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

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Message-ID: <20191003203250.GE32665@bombadil.infradead.org>
Date:   Thu, 3 Oct 2019 13:32:50 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Davidlohr Bueso <dave@...olabs.net>
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, 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 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.
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.