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

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Date:	Mon, 28 Jan 2008 12:43:08 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Shaohua Li <shaohua.li@...el.com>, clameter@....com,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC] some page can't be migrated

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?
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