Re: [PATCH 17/18] writeback: fix dirtied pages accounting on redirty

[Peter Zijlstra]

On Sun, 2011-09-04 at 09:53 +0800, Wu Fengguang wrote:
> De-account the accumulative dirty counters on page redirty.
> 
> Page redirties (very common in ext4) will introduce mismatch between
> counters (a) and (b)
> 
> a) NR_DIRTIED, BDI_DIRTIED, tsk->nr_dirtied
> b) NR_WRITTEN, BDI_WRITTEN
> 
> This will introduce systematic errors in balanced_rate and result in
> dirty page position errors (ie. the dirty pages are no longer balanced
> around the global/bdi setpoints).
> 

So wtf is ext4 doing? Shouldn't a page stay dirty until its written out?

That is, should we really frob around this behaviour or fix ext4 because
its on crack?
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