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 7:22 PM, Aneesh Kumar K.V wrote:
> Anshuman Khandual <anshuman.khandual at arm.com> writes:
> 
>> 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;
>>
>> The following patch has been lightly tested on arm64 and x86 platforms.
>> Could you please verify if this solves the problem on powerpc 4K hash
>> config ? Thank you.
> 
> Tested the patch with the below additional change.
> 
> modified   mm/debug_vm_pgtable.c
> @@ -186,12 +186,13 @@ static void __init pmd_advanced_tests(struct mm_struct *mm,
>  				      unsigned long pfn, unsigned long vaddr,
>  				      pgprot_t prot, pgtable_t pgtable)
>  {
> -	pmd_t pmd = pfn_pmd(pfn, prot);
> +	pmd_t pmd;
>  
>  	if (!has_transparent_hugepage())
>  		return;
>  
>  	pr_debug("Validating PMD advanced\n");
> +	pmd = pfn_pmd(pfn, prot);
>  	/* Align the address wrt HPAGE_PMD_SIZE */
>  	vaddr = (vaddr & HPAGE_PMD_MASK) + HPAGE_PMD_SIZE;
>  
> @@ -334,12 +335,13 @@ static void __init pud_advanced_tests(struct mm_struct *mm,
>  				      unsigned long pfn, unsigned long vaddr,
>  				      pgprot_t prot)
>  {
> -	pud_t pud = pfn_pud(pfn, prot);
> +	pud_t pud;
>  
>  	if (!has_transparent_hugepage())
>  		return;
>  
>  	pr_debug("Validating PUD advanced\n");
> +	pud = pfn_pud(pfn, prot);
>  	/* Align the address wrt HPAGE_PUD_SIZE */
>  	vaddr = (vaddr & HPAGE_PUD_MASK) + HPAGE_PUD_SIZE;
>  

Right. But this change is already on the linux-next (20210514) via another
commit ae7920ce9e9bb ("mm/debug_vm_pgtable: remove redundant pfn_{pmd/pte}()
and fix one comment mistake") which got queued recently. Anyways, now that
we have verified the fix, will send it out against linux-next soon.