linux-kernel - Re: [Experimental][PATCH] putback_lru

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Message-Id: <20080619170059.60b42e73.nishimura@mxp.nes.nec.co.jp>
Date:	Thu, 19 Jun 2008 17:00:59 +0900
From:	Daisuke Nishimura <nishimura@....nes.nec.co.jp>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Lee Schermerhorn <lee.schermerhorn@...com>,
	Nick Piggin <npiggin@...e.de>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, kernel-testers@...r.kernel.org
Subject: Re: [Experimental][PATCH] putback_lru_page rework

> > > -		unlock = putback_lru_page(newpage);
> > > +		putback_lru_page(newpage);
> > >  	} else
> > >  		newpage->mapping = NULL;
> > 
> > originally move_to_lru() called in unmap_and_move().
> > unevictable infrastructure patch move to this point for 
> > calling putback_lru_page() under page locked.
> > 
> > So, your patch remove page locked dependency.
> > move to unmap_and_move() again is better.
> > 
> > it become page lock holding time reducing.
> > 
> ok, will look into again.
> 

I agree with Kosaki-san.

And VM_BUG_ON(page_count(newpage) != 1) in unmap_and_move()
is not correct again, IMHO.
I got this BUG actually when testing this patch(with
migratin_entry_wait fix).

unmap_and_move()
	move_to_new_page()
		migrate_page()
		remove_migration_ptes()
		putback_lru_page()			(*1)
	  :
        if (!newpage->mapping)				(*2)
		VM_BUG_ON(page_count(newpage) != 1)

If a anonymous page(without mapping) is migrated successfully,
this page is moved back to lru by putback_lru_page()(*1),
and the page count becomes 1(pte only).

At the same time(between *1 and *2), if the process
that owns this page are freeing this page, the page count
becomes 0 and ->mapping becomes NULL by free_hot_cold_page(),
so this BUG is caused.

I've not seen this BUG on real HW yet(seen twice on fake-numa
hvm guest of Xen), but I think it can happen theoretically.

Thanks,
Daisuke Nishimura.
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