mm/debug: use valid physical memory for pmd/pud tests

mm/debug: use valid physical memory for pmd/pud tests

The page table debug tests need a physical address to validate low-level
page table manipulation with.  The memory at this address is not actually
touched, it just encoded in the page table entries at various levels
during the tests only.

Since the memory is not used, the code just picks the physical address of
the start_kernel symbol.  This value is then truncated to get a properly
aligned address that is to be used for various tests.  Because of the
truncation, the address might not actually exist, or might not describe a
complete huge page.  That's not a problem for most tests, but the
arch-specific code may check for attribute validity and consistency.  The
x86 version of {pud,pmd}_set_huge actually validates the MTRRs for the
PMD/PUD range.  This may fail with an address derived from start_kernel,
depending on where the kernel was loaded and what the physical memory
layout of the system is.  This then leads to false negatives for the
{pud,pmd}_set_huge tests.

Avoid this by finding a properly aligned memory range that exists and is
usable.  If such a range is not found, skip the tests that needed it.

[fvdl@google.com: v3]
Link: https://lkml.kernel.org/r/20230110181208.1633879-1-fvdl@google.com
Link: https://lkml.kernel.org/r/20230109174332.329366-1-fvdl@google.com
Fixes: 399145f9eb6c ("mm/debug: add tests validating architecture page table helpers")
Signed-off-by: Frank van der Linden <fvdl@google.com>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>