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

From: Wei Yang <richard.weiyang@linux.alibaba.com>
To: Dan Williams <dan.j.williams@intel.com>
Cc: Wei Yang <richard.weiyang@linux.alibaba.com>,
	Michal Hocko <mhocko@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Oscar Salvador <osalvador@suse.de>, Linux MM <linux-mm@kvack.org>,
	Baoquan He <bhe@redhat.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	David Hildenbrand <david@redhat.com>
Subject: Re: [PATCH] mm/spase: never partially remove memmap for early section
Date: Thu, 25 Jun 2020 06:05:52 +0800	[thread overview]
Message-ID: <20200624220552.GA15016@L-31X9LVDL-1304.local> (raw)
In-Reply-To: <CAPcyv4gEUq0yyz00AbQQSyo_acj1wUCZPse5vEff_Bq7GmeksQ@mail.gmail.com>

On Wed, Jun 24, 2020 at 09:10:09AM -0700, Dan Williams wrote:
>On Tue, Jun 23, 2020 at 11:14 PM Wei Yang
><richard.weiyang@linux.alibaba.com> wrote:
>>
>> On Tue, Jun 23, 2020 at 05:18:28PM +0200, Michal Hocko wrote:
>> >On Tue 23-06-20 17:42:58, Wei Yang wrote:
>> >> For early sections, we assumes its memmap will never be partially
>> >> removed. But current behavior breaks this.
>> >>
>> >> Let's correct it.
>> >>
>> >> Fixes: ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
>> >> Signed-off-by: Wei Yang <richard.weiyang@linux.alibaba.com>
>> >
>> >Can a user trigger this or is this a theoretical bug?
>>
>> Let me rewrite the changelog a little. Look forward any comments.
>>
>>    For early sections, its memmap is handled specially even sub-section is
>>    enabled. The memmap could only be populated as a whole.
>>
>>    Quoted from the comment of section_activate():
>>
>>        * The early init code does not consider partially populated
>>        * initial sections, it simply assumes that memory will never be
>>        * referenced.  If we hot-add memory into such a section then we
>>        * do not need to populate the memmap and can simply reuse what
>>        * is already there.
>>
>>    While current section_deactivate() breaks this rule. When hot-remove a
>>    sub-section, section_deactivate() would depopulate its memmap. The
>>    consequence is if we hot-add this subsection again, its memmap never get
>>    proper populated.
>
>Ok, forgive the latency as re-fetched this logic into my mental cache.
>So what I was remembering was the initial state of the code that
>special cased early sections, and that still seems to be the case in
>pfn_valid(). IIRC early_sections / bootmem are blocked from being
>removed entirely. Partial / subsection removals are ok.

Would you mind giving more words? Partial subsection removal is ok, so no need
to fix this?

-- 
Wei Yang
Help you, Help me