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Subject: Re: [PATCH 17/23] mm: count writeback pages per BDI
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Christoph Lameter <clameter@sgi.com>
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Date: Thu, 09 Aug 2007 21:23:36 +0200
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On Thu, 2007-08-09 at 12:15 -0700, Christoph Lameter wrote:
> On Fri, 3 Aug 2007, Peter Zijlstra wrote:
> 
> >  						page_index(page),
> >  						PAGECACHE_TAG_WRITEBACK);
> > +			if (bdi_cap_writeback_dirty(bdi))
> > +				__dec_bdi_stat(bdi, BDI_WRITEBACK);
> 
> Why are these not incremented and decremented in the exact location of 
> NR_WRITEBACK?

int test_clear_page_writeback(struct page *page)
{
	struct address_space *mapping = page_mapping(page);
	int ret;

	if (mapping) {
		struct backing_dev_info *bdi = mapping->backing_dev_info;
		unsigned long flags;

		write_lock_irqsave(&mapping->tree_lock, flags);
		ret = TestClearPageWriteback(page);
		if (ret) {
			radix_tree_tag_clear(&mapping->page_tree,
						page_index(page),
						PAGECACHE_TAG_WRITEBACK);
			if (bdi_cap_writeback_dirty(bdi)) {
				__dec_bdi_stat(bdi, BDI_WRITEBACK);
				__bdi_writeout_inc(bdi);
			}
		}
		write_unlock_irqrestore(&mapping->tree_lock, flags);
	} else {
		ret = TestClearPageWriteback(page);
	}
	if (ret)
		dec_zone_page_state(page, NR_WRITEBACK);
	return ret;
}

Less conditionals. We already have a branch for mapping, why create
another?