Re: [PATCH 04/12] bdi: initialize ->ra_pages and ->io_pages in bdi_init

[Christoph Hellwig]

On Thu, Sep 17, 2020 at 12:04:59PM +0200, Jan Kara wrote:
> On Thu 10-09-20 16:48:24, Christoph Hellwig wrote:
> > Set up a readahead size by default, as very few users have a good
> > reason to change it.
> > 
> > Signed-off-by: Christoph Hellwig <hch@xxxxxx>
> > Acked-by: David Sterba <dsterba@xxxxxxxx> [btrfs]
> > Acked-by: Richard Weinberger <richard@xxxxxx> [ubifs, mtd]
> 
> Looks good but what about coda, ecryptfs, and orangefs? Currenly they have
> readahead disabled and this patch would seem to enable it?

When going through this I pinged all maintainers and asked if anyone
had a reason to actually disable the readahead, and only vbox and
the mtd/ubifs maintainers came up with a reason.

> 
> > diff --git a/mm/backing-dev.c b/mm/backing-dev.c
> > index 8e8b00627bb2d8..2dac3be6127127 100644
> > --- a/mm/backing-dev.c
> > +++ b/mm/backing-dev.c
> > @@ -746,6 +746,8 @@ struct backing_dev_info *bdi_alloc(int node_id)
> >  		kfree(bdi);
> >  		return NULL;
> >  	}
> > +	bdi->ra_pages = VM_READAHEAD_PAGES;
> > +	bdi->io_pages = VM_READAHEAD_PAGES;
> 
> Won't this be more logical in bdi_init() than in bdi_alloc()?

bdi_init is also used for noop_backing_dev_info, which should not
have readahead enabled.  In fact the only caller except for
bdi_alloc is the initialization of noop_backing_dev_info.