Re: [PATCH] mm/debug_vm_pgtable: Fix kernel crash with page table validate

["Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>]

On 6/8/20 4:31 PM, Anshuman Khandual wrote:
> Hi Aneesh,
> 
> On 06/08/2020 11:57 AM, Aneesh Kumar K.V wrote:
>> Architectures can have CONFIG_TRANSPARENT_HUGEPAGE enabled but
>> no THP support enabled based on platforms. For ex: with 4K
>> PAGE_SIZE ppc64 supports THP only with radix translation.
> 
> Good catch, never hit this before.
> 
>>
>> This results in below crash when running with hash translation and
>> 4K PAGE_SIZE.
>>
>> kernel BUG at arch/powerpc/include/asm/book3s/64/hash-4k.h:140!
>> cpu 0x61: Vector: 700 (Program Check) at [c000000ff948f860]
>>      pc: c0000000018810f8: debug_vm_pgtable+0x480/0x8b0
>>      lr: c0000000018810ec: debug_vm_pgtable+0x474/0x8b0
>> ...
>> [c000000ff948faf0] c000000001880fec debug_vm_pgtable+0x374/0x8b0 (unreliable)
>> [c000000ff948fbf0] c000000000011648 do_one_initcall+0x98/0x4f0
>> [c000000ff948fcd0] c000000001843928 kernel_init_freeable+0x330/0x3fc
>> [c000000ff948fdb0] c0000000000122ac kernel_init+0x24/0x148
>> [c000000ff948fe20] c00000000000cc44 ret_from_kernel_thread+0x5c/0x78
>>
>> Check for THP support correctly
> 
> Makes sense, is this the only configuration which hit the problem ?

4K hash ppc64 is the only config i guess.

> 
>>
>> Cc: anshuman.khandual@arm.com
>> Fixes: 399145f9eb6c ("mm/debug: add tests validating architecture page table helpers")
>> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
>> ---
>>   mm/debug_vm_pgtable.c | 3 +++
>>   1 file changed, 3 insertions(+)
>>
>> diff --git a/mm/debug_vm_pgtable.c b/mm/debug_vm_pgtable.c
>> index 188c18908964..e60151c5e997 100644
>> --- a/mm/debug_vm_pgtable.c
>> +++ b/mm/debug_vm_pgtable.c
>> @@ -61,6 +61,9 @@ static void __init pmd_basic_tests(unsigned long pfn, pgprot_t prot)
>>   {
>>   	pmd_t pmd = pfn_pmd(pfn, prot);
>>   
>> +	if (!has_transparent_hugepage())
>> +		return;
>> +
> 
> We should also add this check to pud_basic_tests() as well.


Do we have a function that check for runtime support for pud level THP? 
ppc64 don't do pud level THP yet. So  we have 
CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD=n

are you suggesting we do the same check for pud level THP too?


> 
>>   	WARN_ON(!pmd_same(pmd, pmd));
>>   	WARN_ON(!pmd_young(pmd_mkyoung(pmd_mkold(pmd))));
>>   	WARN_ON(!pmd_dirty(pmd_mkdirty(pmd_mkclean(pmd))));
>>
> 
> The subject line here should mention about correct THP support
> detection which fixes the problem. Probably something like this
> or similar ("Fix kernel crash with correct THP support check").


Not sure about that. This fix a kernel crash with page table validate code.


-aneesh