linux-kernel - Re: [PATCH v2 4/7] ARM: mm: print out correct page table entries

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Date:   Wed, 2 Jun 2021 19:24:47 +0800
From:   Kefeng Wang <wangkefeng.wang@...wei.com>
To:     "Russell King (Oracle)" <linux@...linux.org.uk>
CC:     <linux-arm-kernel@...ts.infradead.org>,
        Catalin Marinas <catalin.marinas@....com>,
        <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jungseung Lee <js07.lee@...il.com>
Subject: Re: [PATCH v2 4/7] ARM: mm: print out correct page table entries


On 2021/6/2 18:44, Russell King (Oracle) wrote:
> On Wed, Jun 02, 2021 at 03:02:43PM +0800, Kefeng Wang wrote:
>> Like commit 67ce16ec15ce ("arm64: mm: print out correct page table entries")
>> does, drop the struct mm_struct argument of show_pte(), print the tables
>> based on the faulting address.
>>
>> Signed-off-by: Kefeng Wang <wangkefeng.wang@...wei.com>
> This can be misleading on 32-bit ARM.
>
> The effective page tables for each thread are the threads *own* page
> tables. There is no hardware magic for addresses above PAGE_OFFSET being
> directed to the init_mm page tables.
>
> So, when we hit a fault in kernel space, we need to be printing the
> currently in-use page tables associated with the running thread.
>
> Hence:
>
>>   /*
>> - * This is useful to dump out the page tables associated with
>> - * 'addr' in mm 'mm'.
>> + * Dump out the page tables associated with 'addr' in the currently active mm
>>    */
>> -void show_pte(const char *lvl, struct mm_struct *mm, unsigned long addr)
>> +void show_pte(const char *lvl, unsigned long addr)
>>   {
>>   	pgd_t *pgd;
>> -
>> -	if (!mm)
>> +	struct mm_struct *mm;
>> +
>> +	if (addr < TASK_SIZE) {
>> +		mm = current->active_mm;
>> +		if (mm == &init_mm) {
>> +			printk("%s[%08lx] user address but active_mm is swapper\n",
>> +				lvl, addr);
>> +			return;
>> +		}
>> +	} else {
>>   		mm = &init_mm;
>> +	}
> is incorrect here.
>
> It's completely fine for architectures where kernel accesses always go
> to the init_mm page tables, but for 32-bit ARM that is not the case.
OK, I will drop this one, thanks
>