update_page(struct kiocb *iocb,
> >  }
> >  
> >  static int filemap_create_folio(struct file *file,
> > -		struct address_space *mapping, pgoff_t index,
> > +		struct address_space *mapping, loff_t pos,
> >  		struct folio_batch *fbatch)
> >  {
> > +	pgoff_t index;
> > +	unsigned int min_order;
> >  	struct folio *folio;
> >  	int error;
> >  
> > @@ -2451,6 +2453,8 @@ static int filemap_create_folio(struct file *file,
> >  	 * well to keep locking rules simple.
> >  	 */
> >  	filemap_invalidate_lock_shared(mapping);
> > +	min_order = mapping_min_folio_order(mapping);
> > +	index = (pos >> (min_order + PAGE_SHIFT)) << min_order;
> 
> That is some cool mathfu. I will add a comment here as it might not be
> that obvious to some people (i.e me).

you guys are both wrong, just use rounddown()