Re: update BDI {io,ra}_pages values based on the RT device limits

[Christoph Hellwig <hch@lst.de>]

On Thu, Jun 25, 2026 at 01:12:05PM +0200, Jan Kara wrote:
> Hum, right. XFS users are used to tweak
> /sys/block/<dev>/queue/read_ahead_kb
> which would stop influencing anything. I guess that could indeed break some
> people's expectations.

Yeah.

> One more idea: We already cache bdi's ra_pages in struct file_ra_state on
> file open. It wouldn't be that hard to allow filesystem to override the
> values in its open method (currently it isn't possible only because
> file_ra_state_init() is called after ->open). For io_pages we currently
> always go to the bdi so we'd have to add it to struct file_ra_state.

It has to be called after ->open because ->open can change
file->f_mapping.  Now we could require any instance changing it to also
call file_ra_state_init, but I think bloating every struct file with a
new field might not win a lot of friends.  OTOH having a local copy
of ra_pages but not io_pages is really weird.  But so is this whole
cascading set of indirections for the readahead parameters.

> 
> 								Honza
> -- 
> Jan Kara <jack@suse.com>
> SUSE Labs, CR
---end quoted text---