Re: [PATCH 01/45] writeback: reduce calls to global_page_state inbalance_dirty_pages()

[Peter Zijlstra]

On Sun, 2009-10-11 at 18:50 +0800, Wu Fengguang wrote:
> 
> Sorry for the confusion, but I mean, filesystems have to limit
> nr_writeback (directly or indirectly via the block io queue),
> otherwise it either hit nr_dirty to 0 (with the loop), or let
> nr_writeback grow out of control (without the loop).

Doesn't this require the writeback queue to have a limit < dirty_thresh?

Or more specifically, for the bdi case:

 bdi_dirty + bdi_writeback + bdi_unstable <= bdi_thresh

we require that the writeback queue be smaller than bdi_thresh, which
could be quite difficult, since bdi_thresh can easily be 0.

Without observing the bdi_thresh constraint we can have:

  \Sum_(over bdis) writeback_queue_size

dirty pages outstanding, which could be significantly higher than
dirty_thresh.

Or am I confused again?

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