Re: [PATCH] mm/spase: never partially remove memmap for early section

[Wei Yang <richard.weiyang@linux.alibaba.com>]

On Wed, Jun 24, 2020 at 10:51:08AM +0200, David Hildenbrand wrote:
>On 24.06.20 05:56, Wei Yang wrote:
>> On Wed, Jun 24, 2020 at 11:52:36AM +0800, Baoquan He wrote:
>>> On 06/24/20 at 11:46am, Wei Yang wrote:
>>>> On Wed, Jun 24, 2020 at 09:47:37AM +0800, Baoquan He wrote:
>>>>> On 06/23/20 at 05:21pm, Dan Williams wrote:
>>>>>> On Tue, Jun 23, 2020 at 2:43 AM Wei Yang
>>>>>> <richard.weiyang@linux.alibaba.com> wrote:
>>>>>>>
>>>>>>> For early sections, we assumes its memmap will never be partially
>>>>>>> removed. But current behavior breaks this.
>>>>>>
>>>>>> Where do we assume that?
>>>>>>
>>>>>> The primary use case for this was mapping pmem that collides with
>>>>>> System-RAM in the same 128MB section. That collision will certainly be
>>>>>> depopulated on-demand depending on the state of the pmem device. So,
>>>>>> I'm not understanding the problem or the benefit of this change.
>>>>>
>>>>> I was also confused when review this patch, the patch log is a little
>>>>> short and simple. From the current code, with SPARSE_VMEMMAP enabled, we
>>>>> do build memmap for the whole memory section during boot, even though
>>>>> some of them may be partially populated. We just mark the subsection map
>>>>> for present pages. 
>>>>>
>>>>> Later, if pmem device is mapped into the partially boot memory section,
>>>>> we just fill the relevant subsection map, do return directly, w/o building
>>>>> the memmap for it, in section_activate(). Because the memmap for the
>>>>> unpresent RAM part have been there. I guess this is what Wei is trying to 
>>>>> do to keep the behaviour be consistent for pmem device adding, or
>>>>> pmem device removing and later adding again.
>>>>>
>>>>> Please correct me if I am wrong.
>>>>
>>>> You are right here.
>>>>
>>>>>
>>>>> To me, fixing it looks good. But a clear doc or code comment is
>>>>> necessary so that people can understand the code with less time.
>>>>> Leaving it as is doesn't cause harm. I personally tend to choose
>>>>> the former.
>>>>>
>>>>
>>>> The former is to add a clear doc?
>>>
>>> Sorry for the confusion. The former means the fix in your patch. Maybe a
>>> improved log and some code comment adding can make it more perfect.
>>>
>> 
>> Sure, I would try to add more log and comments, in case you have some good
>> suggestion, just let me know :)
>> 
>
>We have documented this is section_activate() and pfn_valid()
>sufficiently. Maybe add a pointer like
>
>/*
> * The memmap of early sections is always fully populated. See
> * section_activate() and pfn_valid() .
> */

Thanks, I have added this above the "if" check.

>
>-- 
>Thanks,
>
>David / dhildenb

-- 
Wei Yang
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