Re: [PATCH -next 00/11] lib/interval-tree: move to half closed intervals

[Jason Gunthorpe <jgg@ziepe.ca>]

On Thu, Oct 03, 2019 at 01:18:47PM -0700, Davidlohr Bueso wrote:
> Hi,
> 
> It has been discussed[1,2] that almost all users of interval trees would better
> be served if the intervals were actually not [a,b], but instead [a, b). This
> series attempts to convert all callers by way of transitioning from using
> "interval_tree_generic.h" to "interval_tree_gen.h". Once all users are converted,
> we remove the former.
> 
> Patch 1: adds a call that will make patch 8 easier to review by introducing stab
>          queries for the vma interval tree.
> 
> Patch 2: adds the new interval_tree_gen.h which is the same as the old one but
>          uses [a,b) intervals.
> 
> Patch 3-9: converts, in baby steps (as much as possible), each interval tree to
> 	   the new [a,b) one. It is done this way also to maintain bisectability.
> 	   Most conversions are pretty straightforward, however, there are some
> 	   creative ways in which some callers use the interval 'end' when going
> 	   through intersecting ranges within a tree. Ie: patch 3, 6 and 9.
> 
> Patch 10: deletes the interval_tree_generic.h header; there are no longer any users.
> 
> Patch 11: finally simplifies x86 pat tree to use the new interval tree machinery.
> 
> This has been lightly tested, and certainly not on driver paths that do non
> trivial conversions. Also needs more eyeballs as conversions can be easily
> missed (even when I've tried mitigating this by renaming the endpoint from 'last'
> to 'end' in each corresponding structure).
> 
> Because this touches a lot of drivers, I'm Cc'ing the whole thing to a couple of
> relevant lists (mm, dri, rdma); sorry if you consider this spam.
> 
> Applies on top of today's linux-next tree. Please consider for v5.5.
> 
> Thanks!
> 
> [1] https://lore.kernel.org/lkml/CANN689HVDJXKEwB80yPAVwvRwnV4HfiucQVAho=dupKM_iKozw@mail.gmail.com/

Hurm, this is not entirely accurate. Most users do actually want
overlapping and multiple ranges. I just studied this extensively:

radeon_mn actually wants overlapping but seems to mis-understand the
interval_tree API and actively tries hard to prevent overlapping at
great cost and complexity. I have a patch to delete all of this and
just be overlapping.

amdgpu_mn copied the wrongness from radeon_mn

All the DRM drivers are basically the same here, tracking userspace
controlled VAs, so overlapping is essential

hfi1/mmu_rb definitely needs overlapping as it is dealing with
userspace VA ranges under control of userspace. As do the other
infiniband users.

vhost probably doesn't overlap in the normal case, but again userspace
could trigger overlap in some pathalogical case.

The [start,last] allows the interval to cover up to ULONG_MAX. I don't
know if this is needed however. Many users are using userspace VAs
here. Is there any kernel configuration where ULONG_MAX is a valid
userspace pointer? Ie 32 bit 4G userspace? I don't know. 

Many users seemed to have bugs where they were taking a userspace
controlled start + length and converting them into a start/end for
interval tree without overflow protection (woops)

Also I have a series already cooking to delete several of these
interval tree users, which will terribly conflict with this :\

Is it really necessary to make such churn for such a tiny API change?

Jason