Re: [PATCH v2] mm/hugetlb: fix a addressing exception caused by huge_pte_offset()

[Mike Kravetz]

On 3/24/20 8:55 AM, Jason Gunthorpe wrote:
> On Tue, Mar 24, 2020 at 08:25:09AM -0700, Mike Kravetz wrote:
>> On 3/24/20 4:55 AM, Jason Gunthorpe wrote:
>>> Also, since CH moved all the get_user_pages_fast code out of the
>>> arch's many/all archs can drop their arch specific version of this
>>> routine. This is really just a specialized version of gup_fast's
>>> algorithm..
>>>
>>> (also the arch versions seem different, why do some return actual
>>>  ptes, not null?)
>>
>> Not sure I understand that last question.  The return value should be
>> a *pte or null.
> 
> I mean the common code ends like this:
> 
> 	pmd = pmd_offset(pud, addr);
> 	if (sz != PMD_SIZE && pmd_none(*pmd))
> 		return NULL;
> 	/* hugepage or swap? */
> 	if (pmd_huge(*pmd) || !pmd_present(*pmd))
> 		return (pte_t *)pmd;
> 
> 	return NULL;
> 
> So it always returns a pointer into a PUD or PMD, while say, ppc
> in __find_linux_pte() ends like:
> 
> 	return pte_offset_kernel(&pmd, ea);
> 
> Which is pointing to a PTE

Ok, now I understand the question.  huge_pte_offset will/should only be
called for addresses that are in a vma backed by hugetlb pages.  So,
pte_offset_kernel() will only return page table type (PUD/PMD/etc) associated
with a huge page supported by the particular arch.

> So does sparc:
> 
>         pmd = pmd_offset(pud, addr);
>         if (pmd_none(*pmd))
>                 return NULL;
>         if (is_hugetlb_pmd(*pmd))
>                 return (pte_t *)pmd;
>         return pte_offset_map(pmd, addr);
> 
> Which is even worse because it is leaking a kmap..
> 
> etc
> 
>> /*
>>  * huge_pte_offset() - Walk the page table to resolve the hugepage
>>  * entry at address @addr
>>  *
>>  * Return: Pointer to page table or swap entry (PUD or PMD) for
>                                               ^^^^^^^^^^^^^^^^^^^
> 
> Ie the above is not followed by the archs
> 
> I'm also scratching my head that a function that returns a pte_t *
> always returns a PUD or PMD. Strange bit of type casting..

Yes, the casting is curious.  The casting continues in potential subsequent
calls to huge_pte_alloc().
-- 
Mike Kravetz