linux-kernel - Re: [PATCH v2 1/3] mm/debug_vm_pgtable: remove pte entry from the page table

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Message-ID: <406f41ab-5ed9-7c2e-6bc8-afcae32164c5@arm.com>
Date:   Fri, 21 Jan 2022 09:07:25 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Pasha Tatashin <pasha.tatashin@...een.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, rientjes@...gle.com, pjt@...gle.com,
        weixugc@...gle.com, gthelen@...gle.com, mingo@...hat.com,
        will@...nel.org, rppt@...nel.org, dave.hansen@...ux.intel.com,
        hpa@...or.com, aneesh.kumar@...ux.ibm.com, jirislaby@...nel.org,
        songmuchun@...edance.com, qydwhotmail@...il.com, hughd@...gle.com,
        ziy@...dia.com
Subject: Re: [PATCH v2 1/3] mm/debug_vm_pgtable: remove pte entry from the
 page table



On 1/21/22 12:42 AM, Pasha Tatashin wrote:
> The pte entry that is used in pte_advanced_tests() is never removed from
> the page table at the end of the test.
> 
> The issue is detected by page_table_check, to repro compile kernel with
> the following configs:
> 
> CONFIG_DEBUG_VM_PGTABLE=y
> CONFIG_PAGE_TABLE_CHECK=y
> CONFIG_PAGE_TABLE_CHECK_ENFORCED=y

Assuming this is on latest mainline.

I could enable PAGE_TABLE_CHECK on arm64 after some hacks. It did not build
on the platform otherwise. But enabling DEBUG_VM_PGTABLE afterwards did not
create below mentioned problems. Is the problem x86 specific ?

> 
> During the boot the following BUG is printed:
> 
> [    7.483050][    T1] debug_vm_pgtable: [debug_vm_pgtable         ]:
> Validating architecture page tabs
> [    7.490930][    T1] ------------[ cut here ]------------
> [    7.494926][    T1] kernel BUG at mm/page_table_check.c:194!

Which BUG() is this ? mm/page_table_check.c:194 on latest mainline ..

void __page_table_check_pud_clear(struct mm_struct *mm, unsigned long addr,
                                  pud_t pud) <----

> [    7.499172][    T1] invalid opcode: 0000 [#1] PREEMPT SMP KASAN
> [    7.503610][    T1] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.16.0+
> [    7.508600][    T1] Hardware name: QEMU Standard PC (i440FX + PIIX,
> ...
> 
> The entry should be properly removed from the page table before the page
> is released to the free list.
> 
> Fixes: a5c3b9ffb0f4 ("mm/debug_vm_pgtable: add tests validating advanced arch page table helpers")
I am not sure whether this really fixes an existing problem.

> 
> Signed-off-by: Pasha Tatashin <pasha.tatashin@...een.com>
> Reviewed-by: Zi Yan <ziy@...dia.com>
> Tested-by: Zi Yan <ziy@...dia.com>
> ---
>  mm/debug_vm_pgtable.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/mm/debug_vm_pgtable.c b/mm/debug_vm_pgtable.c
> index a7ac97c76762..db2abd9e415b 100644
> --- a/mm/debug_vm_pgtable.c
> +++ b/mm/debug_vm_pgtable.c
> @@ -171,6 +171,8 @@ static void __init pte_advanced_tests(struct pgtable_debug_args *args)
>  	ptep_test_and_clear_young(args->vma, args->vaddr, args->ptep);
>  	pte = ptep_get(args->ptep);
>  	WARN_ON(pte_young(pte));
> +
> +	ptep_get_and_clear_full(args->mm, args->vaddr, args->ptep, 1);
>  }

Although I dont see any problem on arm64 after this change.

>  
>  static void __init pte_savedwrite_tests(struct pgtable_debug_args *args)
>