Subject: 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]

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Mon, 17 May 2021 19:22:03 +0530

Anshuman Khandual <[email-masked]> 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;

-aneesh