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

[Pankaj Raghav (Samsung)]

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@xxxxxxxxxx>
> > 
> > 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@xxxxxxxxxx>
> > ---
> >  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?

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@xxxxxxxxxxxxx