From mboxrd@z Thu Jan  1 00:00:00 1970
Date: Thu, 8 Feb 2007 13:19:37 -0800 (PST)
From: Christoph Lameter <clameter@sgi.com>
Subject: Re: Drop PageReclaim()
In-Reply-To: <20070207092517.15071f04.akpm@linux-foundation.org>
Message-ID: <Pine.LNX.4.64.0702081317200.12048@schroedinger.engr.sgi.com>
References: <Pine.LNX.4.64.0702070612010.14171@schroedinger.engr.sgi.com>
 <20070207092517.15071f04.akpm@linux-foundation.org>
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To: Andrew Morton <akpm@linux-foundation.org>
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On Wed, 7 Feb 2007, Andrew Morton wrote:

> On Wed, 7 Feb 2007 06:13:48 -0800 (PST) Christoph Lameter <clameter@sgi.com> wrote:
> 
> > Am I missing something here? I cannot see PageReclaim have any effect?
> > 
> > 
> > 
> > PageReclaim is only used for dead code. The only current user is
> > end_page_writeback() which has the following lines:
> > 
> >  if (!TestClearPageReclaim(page) || rotate_reclaimable_page(page)) {
> >          if (!test_clear_page_writeback(page))
> >                   BUG();
> >  }
> > 
> > So the if statement is performed if !PageReclaim(page).
> > If PageReclaim is set then we call rorate_reclaimable(page) which
> > does:
> > 
> >  if (!PageLRU(page))
> >        return 1;
> > 
> > The only user of PageReclaim is shrink_list(). The pages processed
> > by shrink_list have earlier been taken off the LRU. So !PageLRU is always 
> > true.
> > 
> > The if statement is therefore always true and the rotating code
> > is never executed.
> 
> end_page_writeback() is amazingly obscure for such a short function.  For
> which I apologise, but on revisit, it's still not obvious how to clean it
> up.
> 
> It does:
> 
> 	if (!PageReclaim(page)) {
> 		clear_page_writeback();

Well it does
   if (!test_clear_page_writeback(page))
                  BUG();

> 	if (PageRecaim(page)) {
> 		ClearPageReclaim(page);
> 		foo = rotate_reclaimable_page(page);

PageReclaim is only used while the page is off the LRU. In that case 
rotate_reclaimable_page is always returning 1. So foo = 1 and 
rotate_reclaimable_page is useless.

> 		if (foo == 0) {

> 			/*
> 			 * rotate_reclaimable_page has already done
> 			 * clear_page_writeback()
> 			 */
> 		} else {
> 			clear_page_writeback(page);

This is also a test_clear ...

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