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Subject2.6.29 pat issue

The function in include/linux/mm.h:

doesn't seem valid to me.

In particular, we have VMAs to graphics devices in which vma->vm_pgoff
is non-zero (Points to an offset in the drm device node), and the VMA is
sparsely populated with pfns pointing to uncached discontigous RAM pages.

This causes the X86 PAT code to hit the optimized path when it
shouldn't, and issue a warning.

Also a question about the philosofy behind this strict checking that all
PTEs have the same caching attributes: I guess this is only to catch
bugs in kernel drivers that don't get this right. At the same time, now
that also user-space VMAs are checked this will probably have a
significant performance impact. Shouldn't this checking really live
behind a debug define?


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