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

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Message-ID: <20200919070117.GB8237@lst.de>
Date:   Sat, 19 Sep 2020 09:01:17 +0200
From:   Christoph Hellwig <hch@....de>
To:     Jan Kara <jack@...e.cz>
Cc:     Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>,
        Song Liu <song@...nel.org>,
        Hans de Goede <hdegoede@...hat.com>,
        Richard Weinberger <richard@....at>,
        Minchan Kim <minchan@...nel.org>,
        linux-mtd@...ts.infradead.org, dm-devel@...hat.com,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        drbd-dev@...ts.linbit.com, linux-raid@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        cgroups@...r.kernel.org, David Sterba <dsterba@...e.com>
Subject: Re: [PATCH 04/12] bdi: initialize ->ra_pages and ->io_pages in
 bdi_init

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@....de>
> > Acked-by: David Sterba <dsterba@...e.com> [btrfs]
> > Acked-by: Richard Weinberger <richard@....at> [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.