Re: page_lock_anon_vma(): remove check for mapped page

[Christoph Lameter <clameter@engr.sgi.com>]

On Mon, 27 Feb 2006, Hugh Dickins wrote:

> > At least my tests show that this codepath is valid and its for new 
> > functionality in 2.6.16. So I guess its suitable for 2.6.16.
> 
> Well, it's certainly not for me to decide: I just didn't want my signoff
> to be interpreted as a request to push it into 2.6.16.  It seemed to me
> rather late to be enabling this new functionality in 2.6.16, even though
> it's a bug that it wasn't already enabled in 2.6.16-rc: you'll have to
> argue that one without me.  Perhaps it doesn't matter if the vast
> majority have CONFIG_MIGRATION configured off.

There are only a very few users of page migration since this is new 
functionality. Even those with CONFIG_MIGRATION need to do special 
modifications to their systems (installiing a new numactl package, setting 
up their cpusets differently) to use this functionality.

Also the functionality in question has been available in the hotplug 
project in the past.

> I'm not sure that I've understood your doubt correctly.  But I think
> you're missing that rcu_read_lock is just another name for preempt_disable,
> plus we always disable preemption when taking a spin lock: so in effect
> we have rcu_read_lock in force until the spin_unlock(&anon_vma->lock).

That is a rather subtle thing not evident from the code. Add another 
comment? Or better do the rcu locking before calling page_lock_anon_vma 
and the unlocking after spin_unlock to have proper nesting of locks?

We have a rather confusing rcu_read_unlock in page_lock_anon_vma....

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