Re: what is the point of nr_pages information for the flusher thread?

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On Wed, Jul 07, 2010 at 04:37:10PM -0700, Andrew Morton wrote:
> On Wed, 7 Jul 2010 19:16:11 -0400
> Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:
> 
> > Currently there's three possible values we pass into the flusher thread
> > for the nr_pages arguments:
> 
> I assume you're referring to wakeup_flusher_threads().

In that context I refer to everything using the per-bdi flusher thread.
That includes wakeup_flusher_threads() and the functions I've mentioned
below.

> There's also free_more_memory() and do_try_to_free_pages().

Indeed.  So we still have some special cases that want a specific
number to be written back globally.

> wakeup_flusher_threads() apepars to have been borked.  It passes
> nr_pages() into *each* bdi hence can write back far more than it was
> asked to.

> > But seriously, how is the _global_ number of dirty and unstable pages
> > a good indicator for the amount of writeback per-bdi or superblock
> > anyway?
> 
> It isn't.  This appears to have been an attempt to transport the
> wakeup_pdflush() functionality into the new wakeup_flusher_threads()
> regime.  Badly.

Unfortunately we don't just use it for wakeup_flusher_threads() but
also for various bits of per-bdi and per-sb writeback.

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