On 06/24/20 at 09:47am, Baoquan He wrote:
> On 06/23/20 at 05:21pm, Dan Williams wrote:
> > On Tue, Jun 23, 2020 at 2:43 AM Wei Yang
> > <richard.weiyang@xxxxxxxxxxxxxxxxx> wrote:
> > >
> > > For early sections, we assumes its memmap will never be partially
> > > removed. But current behavior breaks this.
> >
> > Where do we assume that?
> >
> > The primary use case for this was mapping pmem that collides with
> > System-RAM in the same 128MB section. That collision will certainly be
> > depopulated on-demand depending on the state of the pmem device. So,
> > I'm not understanding the problem or the benefit of this change.
>
> I was also confused when review this patch, the patch log is a little
> short and simple. From the current code, with SPARSE_VMEMMAP enabled, we
> do build memmap for the whole memory section during boot, even though
> some of them may be partially populated. We just mark the subsection map
> for present pages.
>
> Later, if pmem device is mapped into the partially boot memory section,
> we just fill the relevant subsection map, do return directly, w/o building
> the memmap for it, in section_activate(). Because the memmap for the
> unpresent RAM part have been there. I guess this is what Wei is trying to
> do to keep the behaviour be consistent for pmem device adding, or
OK, from Wei's reply I realized this patch is a necessary fix. If we
depoluate the partial memmap for pmem removing, the later pmem re-adding
won't have a valid memmap.
> pmem device removing and later adding again.
>
> Please correct me if I am wrong.
>
> To me, fixing it looks good. But a clear doc or code comment is
> necessary so that people can understand the code with less time.
> Leaving it as is doesn't cause harm. I personally tend to choose
> the former.
>
> paging_init()
> ->sparse_init()
> ->sparse_init_nid()
> {
> ...
> for_each_present_section_nr(pnum_begin, pnum) {
> ...
> map = __populate_section_memmap(pfn, PAGES_PER_SECTION,
> nid, NULL);
> ...
> }
> }
> ...
> ->zone_sizes_init()
> ->free_area_init()
> {
> for_each_mem_pfn_range(i, MAX_NUMNODES, &start_pfn, &end_pfn, &nid) {
> subsection_map_init(start_pfn, end_pfn - start_pfn);
> }
> {
>
> __add_pages()
> ->sparse_add_section()
> ->section_activate()
> {
> ...
> fill_subsection_map();
> if (nr_pages < PAGES_PER_SECTION && early_section(ms)) <----------*********
> return pfn_to_page(pfn);
> ...
> }
> >
>
>
