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SubjectRe: [PATCH 00 of 10] x86: unify asm/pgtable.h
Andi Kleen wrote:
>> Or to put it another way, what's the underlying rationale for making
>> __PAGE_KERNEL_* not include the GLOBAL flag, but including it in the pgprot
>> versions? It means that code like this in ioremap_64.c:
> There is none, but that is not what change_page_attr() cares about.
> It just cares that you pass in the correct bits and you don't currently.
> I think you're missing the forrest because of all the trees currently.

Yeah, that may be true, but this particular tree is weird, and I'm
trying to understand what's going on here. Specifically, 64-bit
ioremap()s *don't* set _PAGE_GLOBAL, which appears to be an accident
resulting from the strange definitions of __PAGE_KERNEL_* vs

For example, ioremap_64.c:__ioremap() creates a vma for the io mapping,
and explicitly sets _PAGE_GLOBAL in the vma's version of pgprot - but
then it calls ioremap_page_range() to actually create the mapping, which
ends up making a non-global mapping, because its rolling its own version
of PAGE_KERNEL by using pgprot(__PAGE_KERNEL) - which is not the actual
definition of PAGE_KERNEL.

I think there's a bug around here, but I think its currently being
hidden by accident. I think my changes are correct, and they're
exposing some other bug. But I don't really understand how all this
stuff is supposed to fit together, so I'm looking for an explanation of
what's supposed to be happening - and ideally - why the current code
isn't actually buggy.

For example: is ioremap_change_attr() actually *deliberately* creating
non-global mappings? Or is it an accident? And if it really intends to
create non-global mappings, why? And why is it buggy for it to create
global mappings?


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