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]

[Aneesh Kumar K.V]

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.

-aneesh