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    SubjectRe: [PATCH 2/3] mm/sparsemem: get physical address to page struct instead of virtual address to pfn
    On Fri, Feb 7, 2020 at 4:15 AM Wei Yang <richard.weiyang@gmail.com> wrote:
    >
    > On Thu, Feb 06, 2020 at 07:21:49PM -0800, Dan Williams wrote:
    > >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.
    >
    > Just one question from curiosity. Why we don't want sub-section for !vmemmap
    > case? Because it will wast memory for memmap?

    The effort and maintenance burden outweighs the benefit.

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