linux-kernel - Re: [RFC] some page can't be migrated

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Message-Id: <1201484908.1737.0.camel@sli10-desk.sh.intel.com>
Date:	Mon, 28 Jan 2008 09:48:28 +0800
From:	Shaohua Li <shaohua.li@...el.com>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, clameter@....com,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC] some page can't be migrated


On Mon, 2008-01-28 at 12:43 +1100, Nick Piggin wrote:
> On Sunday 27 January 2008 17:03, Andrew Morton wrote:
> > > On Fri, 25 Jan 2008 14:03:25 +0800 Shaohua Li <shaohua.li@...el.com>
> > > wrote:
> > >
> > > -	if (!page->mapping)
> > > +	if (!page->mapping) {
> > > +		if (!PageAnon(page) && PagePrivate(page))
> > > +			try_to_release_page(page, GFP_KERNEL);
> > >  		goto rcu_unlock;
> > > +	}
> >
> > We call something(GFP_KERNEL) under rcu_read_lock()?  I've lost track of
> > the myriad flavours of rcu which we purport to support, but I don't think
> > they'll all like us blocking under rcu_read_lock().
> >
> > We _won't_ block, because try_to_release_page() will see the NULL ->mapping
> > and will call the non-blocking try_to_free_buffers().  But still, it looks
> > bad, and will cause problems if someone decides to add a might_sleep_if()
> > to try_to_release_page().
> >
> > So...  I'd suggest that it would be better to add an apologetic comment and
> > call direct into try_to_free_buffers().
> 
> You're right, but can't we just rcu_read_unlock() before try_to_release_page?
or we could move the code above before doing rcu_read_lock()?

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