Subject: Re: [PATCH v4 13/13] mm/debug_vm_pgtable: Avoid none pte in pte_clear_test

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Fri, 11 Sep 2020 10:51:04 +0530

Nathan Chancellor <[email-masked]> writes:

> On Wed, Sep 02, 2020 at 05:12:22PM +0530, Aneesh Kumar K.V wrote:
>> pte_clear_tests operate on an existing pte entry. Make sure that
>> is not a none pte entry.
>> 
>> Signed-off-by: Aneesh Kumar K.V <[email-masked]>
>> ---
>>  mm/debug_vm_pgtable.c | 7 ++++---
>>  1 file changed, 4 insertions(+), 3 deletions(-)
>> 
>> diff --git a/mm/debug_vm_pgtable.c b/mm/debug_vm_pgtable.c
>> index 9afa1354326b..c36530c69e33 100644
>> --- a/mm/debug_vm_pgtable.c
>> +++ b/mm/debug_vm_pgtable.c
>> @@ -542,9 +542,10 @@ static void __init pgd_populate_tests(struct mm_struct *mm, pgd_t *pgdp,
>>  #endif /* PAGETABLE_P4D_FOLDED */
>>  
>>  static void __init pte_clear_tests(struct mm_struct *mm, pte_t *ptep,
>> -				   unsigned long vaddr)
>> +				   unsigned long pfn, unsigned long vaddr,
>> +				   pgprot_t prot)
>>  {
>> -	pte_t pte = ptep_get(ptep);
>> +	pte_t pte = pfn_pte(pfn, prot);
>>  
>>  	pr_debug("Validating PTE clear\n");
>>  	pte = __pte(pte_val(pte) | RANDOM_ORVALUE);
>> @@ -1049,7 +1050,7 @@ static int __init debug_vm_pgtable(void)
>>  
>>  	ptl = pte_lockptr(mm, pmdp);
>>  	spin_lock(ptl);
>> -	pte_clear_tests(mm, ptep, vaddr);
>> +	pte_clear_tests(mm, ptep, pte_aligned, vaddr, prot);
>>  	pte_advanced_tests(mm, vma, ptep, pte_aligned, vaddr, prot);
>>  	pte_unmap_unlock(ptep, ptl);
>>  
>> -- 
> This patch causes a panic at boot for RISC-V defconfig. The rootfs is here if it is needed:
> https://github.com/ClangBuiltLinux/boot-utils/blob/3b21a5b71451742866349ba4f18638c5a754e660/images/riscv/rootfs.cpio.zst
>
> $ make -skj"$(nproc)" ARCH=riscv CROSS_COMPILE=riscv64-linux- O=out/riscv distclean defconfig Image
>
> $ qemu-system-riscv64 -bios default -M virt -display none -initrd rootfs.cpio -kernel Image -m 512m -nodefaults -serial mon:stdio
> ...
>
> OpenSBI v0.6
>    ____                    _____ ____ _____
>   / __ \                  / ____|  _ \_   _|
>  | |  | |_ __   ___ _ __ | (___ | |_) || |
>  | |  | | '_ \ / _ \ '_ \ \___ \|  _ < | |
>  | |__| | |_) |  __/ | | |____) | |_) || |_
>   \____/| .__/ \___|_| |_|_____/|____/_____|
>         | |
>         |_|
>
> Platform Name          : QEMU Virt Machine
> Platform HART Features : RV64ACDFIMSU
> Platform Max HARTs     : 8
> Current Hart           : 0
> Firmware Base          : 0x80000000
> Firmware Size          : 120 KB
> Runtime SBI Version    : 0.2
>
> MIDELEG : 0x0000000000000222
> MEDELEG : 0x000000000000b109
> PMP0    : 0x0000000080000000-0x000000008001ffff (A)
> PMP1    : 0x0000000000000000-0xffffffffffffffff (A,R,W,X)
> [    0.000000] Linux version 5.9.0-rc4-next-20200910 (nathan@ubuntu-n2-xlarge-x86) (riscv64-linux-gcc (GCC) 10.2.0, GNU ld (GNU Binutils) 2.35) #1 SMP Thu Sep 10 19:10:43 MST 2020
> ...
> [    0.294593] NET: Registered protocol family 17
> [    0.295781] 9pnet: Installing 9P2000 support
> [    0.296153] Key type dns_resolver registered
> [    0.296694] debug_vm_pgtable: [debug_vm_pgtable         ]: Validating architecture page table helpers
> [    0.297635] Unable to handle kernel paging request at virtual address 0a7fffe01dafefc8
> [    0.298029] Oops [#1]
> [    0.298153] Modules linked in:
> [    0.298433] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.9.0-rc4-next-20200910 #1
> [    0.298792] epc: ffffffe000205afc ra : ffffffe0008be0aa sp : ffffffe01ae73d40
> [    0.299078]  gp : ffffffe0010b9b48 tp : ffffffe01ae68000 t0 : ffffffe008152000
> [    0.299362]  t1 : 0000000000000000 t2 : 0000000000000000 s0 : ffffffe01ae73d60
> [    0.299648]  s1 : bffffffffffffffb a0 : 0a7fffe01dafefc8 a1 : bffffffffffffffb
> [    0.299948]  a2 : ffffffe0010a2698 a3 : 0000000000000001 a4 : 0000000000000003
> [    0.300231]  a5 : 0000000000000800 a6 : fffffffff0000080 a7 : 000000001b642000
> [    0.300521]  s2 : ffffffe0081517b8 s3 : ffffffe008150a80 s4 : ffffffe01af30000
> [    0.300806]  s5 : ffffffe01f8ca9b8 s6 : ffffffe008150000 s7 : ffffffe0010bb100
> [    0.301161]  s8 : ffffffe0010bb108 s9 : 0000000000080202 s10: ffffffe0010bb928
> [    0.301481]  s11: 000000002008085b t3 : 0000000000000000 t4 : 0000000000000000
> [    0.301722]  t5 : 0000000000000000 t6 : ffffffe008150000
> [    0.301947] status: 0000000000000120 badaddr: 0a7fffe01dafefc8 cause: 000000000000000f
> [    0.302569] ---[ end trace 7ffb153d816164cf ]---
> [    0.302797] note: swapper/0[1] exited with preempt_count 1
> [    0.303101] Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b
> [    0.303614] ---[ end Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b ]---

I guess it is the combination of a valid pte and usage of
RANDOM_ORVALUE. The below change get the kernel to boot. Can somebody
faimilar with riscv pte format take a look at the RANDOM_ORVALUE?

modified   mm/debug_vm_pgtable.c
@@ -548,7 +548,7 @@ static void __init pte_clear_tests(struct mm_struct *mm, pte_t *ptep,
 	pte_t pte = pfn_pte(pfn, prot);
 
 	pr_debug("Validating PTE clear\n");
-	pte = __pte(pte_val(pte) | RANDOM_ORVALUE);
+//	pte = __pte(pte_val(pte) | RANDOM_ORVALUE);
 	set_pte_at(mm, vaddr, ptep, pte);
 	barrier();
 	pte_clear(mm, vaddr, ptep);