Re: [Lsf] [RFC] writeback and cgroup

[Peter Zijlstra <peterz@infradead.org>]

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.