Re: [ATTEND] [LSF/MM TOPIC] Buffered writes throttling

[Andrea Righi <andrea@betterlinux.com>]

On Mon, Mar 05, 2012 at 02:30:29PM -0800, Fengguang Wu wrote:
> On Mon, Mar 05, 2012 at 04:11:15PM -0500, Vivek Goyal wrote:
...
> > But looks like we don't much choice. As buffered writes can be controlled
> > at two levels, we probably need two knobs. Also controlling writes while
> > entring cache limits will be global and not per device (unlinke currnet
> > per device limit in blkio controller). Having separate control for "dirty
> > rate limit" leaves the scope for implementing write control at device
> > level in the future (As some people prefer that). In possibly two 
> > solutions can co-exist in future.
> 
> Good point. balance_dirty_pages() has no idea about the devices at
> all. So the rate limit for buffered writes can hardly be unified with
> the per-device rate limit for direct writes.
> 

I think balance_dirty_pages() can have an idea about devices. We can get
a reference to the right block device / request queue from the
address_space:

  bdev = mapping->host->i_sb->s_bdev;
  q = bdev_get_queue(bdev);

(NULL pointer dereferences apart).

-Andrea