Re: 64-bit kdev_t - just for playing

[H. Peter Anvin]

Followup to:  <[e-mail]>
By author:    David Lang <[e-mail]>
In newsgroup: linux.dev.kernel
>
> the biggest problem I see with dynamic numbers is that it needs a
> userspace devfs type solution for creating and maintaining the device
> nodes that are then used. While this isn't rocket science it's also
> somthing that is hard to get people to agree to (remember the devfs names
> that everyone gripes about are not what richard started with it's what he
> switched to to get things into the kernel, they changed many times during
> that process)
> 
> I don't think many people will argue that dynamic assignments are evil,
> but I think you will find a lot of people very nervous about switching to
> them and the risk involved with doing so.
> 

They aren't evil, they're just very, very hard to get right.

So far, *none* of the schemes used for dynamics have gotten it right.
They just ignore a fair number of the problems.  People keep focusing
on disks, and they are nearly uniformly the almost-trivial case in
comparison with especially character devices, where you don't have the
layer of indirection called /etc/fstab, persistent labels, etc.

It is also independent of the need to switch to a larger dev_t.
Claiming that we can squeeze more out of the existing device scheme if
we have an ideal-world dynamic scheme is unrealistic because:

a) There are, genuinely, systems with more than 65,536 devices or
anonymous mounts.  That rules out the current dev_t just by itself.

b) Despite the fact that people have tried since the mid-90's, we
still don't have a sane way to manage such dynamicity.

c)  We are now in what pretty much amounts to a crisis situation.  We
have needed to enlarge dev_t for well over half a decade.  Therefore,
it is too late to say "well, given X we wouldn't need it."  We need
something done in *this* kernel cycle.

Given that it has taken, literally, 8 years to get to this point, and
based on collective global experience with numberspaces, I'm arguing
for enlarging it far more than anyone can currently imagine being
necessary.

dev_t is already 64 bits in glibc, and the glibc<->kernel interface
needs to be fixed *anyway*.  We have to take the pain of migration, we
might as well go all the way.

	-hpa
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