Re: [PATCH] mm/debug_vm_pgtable: Fix BUG_ON with pud advanced test

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

On 1/29/24 11:56, Aneesh Kumar K.V wrote:
> On 1/29/24 11:52 AM, Anshuman Khandual wrote:
>>
>>
>> On 1/29/24 11:30, Aneesh Kumar K.V (IBM) wrote:
>>> Architectures like powerpc add debug checks to ensure we find only devmap
>>> PUD pte entries. These debug checks are only done with CONFIG_DEBUG_VM.
>>> This patch marks the ptes used for PUD advanced test devmap pte entries
>>> so that we don't hit on debug checks on architecture like ppc64 as
>>> below.
>>>
>>> WARNING: CPU: 2 PID: 1 at arch/powerpc/mm/book3s64/radix_pgtable.c:1382 radix__pud_hugepage_update+0x38/0x138
>>> ....
>>> NIP [c0000000000a7004] radix__pud_hugepage_update+0x38/0x138
>>> LR [c0000000000a77a8] radix__pudp_huge_get_and_clear+0x28/0x60
>>> Call Trace:
>>> [c000000004a2f950] [c000000004a2f9a0] 0xc000000004a2f9a0 (unreliable)
>>> [c000000004a2f980] [000d34c100000000] 0xd34c100000000
>>> [c000000004a2f9a0] [c00000000206ba98] pud_advanced_tests+0x118/0x334
>>> [c000000004a2fa40] [c00000000206db34] debug_vm_pgtable+0xcbc/0x1c48
>>> [c000000004a2fc10] [c00000000000fd28] do_one_initcall+0x60/0x388
>>>
>>> Also
>>>
>>>  kernel BUG at arch/powerpc/mm/book3s64/pgtable.c:202!
>>>  ....
>>>
>>>  NIP [c000000000096510] pudp_huge_get_and_clear_full+0x98/0x174
>>>  LR [c00000000206bb34] pud_advanced_tests+0x1b4/0x334
>>>  Call Trace:
>>>  [c000000004a2f950] [000d34c100000000] 0xd34c100000000 (unreliable)
>>>  [c000000004a2f9a0] [c00000000206bb34] pud_advanced_tests+0x1b4/0x334
>>>  [c000000004a2fa40] [c00000000206db34] debug_vm_pgtable+0xcbc/0x1c48
>>>  [c000000004a2fc10] [c00000000000fd28] do_one_initcall+0x60/0x388
>>>
>>> Fixes: 27af67f35631 ("powerpc/book3s64/mm: enable transparent pud hugepage")
>>> Signed-off-by: Aneesh Kumar K.V (IBM) <aneesh.kumar@kernel.org>
>>> ---
>>>  mm/debug_vm_pgtable.c | 8 ++++++++
>>>  1 file changed, 8 insertions(+)
>>>
>>> diff --git a/mm/debug_vm_pgtable.c b/mm/debug_vm_pgtable.c
>>> index 5662e29fe253..65c19025da3d 100644
>>> --- a/mm/debug_vm_pgtable.c
>>> +++ b/mm/debug_vm_pgtable.c
>>> @@ -362,6 +362,12 @@ static void __init pud_advanced_tests(struct pgtable_debug_args *args)
>>>  	vaddr &= HPAGE_PUD_MASK;
>>>  
>>>  	pud = pfn_pud(args->pud_pfn, args->page_prot);
>>> +	/*
>>> +	 * Some architectures have debug checks to make sure
>>> +	 * huge pud mapping are only found with devmap entries
>>> +	 * For now test with only devmap entries.
>>> +	 */
>> Do you see this behaviour to be changed in powerpc anytime soon ? Otherwise
>> these pud_mkdevmap() based work arounds, might be required to stick around
>> for longer just to prevent powerpc specific triggers. Given PUD transparent
>> huge pages i.e HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD are just supported on x86
>> and powerpc platforms, could not this problem be solved in a more uniform
>> manner.
>>
> 
> 
> IIUC pud level transparent hugepages are only supported with devmap entries even
> on x86. We don't do anonymous pud hugepage.

There are some 'pud_trans_huge(orig_pud) || pud_devmap(orig_pud)' checks in
core paths i.e in mm/memory.c which might suggest pud_trans_huge() to exist
without also being a devmap. I might be missing something here, but on x86
platform following helpers suggest pud_trans_huge() to exist without being
a devmap as well.

#ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
static inline int pud_trans_huge(pud_t pud)
{
        return (pud_val(pud) & (_PAGE_PSE|_PAGE_DEVMAP)) == _PAGE_PSE;
}
#endif

#ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
static inline int pud_devmap(pud_t pud)
{
        return !!(pud_val(pud) & _PAGE_DEVMAP);
}
#else
static inline int pud_devmap(pud_t pud)
{
        return 0;
}
#endif

We might need some more clarity on this regarding x86 platform's pud huge
page implementation.

> 
>>> +	pud = pud_mkdevmap(pud);
>>>  	set_pud_at(args->mm, vaddr, args->pudp, pud);
>>>  	flush_dcache_page(page);
>>>  	pudp_set_wrprotect(args->mm, vaddr, args->pudp);
>>> @@ -374,6 +380,7 @@ static void __init pud_advanced_tests(struct pgtable_debug_args *args)
>>>  	WARN_ON(!pud_none(pud));
>>>  #endif /* __PAGETABLE_PMD_FOLDED */
>>>  	pud = pfn_pud(args->pud_pfn, args->page_prot);
>>> +	pud = pud_mkdevmap(pud);
>>>  	pud = pud_wrprotect(pud);
>>>  	pud = pud_mkclean(pud);
>>>  	set_pud_at(args->mm, vaddr, args->pudp, pud);
>>> @@ -391,6 +398,7 @@ static void __init pud_advanced_tests(struct pgtable_debug_args *args)
>>>  #endif /* __PAGETABLE_PMD_FOLDED */
>>>  
>>>  	pud = pfn_pud(args->pud_pfn, args->page_prot);
>>> +	pud = pud_mkdevmap(pud);
>>>  	pud = pud_mkyoung(pud);
>>>  	set_pud_at(args->mm, vaddr, args->pudp, pud);
>>>  	flush_dcache_page(page);
> 
> 
> -aneesh
>