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

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