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

Anshuman Khandual
2019-09-13T08:42:46Z

On 09/13/2019 12:41 PM, Christophe Leroy wrote:
>
>
> Le 13/09/2019 à 09:03, Christophe Leroy a écrit :
>>
>>
>> Le 13/09/2019 à 08:58, Anshuman Khandual a écrit :
>>> On 09/13/2019 11:53 AM, Christophe Leroy wrote:
>>>> Fix build failure on powerpc.
>>>>
>>>> Fix preemption imbalance.
>>>>
>>>> Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
>>>> ---
>>>>   mm/arch_pgtable_test.c | 3 +++
>>>>   1 file changed, 3 insertions(+)
>>>>
>>>> diff --git a/mm/arch_pgtable_test.c b/mm/arch_pgtable_test.c
>>>> index 8b4a92756ad8..f2b3c9ec35fa 100644
>>>> --- a/mm/arch_pgtable_test.c
>>>> +++ b/mm/arch_pgtable_test.c
>>>> @@ -24,6 +24,7 @@
>>>>   #include <linux/swap.h>
>>>>   #include <linux/swapops.h>
>>>>   #include <linux/sched/mm.h>
>>>> +#include <linux/highmem.h>
>>>
>>> This is okay.
>>>
>>>>   #include <asm/pgalloc.h>
>>>>   #include <asm/pgtable.h>
>>>> @@ -400,6 +401,8 @@ static int __init arch_pgtable_tests_init(void)
>>>>       p4d_clear_tests(p4dp);
>>>>       pgd_clear_tests(mm, pgdp);
>>>> +    pte_unmap(ptep);
>>>> +
>>>
>>> Now the preemption imbalance via pte_alloc_map() path i.e
>>>
>>> pte_alloc_map() -> pte_offset_map() -> kmap_atomic()
>>>
>>> Is not this very much powerpc 32 specific or this will be applicable
>>> for all platform which uses kmap_XXX() to map high memory ?
>>>
>>
>> See https://elixir.bootlin.com/linux/v5.3-rc8/source/include/linux/highmem.h#L91
>>
>> I think it applies at least to all arches using the generic implementation.
>>
>> Applies also to arm:
>> https://elixir.bootlin.com/linux/v5.3-rc8/source/arch/arm/mm/highmem.c#L52
>>
>> Applies also to mips:
>> https://elixir.bootlin.com/linux/v5.3-rc8/source/arch/mips/mm/highmem.c#L47
>>
>> Same on sparc:
>> https://elixir.bootlin.com/linux/v5.3-rc8/source/arch/sparc/mm/highmem.c#L52
>>
>> Same on x86:
>> https://elixir.bootlin.com/linux/v5.3-rc8/source/arch/x86/mm/highmem_32.c#L34
>>
>> I have not checked others, but I guess it is like that for all.
>>
>
>
> Seems like I answered too quickly. All kmap_atomic() do preempt_disable(), but not all pte_alloc_map() call kmap_atomic().
>
> However, for instance ARM does:
>
> https://elixir.bootlin.com/linux/v5.3-rc8/source/arch/arm/include/asm/pgtable.h#L200
>
> And X86 as well:
>
> https://elixir.bootlin.com/linux/v5.3-rc8/source/arch/x86/include/asm/pgtable_32.h#L51
>
> Microblaze also:
>
> https://elixir.bootlin.com/linux/v5.3-rc8/source/arch/microblaze/include/asm/pgtable.h#L495

All the above platforms checks out to be using k[un]map_atomic(). I am wondering whether
any of the intermediate levels will have similar problems on any these 32 bit platforms
or any other platforms which might be using generic k[un]map_atomic(). There can be many
permutations here.

p4dp = p4d_alloc(mm, pgdp, vaddr);
pudp = pud_alloc(mm, p4dp, vaddr);
pmdp = pmd_alloc(mm, pudp, vaddr);

Otherwise pte_alloc_map()/pte_unmap() looks good enough which will atleast take care of
a known failure.