Re: [PATCH 01/11] staging: lustre: simplify use of interval-tree.

[James Simmons <jsimmons@infradead.org>]

> >> Lustre has a private interval-tree implementation.  This
> >> implementation (inexplicably) refuses to insert an interval if an
> >> identical interval already exists.  It is OK with all sorts of
> >> overlapping intervals, but identical intervals are rejected.
> >
> > I talked to Oleg about this since this changes the behavior. He is worried
> > about having identical items that would end up being merged.  If we can 
> > guarantee by some other means there are no identical nodes, we are 
> > probably fine with the interval tree code allowing this. Oleg can explain 
> > better than me in this case.
> 
> I don't think this is a change in behaviour.
> In the driver/staging client code, interval tree is being used in two
> places and both of them have clumsy work-arounds for the fact that they
> cannot insert duplicates in the interval tree.
> The patch just cleans his up.
> 
> However if I have missed something, please provide details.
> What "identical items" might get merged?

Oleg could you fill in detail what your concerns are?
 
> >  
> >> Both users of interval-tree in lustre would be simpler if this was not
> >> the case.  They need to store all intervals, even if some are
> >> identical.
> >> 
> >> llite/range_lock.c add a rl_next_lock list_head to each lock.
> >> If it cannot insert a new lock because the range is in use, it
> >> attached the new lock to the existing lock using rl_next_lock.
> >> This requires extra code to iterate over the rl_next_lock lists when
> >> iterating over locks, and to update the list when deleting a lock from
> >> the tree.
> >> 
> >> ldlm_extend allocates a separate ldlm_interval which as a list of
> >> ldlm_locks which share the same interval.  This is linked together
> >> by over-loading the l_sl_policy which, for non-extent locks, is used
> >> for linking together locks with the same policy.
> >> This doesn't only require extra code, but also an extra memory
> >> allocation.
> >> 
> >> This patch removes all that complexity.
> >> - interval_insert() now never fails.
> >
> > Its not really a failure. What it does is if it finds a already existing
> > node with the range requested it returns the already existing node 
> > pointer. If not it just creates a new node and returns NULL. Sometimes
> > identical request can happen. A good example of this is with HSM request
> > on the MDS server. In that case sometimes we get identical progress
> > reports which we want to filter out so not add the same data.
> 
> This example is server-side code which is not a focus at present.
> Having a quick look, it looks like it would be easy enough to do a
> lookup first and then only insert if the lookup failed.
> I think this is a much nicer approach than never allowing duplicates in
> the interval tree.
> 
> Thanks,
> NeilBrown
>