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

Michel Lespinasse
2019-10-04T12:43:31Z

On Thu, Oct 03, 2019 at 01:32:50PM -0700, 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 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.

Yeah, I wanted to ask - is the case where pgoff == ULONG_MAX (i.e.,
last block of a file that is exactly 16TB) currently supported on
32-bit archs ?
I have no idea if I am supposed to care about this or not...

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