Re: [RFC v2 04/14] readahead: set file_ra_state->ra_pages to be at least mapping_min_order

["Pankaj Raghav (Samsung)" <kernel@pankajraghav.com>]

On Wed, Feb 14, 2024 at 02:32:20PM +0100, Pankaj Raghav (Samsung) wrote:
> On Wed, Feb 14, 2024 at 09:09:53AM +1100, Dave Chinner wrote:
> > On Tue, Feb 13, 2024 at 10:37:03AM +0100, Pankaj Raghav (Samsung) wrote:
> > > From: Luis Chamberlain <mcgrof@kernel.org>
> > > 
> > > Set the file_ra_state->ra_pages in file_ra_state_init() to be at least
> > > mapping_min_order of pages if the bdi->ra_pages is less than that.
> > > 
> > > Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
> > > ---
> > >  mm/readahead.c | 5 +++++
> > >  1 file changed, 5 insertions(+)
> > > 
> > > diff --git a/mm/readahead.c b/mm/readahead.c
> > > index 2648ec4f0494..4fa7d0e65706 100644
> > > --- a/mm/readahead.c
> > > +++ b/mm/readahead.c
> > > @@ -138,7 +138,12 @@
> > >  void
> > >  file_ra_state_init(struct file_ra_state *ra, struct address_space *mapping)
> > >  {
> > > +	unsigned int min_nrpages = mapping_min_folio_nrpages(mapping);
> > > +	unsigned int max_pages = inode_to_bdi(mapping->host)->io_pages;
> > > +
> > >  	ra->ra_pages = inode_to_bdi(mapping->host)->ra_pages;
> > > +	if (ra->ra_pages < min_nrpages && min_nrpages < max_pages)
> > > +		ra->ra_pages = min_nrpages;
> > 
> > Why do we want to clamp readahead in this case to io_pages?
> > 
> > We're still going to be allocating a min_order folio in the page
> > cache, but it is far more efficient to initialise the entire folio
> > all in a single readahead pass than it is to only partially fill it
> > with data here and then have to issue and wait for more IO to bring
> > the folio fully up to date before we can read out data out of it,
> > right?

I think I misunderstood your question. I got more context after seeing
your next response.

You are right, I will remove the clamp to io_pages. So a single FSB
might be split into multiple IOs if the underlying block device has
io_pages < min_nrpages.

> 
> We are not clamping it to io_pages. ra_pages is set to min_nrpages if
> bdi->ra_pages is less than the min_nrpages. The io_pages parameter is
> used as a sanity check so that min_nrpages does not go beyond it.
> 
> So maybe, this is not the right place to check if we can at least send
> min_nrpages to the backing device but instead do it during mount?
> 
> > 
> > -Dave.
> > -- 
> > Dave Chinner
> > david@fromorbit.com