Re: [PATCH v4 00/13] mm/debug_vm_pgtable fixes

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

On 09/04/2020 09:31 PM, Gerald Schaefer wrote:
> On Fri, 4 Sep 2020 17:26:47 +0200
> Gerald Schaefer <gerald.schaefer@linux.ibm.com> wrote:
> 
>> On Fri, 4 Sep 2020 12:18:05 +0530
>> Anshuman Khandual <anshuman.khandual@arm.com> wrote:
>>
>>>
>>>
>>> On 09/02/2020 05:12 PM, Aneesh Kumar K.V wrote:
>>>> This patch series includes fixes for debug_vm_pgtable test code so that
>>>> they follow page table updates rules correctly. The first two patches introduce
>>>> changes w.r.t ppc64. The patches are included in this series for completeness. We can
>>>> merge them via ppc64 tree if required.
>>>>
>>>> Hugetlb test is disabled on ppc64 because that needs larger change to satisfy
>>>> page table update rules.
>>>>
>>>> These tests are broken w.r.t page table update rules and results in kernel
>>>> crash as below. 
>>>>
>>>> [   21.083519] kernel BUG at arch/powerpc/mm/pgtable.c:304!
>>>> cpu 0x0: Vector: 700 (Program Check) at [c000000c6d1e76c0]
>>>>     pc: c00000000009a5ec: assert_pte_locked+0x14c/0x380
>>>>     lr: c0000000005eeeec: pte_update+0x11c/0x190
>>>>     sp: c000000c6d1e7950
>>>>    msr: 8000000002029033
>>>>   current = 0xc000000c6d172c80
>>>>   paca    = 0xc000000003ba0000   irqmask: 0x03   irq_happened: 0x01
>>>>     pid   = 1, comm = swapper/0
>>>> kernel BUG at arch/powerpc/mm/pgtable.c:304!
>>>> [link register   ] c0000000005eeeec pte_update+0x11c/0x190
>>>> [c000000c6d1e7950] 0000000000000001 (unreliable)
>>>> [c000000c6d1e79b0] c0000000005eee14 pte_update+0x44/0x190
>>>> [c000000c6d1e7a10] c000000001a2ca9c pte_advanced_tests+0x160/0x3d8
>>>> [c000000c6d1e7ab0] c000000001a2d4fc debug_vm_pgtable+0x7e8/0x1338
>>>> [c000000c6d1e7ba0] c0000000000116ec do_one_initcall+0xac/0x5f0
>>>> [c000000c6d1e7c80] c0000000019e4fac kernel_init_freeable+0x4dc/0x5a4
>>>> [c000000c6d1e7db0] c000000000012474 kernel_init+0x24/0x160
>>>> [c000000c6d1e7e20] c00000000000cbd0 ret_from_kernel_thread+0x5c/0x6c
>>>>
>>>> With DEBUG_VM disabled
>>>>
>>>> [   20.530152] BUG: Kernel NULL pointer dereference on read at 0x00000000
>>>> [   20.530183] Faulting instruction address: 0xc0000000000df330
>>>> cpu 0x33: Vector: 380 (Data SLB Access) at [c000000c6d19f700]
>>>>     pc: c0000000000df330: memset+0x68/0x104
>>>>     lr: c00000000009f6d8: hash__pmdp_huge_get_and_clear+0xe8/0x1b0
>>>>     sp: c000000c6d19f990
>>>>    msr: 8000000002009033
>>>>    dar: 0
>>>>   current = 0xc000000c6d177480
>>>>   paca    = 0xc00000001ec4f400   irqmask: 0x03   irq_happened: 0x01
>>>>     pid   = 1, comm = swapper/0
>>>> [link register   ] c00000000009f6d8 hash__pmdp_huge_get_and_clear+0xe8/0x1b0
>>>> [c000000c6d19f990] c00000000009f748 hash__pmdp_huge_get_and_clear+0x158/0x1b0 (unreliable)
>>>> [c000000c6d19fa10] c0000000019ebf30 pmd_advanced_tests+0x1f0/0x378
>>>> [c000000c6d19fab0] c0000000019ed088 debug_vm_pgtable+0x79c/0x1244
>>>> [c000000c6d19fba0] c0000000000116ec do_one_initcall+0xac/0x5f0
>>>> [c000000c6d19fc80] c0000000019a4fac kernel_init_freeable+0x4dc/0x5a4
>>>> [c000000c6d19fdb0] c000000000012474 kernel_init+0x24/0x160
>>>> [c000000c6d19fe20] c00000000000cbd0 ret_from_kernel_thread+0x5c/0x6c
>>>>
>>>> Changes from v3:
>>>> * Address review feedback
>>>> * Move page table depost and withdraw patch after adding pmdlock to avoid bisect failure.
>>>
>>> This version
>>>
>>> - Builds on x86, arm64, s390, arc, powerpc and riscv (defconfig with DEBUG_VM_PGTABLE)
>>> - Runs on arm64 and x86 without any regression, atleast nothing that I have noticed
>>> - Will be great if this could get tested on s390, arc, riscv, ppc32 platforms as well
>>
>> When I quickly tested v3, it worked fine, but now it turned out to
>> only work fine "sometimes", both v3 and v4. I need to look into it
>> further, but so far it seems related to the hugetlb_advanced_tests().
>>
>> I guess there was already some discussion on this test, but we did
>> not receive all of the thread(s). Please always add at least
>> linux-s390@vger.kernel.org and maybe myself and Vasily Gorbik <gor@linux.ibm.com>
>> for further discussions.
> 
> BTW, with myself I mean the new address gerald.schaefer@linux.ibm.com.
> The old gerald.schaefer@de.ibm.com seems to work (again), but is not
> very reliable.

Sure, noted.

> 
> BTW2, a quick test with this change (so far) made the issues on s390
> go away:
> 
> @@ -1069,7 +1074,7 @@ static int __init debug_vm_pgtable(void)
>         spin_unlock(ptl);
>  
>  #ifndef CONFIG_PPC_BOOK3S_64
> -       hugetlb_advanced_tests(mm, vma, ptep, pte_aligned, vaddr, prot);
> +       hugetlb_advanced_tests(mm, vma, (pte_t *) pmdp, pmd_aligned, vaddr, prot);
>  #endif
>  
>         spin_lock(&mm->page_table_lock);
> 
> That would more match the "pte_t pointer" usage for hugetlb code,
> i.e. just cast a pmd_t pointer to it. Also changed to pmd_aligned,
> but I think the root cause is the pte_t pointer.

Ideally, the pte_t pointer used here should be from huge_pte_alloc()
not from pte_alloc_map_lock() as the case currently.

> 
> Not entirely sure though if that would really be the correct fix.
> I somehow lost whatever little track I had about what these tests
> really want to check, and if that would still be valid with that
> change.
>