Jeremy Fitzhardinge wrote: > 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 Thanks for the info. Yes, we've had to live with the splitting of large pages for some time. In the future we'll probably set up a pool of video pages into which we might perhaps try to allocate highmem pages or try allocations with large page sizes. In the end perhaps another zone will be needed.
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