Re: [PATCH V15] mm/debug: Add tests validating architecture page table helpers

On Fri, 2020-03-06 at 05:27 +0530, Anshuman Khandual 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
inside kernel_init() right after async_synchronize_full().

This test gets built and run when CONFIG_DEBUG_VM_PGTABLE is selected. Any
architecture, which is willing to subscribe this test will need to select
ARCH_HAS_DEBUG_VM_PGTABLE. For now this is limited to arc, arm64, x86, s390
and ppc32 platforms where the test is known to build and run successfully.
Going forward, other architectures too can subscribe the test after fixing
any build or runtime problems with their page table helpers. Meanwhile for
better platform coverage, the test can also be enabled with CONFIG_EXPERT
even without ARCH_HAS_DEBUG_VM_PGTABLE.

Folks interested in making sure that a given platform's page table helpers
conform to expected generic MM semantics should enable the above config
which will just trigger this test during boot. Any non conformity here will
be reported as an warning which would need to be fixed. This test will help
catch any changes to the agreed upon semantics expected from generic MM and
enable platforms to accommodate it thereafter.

OK, I get this working on powerpc hash MMU as well, so this?

diff --git a/Documentation/features/debug/debug-vm-pgtable/arch-support.txt

b/Documentation/features/debug/debug-vm-pgtable/arch-support.txt
index 64d0f9b15c49..c527d05c0459 100644

--- a/Documentation/features/debug/debug-vm-pgtable/arch-support.txt
+++ b/Documentation/features/debug/debug-vm-pgtable/arch-support.txt

@@ -22,8 +22,7 @@

     |       nios2: | TODO |
     |    openrisc: | TODO |
     |      parisc: | TODO |
-    |  powerpc/32: |  ok  |
-    |  powerpc/64: | TODO |
+    |     powerpc: |  ok  |
     |       riscv: | TODO |
     |        s390: |  ok  |
     |          sh: | TODO |

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index 2e7eee523ba1..176930f40e07 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig

@@ -116,7 +116,7 @@ config PPC

 	#
 	select ARCH_32BIT_OFF_T if PPC32
 	select ARCH_HAS_DEBUG_VIRTUAL
-	select ARCH_HAS_DEBUG_VM_PGTABLE if PPC32
+	select ARCH_HAS_DEBUG_VM_PGTABLE
 	select ARCH_HAS_DEVMEM_IS_ALLOWED
 	select ARCH_HAS_ELF_RANDOMIZE
 	select ARCH_HAS_FORTIFY_SOURCE

diff --git a/mm/debug_vm_pgtable.c b/mm/debug_vm_pgtable.c
index 96a91bda3a85..98990a515268 100644
--- a/mm/debug_vm_pgtable.c
+++ b/mm/debug_vm_pgtable.c

@@ -256,7 +256,8 @@ static void __init pte_clear_tests(struct mm_struct *mm,

pte_t *ptep,
 	pte_t pte = READ_ONCE(*ptep);
 
 	pte = __pte(pte_val(pte) | RANDOM_ORVALUE);
-	WRITE_ONCE(*ptep, pte);
+	set_pte_at(mm, vaddr, ptep, pte);
+	barrier();
 	pte_clear(mm, vaddr, ptep);
 	pte = READ_ONCE(*ptep);
 	WARN_ON(!pte_none(pte));