A user reported the following oops when a backup process read
BUG: unable to handle kernel paging request at ffffbb00ff33b000
Investigation determined that the bug triggered when reading system RAM
at the 4G mark. On this system, that was the first address using 1G pages
for the virt->phys direct mapping so the PUD is pointing to a physical
address, not a PMD page. The problem is that the page table walker in
kern_addr_valid() is not checking pud_large() and treats the physical
address as if it was a PMD. If it happens to look like pmd_none then it'll
silently fail, probably returning zeros instead of real data. If the data
happens to look like a present PMD though, it will be walked resulting in
the oops above. This patch adds the necessary pud_large() check.
Unfortunately the problem was not readily reproducible and now they are
running the backup program without accessing /proc/kcore so the patch has
not been validated but I think it makes sense. If reviewers agree then it
should also be included in -stable back as far as 3.0-stable.
Signed-off-by: Mel Gorman <mgorman@xxxxxxx>