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    SubjectRe: [PATCH] mm/spase: never partially remove memmap for early section
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    On 23.06.20 11:42, 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>
    > ---
    > mm/sparse.c | 6 +++---
    > 1 file changed, 3 insertions(+), 3 deletions(-)
    >
    > diff --git a/mm/sparse.c b/mm/sparse.c
    > index b2b9a3e34696..1a0069f492f5 100644
    > --- a/mm/sparse.c
    > +++ b/mm/sparse.c
    > @@ -825,10 +825,10 @@ static void section_deactivate(unsigned long pfn, unsigned long nr_pages,
    > ms->section_mem_map &= ~SECTION_HAS_MEM_MAP;
    > }
    >
    > - if (section_is_early && memmap)
    > - free_map_bootmem(memmap);
    > - else
    > + if (!section_is_early)
    > depopulate_section_memmap(pfn, nr_pages, altmap);
    > + else if (memmap)
    > + free_map_bootmem(memmap);
    >
    > if (empty)
    > ms->section_mem_map = (unsigned long)NULL;
    >

    Agreed, that's what pfn_valid() and section_activate() expect.

    "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." - this
    is the case when hot-adding sub-sections into partially populated early
    sections, and has to be the case when re-hot-adding after hot-removing.

    Acked-by: David Hildenbrand <david@redhat.com>


    I am also not convinced that the complicated sparse_decode_mem_map()
    handling in that function is required - ms->section_mem_map &
    SECTION_MAP_MASK is sufficient for this use case of removing the memmap
    of a full early section once empty.

    --
    Thanks,

    David / dhildenb

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