Re: [PATCH 03/13] filemap: align the index to mapping_min_order in the page cache

[Kent Overstreet]

On Tue, Feb 27, 2024 at 05:36:09PM +0100, Pankaj Raghav (Samsung) wrote:
> On Tue, Feb 27, 2024 at 11:22:24AM -0500, Kent Overstreet wrote:
> > 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@xxxxxxxxxx>
> > > > > 
> > > > > 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()
> 
> Umm, what do you mean just use rounddown? rounddown to ...?
> 
> We need to get index that are in PAGE units but aligned to min_order
> pages.
> 
> The original patch did this:
> 
> index = mapping_align_start_index(mapping, iocb->ki_pos >> PAGE_SHIFT);
> 
> Which is essentially a rounddown operation (probably this is what you
> are suggesting?).
> 
> So what willy is proposing will do the same. To me, what I proposed is
> less complicated but to willy it is the other way around.

Ok, I just found the code for mapping_align_start_index() - it is just a
round_down().

Never mind; patch looks fine (aside from perhaps some quibbling over
whether the round_down()) should be done before calling readahead or
within readahead; I think that might have been more what willy was
keying in on)