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]

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.