linux-kernel - Re: mmotm 2009-06-02-16-11 uploaded (readahead)

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Date:	Wed, 3 Jun 2009 13:47:39 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Randy Dunlap <randy.dunlap@...cle.com>
Cc:	linux-kernel@...r.kernel.org, hifumi.hisashi@....ntt.co.jp,
	Jens Axboe <jens.axboe@...cle.com>,
	Wu Fengguang <fengguang.wu@...el.com>
Subject: Re: mmotm 2009-06-02-16-11 uploaded (readahead)

On Tue, 02 Jun 2009 20:54:39 -0700
Randy Dunlap <randy.dunlap@...cle.com> wrote:

> akpm@...ux-foundation.org wrote:
> > The mm-of-the-moment snapshot 2009-06-02-16-11 has been uploaded to
> > 
> >    http://userweb.kernel.org/~akpm/mmotm/
> > 
> > and will soon be available at
> > 
> >    git://git.zen-sources.org/zen/mmotm.git
> 
> 
> readahead-add-blk_run_backing_dev.patch:
> 
> mm/readahead.c: In function 'page_cache_async_readahead':
> mm/readahead.c:559: error: implicit declaration of function 'blk_run_backing_dev'

hm, yeah, CONFIG_BLOCK=n.

Doing a block-specific call from inside page_cache_async_readahead() is
a bit of a layering violation - this may not be a block-backed
filesystem at all.

otoh, perhaps blk_run_backing_dev() is wrongly named and defined in the
wrong place.  Perhaps non-block-backed backing_devs want to implement
an unplug-style function too?  In which case the whole thing should be
renamed and moved outside blkdev.h.

If we don't want to do that, shouldn't backing_dev_info.unplug* be
wrapped in #ifdef CONFIG_BLOCK?  And wasn't it a layering violation to
put block-specific things into the backing_dev_info?

Jens, talk to me!

>From the readahead POV: does it make sense to call the backing-dev's
"unplug" function even if that isn't a block-based device?  Or was this
just a weird block-device-only performance problem?  Hard to say.
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