Re: [PATCH 1/6] [RFC] Interval tree implementation

[Michel Lespinasse]

On Tue, Jun 12, 2012 at 6:10 PM, John Stultz <john.stultz@xxxxxxxxxx> wrote:
> After Andrew suggested something like his mumbletree idea
> to better store a list of intervals, I worked on a few different
> approaches, and this is what I've finally managed to get working.
>
> The idea of storing intervals in a tree is nice, but has a number
> of complications. When adding an interval, its possible that a
> large interval will consume and merge a number of smaller intervals.
> When removing a interval, its possible you may end up splitting an
> existing interval, causing one interval to become two. This makes it
> very difficult to provide generic list_head like behavior, as
> the parent structures would need to be duplicated and removed,
> and that has lots of memory ownership issues.
>
> So, this is a much simplified and more list_head like
> implementation. You can add a node to a tree, or remove a node
> to a tree, but the generic implementation doesn't do the
> merging or splitting for you. But it does provide helpers to
> find overlapping and adjacent intervals.
>
> Andrew also really wanted this interval-tree implementation to be
> resuable so we don't duplicate the file locking logic. I'm not
> totally convinced that the requirements between the volatile
> intervals and file locking are really equivelent, but this reduced
> impelementation may make it possible.

Well, we already have several implementations of interval trees:

* lib/prio_tree.c seems to do what you want. It's often used with
VMAs, but there is also a 'raw' node structure, as used in
mm/kmemleak.c, which supports all the operations you are proposing.

* lib/rb_tree.c has some support for the "augmented tree" structure,
with which you can implement interval trees as is done in
arch/x86/mm/pat_rbtree.c

I don't think it makes sense to have two implementations already, and
I am looking into possibly collapsing these into one. Please don't add
a third one :)


Also, the name "interval tree" typically implies a data structure that
is capable of representing overlapping intervals. Since your
implementation does not support that (it requires its callers to make
sure the intervals are non-overlapping), I think calling it "interval
tree" could cause confusion.

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.
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