Re: [PATCH] mm/debug_vm_pgtable: Fix kernel crash with page table validate

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

Hi Aneesh,

On 06/08/2020 11:57 AM, Aneesh Kumar K.V wrote:
> Architectures can have CONFIG_TRANSPARENT_HUGEPAGE enabled but
> no THP support enabled based on platforms. For ex: with 4K
> PAGE_SIZE ppc64 supports THP only with radix translation.

Good catch, never hit this before.

> 
> This results in below crash when running with hash translation and
> 4K PAGE_SIZE.
> 
> kernel BUG at arch/powerpc/include/asm/book3s/64/hash-4k.h:140!
> cpu 0x61: Vector: 700 (Program Check) at [c000000ff948f860]
>     pc: c0000000018810f8: debug_vm_pgtable+0x480/0x8b0
>     lr: c0000000018810ec: debug_vm_pgtable+0x474/0x8b0
> ...
> [c000000ff948faf0] c000000001880fec debug_vm_pgtable+0x374/0x8b0 (unreliable)
> [c000000ff948fbf0] c000000000011648 do_one_initcall+0x98/0x4f0
> [c000000ff948fcd0] c000000001843928 kernel_init_freeable+0x330/0x3fc
> [c000000ff948fdb0] c0000000000122ac kernel_init+0x24/0x148
> [c000000ff948fe20] c00000000000cc44 ret_from_kernel_thread+0x5c/0x78
> 
> Check for THP support correctly

Makes sense, is this the only configuration which hit the problem ?

> 
> Cc: anshuman.khandual@arm.com
> Fixes: 399145f9eb6c ("mm/debug: add tests validating architecture page table helpers")
> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
> ---
>  mm/debug_vm_pgtable.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/mm/debug_vm_pgtable.c b/mm/debug_vm_pgtable.c
> index 188c18908964..e60151c5e997 100644
> --- a/mm/debug_vm_pgtable.c
> +++ b/mm/debug_vm_pgtable.c
> @@ -61,6 +61,9 @@ static void __init pmd_basic_tests(unsigned long pfn, pgprot_t prot)
>  {
>  	pmd_t pmd = pfn_pmd(pfn, prot);
>  
> +	if (!has_transparent_hugepage())
> +		return;
> +

We should also add this check to pud_basic_tests() as well.

>  	WARN_ON(!pmd_same(pmd, pmd));
>  	WARN_ON(!pmd_young(pmd_mkyoung(pmd_mkold(pmd))));
>  	WARN_ON(!pmd_dirty(pmd_mkdirty(pmd_mkclean(pmd))));
> 

The subject line here should mention about correct THP support
detection which fixes the problem. Probably something like this
or similar ("Fix kernel crash with correct THP support check").

- Anshuman