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Subject: Re: [PATCH] mm/spase: never partially remove memmap for early section
To:     Wei Yang <richard.weiyang@linux.alibaba.com>,
        Baoquan He <bhe@redhat.com>
Cc:     Dan Williams <dan.j.williams@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Oscar Salvador <osalvador@suse.de>,
        Linux MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
References: <20200623094258.6705-1-richard.weiyang@linux.alibaba.com>
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From:   David Hildenbrand <david@redhat.com>
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Organization: Red Hat GmbH
Message-ID: <53f7f04e-9c77-a987-8206-bd572268522b@redhat.com>
Date:   Wed, 24 Jun 2020 10:51:08 +0200
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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,

David / dhildenb