Thomas Hellström wrote:
> Hi!
>
> For a long time now, the agpgart module has been creating illegal
> mapping aliases, since the user-space mappings of the pages in the
> gart are usually write-combined, whereas the kernel linear mapping of
> the same pages are uc for x86, and may even be wb for some architectures.
>
> In order to fix this, and to facilitate fast insertion and removal of
> pages into / from the gart I'd like to disable all default kernel
> mappings for those pages, which would in effect, make them behave as
> highmem pages from our point of view.
>
> As prevously discussed, the x86 set_memory_xxx() interface wasn't
> suitable for this, since it handles only a single mapping, and the
> pages may have more than one default kernel mapping.
>
> But it turns out that there is an interface that does exactly this.
> kernel_map_pages(). But it is only available with
> CONFIG_DEBUG_PAGEALLOC. I'd like to make that function exported by
> default, but with some minor alterations as the original functions
> does some debug checks as well, that aren't desirable for the purpose
> mentioned above:
>
> As with highmem pages, if the driver sets up user-space mappings with
> non-standard caching attributes, those mappings need to be killed at
> suspend time, since the suspend code would otherwise create temporary
> incompatible mappings.
>
> On x86 this all would probably work fine. Does kernel_map_pages() work
> identically on other architectures? Specifically: Will it always work
> with a 4K page granularity?
Well, not all architectures use 4k as their base page size, but
kernel_map_pages should work at the smallest supported page size.
The disadvantage of this is that it will end up shattering any
large-page mappings the kernel has. This is pretty much unavoidable
unless you can arrange to only allocate AGP pages in a physically
distinct area away from other kernel allocations (a mechanism to do this
might be generally useful, though I'm not sure what form it would take -
another zone perhaps?).
J
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