From: Jeremy F. <je...@go...> - 2008-04-06 19:02:34
|
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 |