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    SubjectRe: [PATCH 2/3] mm/sparsemem: get physical address to page struct instead of virtual address to pfn
    On Thu, Feb 6, 2020 at 7:10 PM Baoquan He <bhe@redhat.com> wrote:
    >
    > Hi Dan,
    >
    > On 02/06/20 at 06:19pm, Dan Williams wrote:
    > > On Thu, Feb 6, 2020 at 3:17 PM Wei Yang <richardw.yang@linux.intel.com> wrote:
    > > > diff --git a/mm/sparse.c b/mm/sparse.c
    > > > index b5da121bdd6e..56816f653588 100644
    > > > --- a/mm/sparse.c
    > > > +++ b/mm/sparse.c
    > > > @@ -888,7 +888,7 @@ int __meminit sparse_add_section(int nid, unsigned long start_pfn,
    > > > /* Align memmap to section boundary in the subsection case */
    > > > if (IS_ENABLED(CONFIG_SPARSEMEM_VMEMMAP) &&
    > > > section_nr_to_pfn(section_nr) != start_pfn)
    > > > - memmap = pfn_to_kaddr(section_nr_to_pfn(section_nr));
    > > > + memmap = pfn_to_page(section_nr_to_pfn(section_nr));
    > >
    > > Yes, this looks obviously correct. This might be tripping up
    > > makedumpfile. Do you see any practical effects of this bug? The kernel
    > > mostly avoids ->section_mem_map in the vmemmap case and in the
    > > !vmemmap case section_nr_to_pfn(section_nr) should always equal
    > > start_pfn.
    >
    > The practical effects is that the memmap for the first unaligned section will be lost
    > when destroy namespace to hot remove it. Because we encode the ->section_mem_map
    > into mem_section, and get memmap from the related mem_section to free it in
    > section_deactivate(). In fact in vmemmap, we don't need to encode the ->section_mem_map
    > with memmap.

    Right, but can you actually trigger that in the SPARSEMEM_VMEMMAP=n case?

    > By the way, sub-section support is only valid in vmemmap case, right?

    Yes.

    > Seems yes from code, but I don't find any document to prove it.

    check_pfn_span() enforces this requirement.

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