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

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Date:   Thu, 3 Oct 2019 14:10:50 -0700
From:   Davidlohr Bueso <dave@...olabs.net>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     akpm@...ux-foundation.org, walken@...gle.com, peterz@...radead.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        dri-devel@...ts.freedesktop.org, linux-rdma@...r.kernel.org
Subject: Re: [PATCH -next 00/11] lib/interval-tree: move to half closed
 intervals

On Thu, 03 Oct 2019, Matthew Wilcox wrote:

>On Thu, Oct 03, 2019 at 01:18:47PM -0700, Davidlohr Bueso wrote:
>> 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
>
>So how does a user represent a range from ULONG_MAX to ULONG_MAX now?

I would assume that any such lookups would be stab queries (anon/vma interval
tree). So both anon and files. And yeah, I blissfully ignored any overflow scenarios.
This should at least be documented.

>
>I think the problem is that large parts of the kernel just don't consider
>integer overflow.  Because we write in C, it's natural to write:
>
>	for (i = start; i < end; i++)
>
>and just assume that we never need to hit ULONG_MAX or UINT_MAX.

Similarly, I did not adjust queries such as 0 to ULONG_MAX, which are actually
real, then again any intersecting ranges will most likely not even be close to
end.

>If we're storing addresses, that's generally true -- most architectures
>don't allow addresses in the -PAGE_SIZE to ULONG_MAX range (or they'd
>have trouble with PTR_ERR).  If you're looking at file sizes, that's
>not true on 32-bit machines, and we've definitely seen filesystem bugs
>with files nudging up on 16TB (on 32 bit with 4k page size).  Or block
>driver bugs with similarly sized block devices.
>
>So, yeah, easier to use.  But damning corner cases.

I agree.

Thanks,
Davidlohr