Comment # 34 on bug 61182 from D. Hugh Redelmeier

Thanks, Alex, for the clear restatement.

Naively, I think of two C/UNIX conventions. For allocations that can either
succeed or fail, typically the result is a pointer which is NULL for failure --
eg. malloc(3)).  For allocations that can partially succeed, the result is the
amount successfully processed (think write(2) which returns the length actually
transferred).

It seems to me that for allocating address space in the VRAM window
(aperture?), success can partial, and anything that deals with that window
needs to be aware that accessing some object may require piecewise operations,
punctuated by adjustment of the mapping.

In other words, this case doesn't sound pathological; it should be handled as a
normal case.

I'm not saying that comment #29 is wrong.  I'm saying that the existing code
ought to have been written to handle this case.  Clearly one solution is to
replace the code as suggested.  But fixing the code ought to be feasible too. 
Are there other lurking bugs where code assumes addressability?

I reiterate: I'm not knowledgeable about modern video architectures or about X
server architectures.

Footnote: Why do I think that VRAM windows cannot always map the whole VRAM? 
Because video cards now routinely have gigabytes of VRAM and 32-bit address
spaces for x86 machines cannot spare enough to allocate for that much VRAM
(ignoring PAE).