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

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

On Wed, Jun 24, 2020 at 03:20:59PM -0700, Dan Williams wrote:
>On Wed, Jun 24, 2020 at 3:06 PM Wei Yang
><richard.weiyang@linux.alibaba.com> wrote:
>>
>> 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?
>
>Early sections establish a memmap for the full section. There's
>conceptually nothing wrong with unplugging the non-system-RAM portion
>of the memmap, but it would need to be careful, at least on x86, to
>map the partial section with PTEs instead of PMDs.
>
>So, you are right that there is a mismatch here, but I think the
>comprehensive fix is to allow early sections to be partially
>depopulated/repopulated rather than have section_activate() and
>section_deacticate() special case early sections. The special casing
>is problematic in retrospect as section_deactivate() can't be
>maintained without understand special rules in section_activate().

Hmm... This means we need to adjust pfn_valid() too, which always return true
for early sections.

-- 
Wei Yang
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