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

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