Re: Fwd: [Bug 213069] New: kernel BUG at arch/powerpc/include/asm/book3s/64/hash-4k.h:147! Oops: Exception in kernel mode, sig: 5 [#1]

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 5/17/21 1:14 PM, Aneesh Kumar K.V wrote:
> On 5/17/21 12:55 PM, Anshuman Khandual wrote:
>>
>>
>> On 5/17/21 11:25 AM, Aneesh Kumar K.V wrote:
>>> On 5/17/21 11:17 AM, Christophe Leroy wrote:
>>>> +aneesh
>>>> +linuxppc-dev list
>>>>
>>>> Le 17/05/2021 à 07:44, Anshuman Khandual a écrit :
>>>>> Hello Christophe,
>>>>>
>>>>> DEBUG_VM_PGTABLE has now been re-enabled on powerpc recently ? was not
>>>>> aware about this. From the error log, it failed explicitly on 4K page
>>>>> size hash config.
>>>>>
>>>>> static inline pmd_t hash__pmd_mkhuge(pmd_t pmd)
>>>>> {
>>>>>           BUG();        ------> Failed
>>>>>           return pmd;
>>>>> }
>>>>>
>>>>> static inline pmd_t __pmd_mkhuge(pmd_t pmd)
>>>>> {
>>>>>           if (radix_enabled())
>>>>>                   return radix__pmd_mkhuge(pmd);
>>>>>           return hash__pmd_mkhuge(pmd);
>>>>> }
>>>>>
>>>>> pmd_t pfn_pmd(unsigned long pfn, pgprot_t pgprot)
>>>>> {
>>>>>           unsigned long pmdv;
>>>>>
>>>>>           pmdv = (pfn << PAGE_SHIFT) & PTE_RPN_MASK;
>>>>>
>>>>>           return __pmd_mkhuge(pmd_set_protbits(__pmd(pmdv), pgprot));
>>>>> }
>>>>>
>>>>> It seems like on powerpc, where pfn_pmd() makes a huge page but which
>>>>> is not supported on 4K hash config thus triggering the BUG(). But all
>>>>> pfn_pmd() call sites inside the debug_vm_pgtable() test are protected
>>>>> with CONFIG_TRANSPARENT_HUGEPAGE. IIUC unlike powerpc, pfn_pmd() does
>>>>> not directly make a huge page on other platforms.
>>>>>
>>>>> Looking at arch/powerpc/include/asm/book3s/64/hash-4k.h, all relevant
>>>>> THP helpers has BUG() or 0 which indicates THP might not be supported
>>>>> on 4K page size hash config ?
>>>>>
>>>>> But looking at arch/powerpc/platforms/Kconfig.cputype, it seems like
>>>>> HAVE_ARCH_TRANSPARENT_HUGEPAGE is invariably selected on PPC_BOOK3S_64
>>>>> platforms which I assume includes 4K page size hash config as well.
>>>>>
>>>>> Is THP some how getting enabled on this 4K page size hash config where
>>>>> it should not be (thus triggering the BUG) ? OR am I missing something
>>>>> here.
>>>>>
>>>>
>>>
>>> We should put those  pfn_pmd()  and pfn_pud() after
>>>
>>>      if (!has_transparent_hugepage())
>>>          return;
>>>
>>>
>>> On hash with 4K page size, we can't support leaf page table entry and PMD and PUD level. Hence we don't support THP for them.
>>
>> But could CONFIG_TRANSPARENT_HUGEPAGE be enabled on such configs ?
>> Otherwise, wondering how the BUG() just got triggered there.
>>
> 
> yes. We do support THP with radix translation and 4K page size.

I was actually wondering about whether 4K page size in hash config
supports THP and could enable CONFIG_TRANSPARENT_HUGEPAGE ? But as
there is a BUG() trigger on hash__pmd_mkhuge(), it seems like THP
config should not have been enabled for such platforms but it does
for some reason. Could not we disable it ?