On Wed, 2020-02-26 at 10:51 -0500, Qian Cai wrote:

quoted

On Wed, 2020-02-26 at 15:45 +0100, Christophe Leroy wrote:

quoted

Le 26/02/2020 à 15:09, Qian Cai a écrit :

quoted

On Mon, 2020-02-17 at 08:47 +0530, Anshuman Khandual wrote:

quoted

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.

How useful is this that straightly crash the powerpc?

[   23.263425][    T1] debug_vm_pgtable: debug_vm_pgtable: Validating
architecture page table helpers
[   23.263625][    T1] ------------[ cut here ]------------
[   23.263649][    T1] kernel BUG at arch/powerpc/mm/pgtable.c:274!

The problem on PPC64 is known and has to be investigated and fixed.

It might be interesting to hear what powerpc64 maintainers would say about it
and if it is actually worth "fixing" in the arch code, but that BUG_ON() was
there since 2009 and had not been exposed until this patch comes alone?

This patch below makes it works on powerpc64 in order to dodge the BUG_ON()s in 
assert_pte_locked() triggered by pte_clear_tests().

diff --git a/mm/debug_vm_pgtable.c b/mm/debug_vm_pgtable.c
index 96dd7d574cef..50b385233971 100644
--- a/mm/debug_vm_pgtable.c
+++ b/mm/debug_vm_pgtable.c

@@ -55,6 +55,8 @@

 #define RANDOM_ORVALUE	GENMASK(BITS_PER_LONG - 1, S390_MASK_BITS)
 #define RANDOM_NZVALUE	GENMASK(7, 0)
 
+unsigned long vaddr;
+
 static void __init pte_basic_tests(unsigned long pfn, pgprot_t prot)
 {
 	pte_t pte = pfn_pte(pfn, prot);

@@ -256,7 +258,7 @@ static void __init pte_clear_tests(struct mm_struct *mm,

pte_t *ptep)
 
 	pte = __pte(pte_val(pte) | RANDOM_ORVALUE);
 	WRITE_ONCE(*ptep, pte);
-	pte_clear(mm, 0, ptep);
+	pte_clear(mm, vaddr, ptep);
 	pte = READ_ONCE(*ptep);
 	WARN_ON(!pte_none(pte));
 }

@@ -310,8 +312,9 @@ void __init debug_vm_pgtable(void)

 	pgtable_t saved_ptep;
 	pgprot_t prot;
 	phys_addr_t paddr;
-	unsigned long vaddr, pte_aligned, pmd_aligned;
+	unsigned long pte_aligned, pmd_aligned;
 	unsigned long pud_aligned, p4d_aligned, pgd_aligned;
+	spinlock_t *ptl;
 
 	pr_info("Validating architecture page table helpers\n");
 	prot = vm_get_page_prot(VMFLAGS);

@@ -344,7 +347,7 @@ void __init debug_vm_pgtable(void)

 	p4dp = p4d_alloc(mm, pgdp, vaddr);
 	pudp = pud_alloc(mm, p4dp, vaddr);
 	pmdp = pmd_alloc(mm, pudp, vaddr);
-	ptep = pte_alloc_map(mm, pmdp, vaddr);
+	ptep = pte_alloc_map_lock(mm, pmdp, vaddr, &ptl);
 
 	/*
 	 * Save all the page table page addresses as the page table

@@ -370,7 +373,7 @@ void __init debug_vm_pgtable(void)

 	p4d_clear_tests(mm, p4dp);
 	pgd_clear_tests(mm, pgdp);
 
-	pte_unmap(ptep);
+	pte_unmap_unlock(ptep, ptl);
 
 	pmd_populate_tests(mm, pmdp, saved_ptep);
 	pud_populate_tests(mm, pudp, saved_pmdp);