linux-kernel - Re: [Lsf] [RFC] writeback and cgroup

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Date:	Sat, 14 Apr 2012 14:15:16 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steve French <smfrench@...il.com>
Cc:	Tejun Heo <tj@...nel.org>, ctalbott@...gle.com, rni@...gle.com,
	andrea@...terlinux.com, containers@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, lsf@...ts.linux-foundation.org,
	linux-mm@...ck.org, jmoyer@...hat.com, lizefan@...wei.com,
	linux-fsdevel@...r.kernel.org, cgroups@...r.kernel.org
Subject: Re: [Lsf] [RFC] writeback and cgroup

On Wed, 2012-04-04 at 14:23 -0500, Steve French wrote:
> Current use of bdi is a little hard to understand since
> there are 25+ fields in the structure.  

Filesystems only need a small fraction of those.

In particular,

  backing_dev_info::name	-- string
  backing_dev_info::ra_pages	-- number of read-ahead-pages
  backing_dev_info::capability	-- see BDI_CAP_*
  
One should properly initialize/destroy the thing using:

  bdi_init()/bdi_destroy()


Furthermore, it has hooks into the regular page-writeback stuff:

  test_{set,clear}_page_writeback()/bdi_writeout_inc()
  set_page_dirty()/account_page_dirtied()
  
but also allows filesystems to do custom stuff, see FUSE for example.

The only other bit is the pressure valve, aka.
{set,clear}_bdi_congested(). Which really is rather broken and of
dubious value.


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