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

[Christoph Hellwig]

On Wed, Sep 07, 2011 at 02:22:22AM +0200, Jan Kara wrote:
> > 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).

The pattern we do in writeback is:

in pageout / write_cache_pages:
	lock_page();
	clear_page_dirty_for_io();

in ->writepage:
	set_page_writeback();
	unlock_page();
	end_page_writeback();

So whenever ->writepage decides it doesn't want to write things back
we have to redirty pages.  We have this happen quite a bit in every
filesystem, but ext4 hits it a lot more than usual because it refuses
to write out delalloc pages from plain ->writepage and only allows
->writepages to do it.

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