On Jan 27, 2020, at 8:28 PM, Anshuman Khandual [off-list ref] wrote:

This adds tests which will validate architecture page table helpers and
other accessors in their compliance with expected generic MM semantics.
This will help various architectures in validating changes to existing
page table helpers or addition of new ones.

This test covers basic page table entry transformations including but not
limited to old, young, dirty, clean, write, write protect etc at various
level along with populating intermediate entries with next page table page
and validating them.

Test page table pages are allocated from system memory with required size
and alignments. The mapped pfns at page table levels are derived from a
real pfn representing a valid kernel text symbol. This test gets called
right after page_alloc_init_late().

This gets build and run when CONFIG_DEBUG_VM_PGTABLE is selected along with
CONFIG_VM_DEBUG. Architectures willing to subscribe this test also need to
select CONFIG_ARCH_HAS_DEBUG_VM_PGTABLE which for now is limited to x86 and
arm64. Going forward, other architectures too can enable this after fixing
build or runtime problems (if any) with their page table helpers.

What’s the value of this block of new code? It only supports x86 and arm64 which are supposed to be good now. Did those tests ever find any regression or this is almost only useful for new architectures which only happened once in a few years? The worry if not many people will use this config and code those that much in the future because it is inefficient to find bugs, it will simply be rotten like a few other debugging options out there we have in the mainline that will be a pain to remove later on.