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SubjectRe: [PATCH 17/18] writeback: fix dirtied pages accounting on redirty
On Tue 06-09-11 18:18:56, Peter Zijlstra wrote:
> 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?
Fengguang, could you please verify your findings with recent kernel? I
believe ext4 got fixed in this regard some time ago already (and yes, old
delalloc writeback code in ext4 was terrible).

Honza
--
Jan Kara <jack@suse.cz>
SUSE Labs, CR


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