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SubjectRe: [RFC] mm/pgtable/debug: Add test validating architecture page table helpers
On Fri, Jul 26, 2019 at 10:17:11AM +0530, Anshuman Khandual wrote:
> > But 'page' isn't necessarily PMD-aligned. I don't think we can rely on
> > architectures doing the right thing if asked to make a PMD for a randomly
> > aligned page.
> >
> > How about finding the physical address of something like kernel_init(),
>
> Physical address corresponding to the symbol in the kernel text segment ?

Yes. We need the address of something that's definitely memory.
The stack might be in vmalloc space. We can't allocate memory from the
allocator that's PUD-aligned. This seems like a reasonable approximation
to something that might work.

> > and using the corresponding pte/pmd/pud/p4d/pgd that encompasses that
>
> So I guess this will help us use pte/pmd/pud/p4d/pgd entries from a real and
> present mapping rather then making them up for test purpose. Although we are
> not creating real page tables here just wondering if this could some how
> affect these real mapping in anyway from some accessors. The current proposal
> stays clear from anything real - allocates, evaluates and releases.

I think that's a mistake. As Russell said, the ARM p*d manipulation
functions expect to operate on tables, not on individual entries
constructed on the stack.

So I think the right thing to do here is allocate an mm, then do the
pgd_alloc / p4d_alloc / pud_alloc / pmd_alloc / pte_alloc() steps giving
you real page tables that you can manipulate.

Then destroy them, of course. And don't access through them.

> >> +#ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
> >> +static void pud_basic_tests(void)
> >
> > Is this the right ifdef?
>
> IIUC THP at PUD is where the pud_t entries are directly operated upon and the
> corresponding accessors are present only when HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
> is enabled. Am I missing something here ?

Maybe I am. I thought we could end up operating on PUDs for kernel mappings,
even without transparent hugepages turned on.

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