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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