Re: [PATCH 1/1] mm/pgtable/debug: Add test validating architecture page table helpers

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Thu, Sep 05, 2019 at 01:48:27PM +0530, Anshuman Khandual wrote:
> >> +#define VADDR_TEST	(PGDIR_SIZE + PUD_SIZE + PMD_SIZE + PAGE_SIZE)
> > 
> > What is special about this address? How do you know if it is not occupied
> > yet?
> 
> We are creating the page table from scratch after allocating an mm_struct
> for a given random virtual address 'VADDR_TEST'. Hence nothing is occupied
> just yet. There is nothing special about this address, just that it tries
> to ensure the page table entries are being created with some offset from
> beginning of respective page table page at all levels ? The idea is to
> have a more representative form of page table structure for test.

Why P4D_SIZE is missing?

Are you sure it will not land into kernel address space on any arch?

I think more robust way to deal with this would be using
get_unmapped_area() instead of fixed address.

> This makes sense for runtime cases but there is a problem here.
> 
> On arm64, pgd_populate() which takes (pud_t *) as last argument instead of
> (p4d_t *) will fail to build when not wrapped in !__PAGETABLE_P4D_FOLDED
> on certain configurations.
> 
> ./arch/arm64/include/asm/pgalloc.h:81:75: note:
> expected ‘pud_t *’ {aka ‘struct <anonymous> *’}
> but argument is of type ‘pgd_t *’ {aka ‘struct <anonymous> *’}
> static inline void pgd_populate(struct mm_struct *mm, pgd_t *pgdp, pud_t *pudp)
>                                                                    ~~~~~~~^~~~
> Wondering if this is something to be fixed on arm64 or its more general
> problem. Will look into this further.

I think you need wrap this into #ifndef __ARCH_HAS_5LEVEL_HACK.

> >> +	pmd_populate_tests(mm, pmdp, (pgtable_t) page);
> > 
> > This is not correct for architectures that defines pgtable_t as pte_t
> > pointer, not struct page pointer.
> 
> Right, a grep on the source confirms that.
> 
> These platforms define pgtable_t as struct page *
> 
> arch/alpha/include/asm/page.h:typedef struct page *pgtable_t;
> arch/arm/include/asm/page.h:typedef struct page *pgtable_t;
> arch/arm64/include/asm/page.h:typedef struct page *pgtable_t;
> arch/csky/include/asm/page.h:typedef struct page *pgtable_t;
> arch/hexagon/include/asm/page.h:typedef struct page *pgtable_t;
> arch/ia64/include/asm/page.h:  typedef struct page *pgtable_t;
> arch/ia64/include/asm/page.h:    typedef struct page *pgtable_t;
> arch/m68k/include/asm/page.h:typedef struct page *pgtable_t;
> arch/microblaze/include/asm/page.h:typedef struct page *pgtable_t;
> arch/mips/include/asm/page.h:typedef struct page *pgtable_t;
> arch/nds32/include/asm/page.h:typedef struct page *pgtable_t;
> arch/nios2/include/asm/page.h:typedef struct page *pgtable_t;
> arch/openrisc/include/asm/page.h:typedef struct page *pgtable_t;
> arch/parisc/include/asm/page.h:typedef struct page *pgtable_t;
> arch/riscv/include/asm/page.h:typedef struct page *pgtable_t;
> arch/sh/include/asm/page.h:typedef struct page *pgtable_t;
> arch/sparc/include/asm/page_32.h:typedef struct page *pgtable_t;
> arch/um/include/asm/page.h:typedef struct page *pgtable_t;
> arch/unicore32/include/asm/page.h:typedef struct page *pgtable_t;
> arch/x86/include/asm/pgtable_types.h:typedef struct page *pgtable_t;
> arch/xtensa/include/asm/page.h:typedef struct page *pgtable_t;
> 
> These platforms define pgtable_t as pte_t *
> 
> arch/arc/include/asm/page.h:typedef pte_t * pgtable_t;
> arch/powerpc/include/asm/mmu.h:typedef pte_t *pgtable_t;
> arch/s390/include/asm/page.h:typedef pte_t *pgtable_t;
> arch/sparc/include/asm/page_64.h:typedef pte_t *pgtable_t;
> 
> Should we need have two pmd_populate_tests() definitions with
> different arguments (struct page pointer or pte_t pointer) and then
> call either one after detecting the given platform ?

Use pte_alloc_one() instead of alloc_mapped_page() to allocate the page
table.

> >> +	pud_populate_tests(mm, pudp, pmdp);
> >> +	p4d_populate_tests(mm, p4dp, pudp);
> >> +	pgd_populate_tests(mm, pgdp, p4dp);
> > 
> > This is wrong. All p?dp points to the second entry in page table entry.
> > This is not valid pointer for page table and triggers p?d_bad() on x86.
> 
> Yeah these are second entries because of the way we create the page table.
> But I guess its applicable only to the second argument in all these above
> cases because the first argument can be any valid entry on previous page
> table level.

Yes:

@@ -397,9 +396,9 @@ static int __init arch_pgtable_tests_init(void)
 	pgd_clear_tests(pgdp);
 
 	pmd_populate_tests(mm, pmdp, (pgtable_t) page);
-	pud_populate_tests(mm, pudp, pmdp);
-	p4d_populate_tests(mm, p4dp, pudp);
-	pgd_populate_tests(mm, pgdp, p4dp);
+	pud_populate_tests(mm, pudp, saved_pmdp);
+	p4d_populate_tests(mm, p4dp, saved_pudp);
+	pgd_populate_tests(mm, pgdp, saved_p4dp);
 
 	p4d_free(mm, saved_p4dp);
 	pud_free(mm, saved_pudp);

-- 
 Kirill A. Shutemov