linux-kernel - Re: [PATCH v3 01/12] mm/debug_vm_pgtable: Introduce struct pgtable_debug

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Date:   Fri, 23 Jul 2021 10:43:20 +1000
From:   Gavin Shan <gshan@...hat.com>
To:     Anshuman Khandual <anshuman.khandual@....com>, linux-mm@...ck.org
Cc:     linux-kernel@...r.kernel.org, catalin.marinas@....com,
        will@...nel.org, akpm@...ux-foundation.org, chuhu@...hat.com,
        shan.gavin@...il.com
Subject: Re: [PATCH v3 01/12] mm/debug_vm_pgtable: Introduce struct
 pgtable_debug_args

Hi Anshuman,

On 7/22/21 5:08 PM, Anshuman Khandual wrote:
> On 7/22/21 11:53 AM, Gavin Shan wrote:
>> On 7/22/21 2:41 PM, Anshuman Khandual wrote:
>>> On 7/21/21 3:50 PM, Gavin Shan wrote:
>>>> On 7/21/21 3:44 PM, Anshuman Khandual wrote:
>>>>> On 7/19/21 6:36 PM, Gavin Shan wrote:
>>>>>> In debug_vm_pgtable(), there are many local variables introduced to
>>>>>> track the needed information and they are passed to the functions for
>>>>>> various test cases. It'd better to introduce a struct as place holder
>>>>>> for these information. With it, what the functions for various test
>>>>>> cases need is the struct, to simplify the code. It also makes code
>>>>>> easier to be maintained.
>>>>>>
>>>>>> Besides, set_xxx_at() could access the data on the corresponding pages
>>>>>> in the page table modifying tests. So the accessed pages in the tests
>>>>>> should have been allocated from buddy. Otherwise, we're accessing pages
>>>>>> that aren't owned by us. This causes issues like page flag corruption.
>>>>>>
>>>>>> This introduces "struct pgtable_debug_args". The struct is initialized
>>>>>> and destroyed, but the information in the struct isn't used yet. They
>>>>>> will be used in subsequent patches.
>>>>>>
>>>>>> Signed-off-by: Gavin Shan <gshan@...hat.com>
>>>>>> ---
>>>>>>     mm/debug_vm_pgtable.c | 197 +++++++++++++++++++++++++++++++++++++++++-
>>>>>>     1 file changed, 196 insertions(+), 1 deletion(-)
>>>>>>

[...]

>>>
>>> IIRC it is also not guaranteed that PMD_SHIFT <= (MAX_ORDER - 1). Hence
>>> this same scheme should be followed for PMD level allocation as well.
>>>
>>
>> In theory, it's possible to have PMD_SHIFT <= (MAX_ORDER - 1) with misconfigured
>> kernel. I will apply the similar logic to PMD huge page in v4.
>>
>>>>       [... The code to release the PUD huge page needs changes based on @args->is_contiguous_pud_page]
>>>
>>> Right, a flag would be needed to call the appropriate free function.
>>>
>>
>> Yes. We need two falgs for PUD and PMD huge pages separately.
> 
> A single flag should be enough, the order would be dependent on
> whether args->pud_pfn or args->pmd_pfn is valid.
> 

Yes, it's correct that one flag is enough as we're sharing the PUD
or PMD huge page.

Thanks,
Gavin