linux-kernel - Re: [PATCH 1/2] mm: use list.h for vma list

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Date:	Thu, 12 Mar 2009 10:58:32 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Daniel Lowengrub <lowdanie@...il.com>
Cc:	linux-mm@...ck.org, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Wolfram Strepp <wstrepp@....de>
Subject: Re: [PATCH 1/2] mm: use list.h for vma list

On Wed, 2009-03-11 at 11:55 +0200, Daniel Lowengrub wrote:
> Use the linked list defined list.h for the list of vmas that's stored
> in the mm_struct structure.  Wrapper functions "vma_next" and
> "vma_prev" are also implemented.  Functions that operate on more than
> one vma are now given a list of vmas as input.
> 
> Signed-off-by: Daniel Lowengrub

While this is the approach I've taken for a patch I'm working on, a
better solution has come up if you keep the RB tree (I don't).

It is, however, even more invasive than the this one ;-)

Wolfram has been working on implementing a threaded RB-tree. This means
rb_prev() and rb_next() will be O(1) operations, so you could simply use
those to iterate the vmas.

The only draw-back is that each and every RB-tree user in the kernel
needs to be adapted because its not quite possible to maintain the
current API.

I was planning to help Wolfram do that, but I'm utterly swamped atm. :-(

What needs to be done is introduce rb_left(), rb_right() and rb_node()
helpers that for now look like:

static inline struct rb_node *rb_left(struct rb_node *n)
{
	return n->rb_left;
}

static inline struct rb_node *rb_right(struct rb_node *n)
{
	return n->rb_right;
}

static inline struct rb_node *rb_node(struct rb_node *n)
{
	return n;
}

We need these because the left and right child pointers will be
over-loaded with threading information.

After that we can flip the implementation of the RB-tree.

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