On 10/01/09 01:41, Jan Beulich wrote:
>> Why, what's wrong with VM_IO? It's documented as being "Memory mapped
>> I/O or similar". VM_PFNMAP is another option, but that seems more
>> dubious. Are there places where VM_IO is being legitimately set on
>> normal page mappings?
>>
> This is what Nick responded on my similar question: "Hmm, good question.
> Unfortunately I think VM_IO was never really intended to set pte protection
> bits, but just protect the vma from get_user_pages (actually it is hard to
> know exactly what some of the vma flags bits were really intended for, but
> that seems to be the current reasonable use)."
>
VM_PFNMAP seems more specifically about preventing get_user_pages (ie,
no struct page), (though I guess that could have changed with
_PAGE_SPECIAL ptes). And there's the ever-mysterious VM_RESERVED. Does
that mean it
> The most prominent reason for not deriving _PAGE_IOMAP from VM_IO
> is probably the fact that remap_pfn_range() sets VM_IO, but clearly can
> be called for local RAM pages (e.g. through mmap_kmem()).
mmap_kmem looks pretty broken anyway. All the CONFIG_STRICT_DEVMEM
checks make sure the pages being mapped are *not* regular pages, and
will end up failing all /dev/kmem mappings as a result. So I think in
that case I think it gets set as an accidental erroneous side-effect.
> And if you
> grep for the uses of VM_IO, you'll find a few more examples (mostly
> arch specific, but anyway) that show that such an implication isn't valid
> at present.
>
Pointers? The overwhelmingly common use is on mappings of hardware
devices, and most of the tests of the bit explicitly reference IO as the
reason they're testing it. Why would you set it if the mapping weren't
for IO anyway? There are other ways to prevent, for example, core
dumping it, or having it participate in normal memory management, or
preventing get_user_pages, or whatever.
The use of remap_pfn_range() is a slightly different matter, but it also
looks like it is only used on device memory too (aside from the kmem
thing mentioned above). But I'm not sure if it is defined to be only
used on IO(-like) memory regions...
J
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