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SubjectRe: [PATCH] mm/spase: never partially remove memmap for early section
On 06/24/20 at 11:46am, Wei Yang wrote:
> On Wed, Jun 24, 2020 at 09:47:37AM +0800, 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@linux.alibaba.com> 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
> >pmem device removing and later adding again.
> >
> >Please correct me if I am wrong.
>
> You are right here.
>
> >
> >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.
> >
>
> The former is to add a clear doc?

Sorry for the confusion. The former means the fix in your patch. Maybe a
improved log and some code comment adding can make it more perfect.

>
> > 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);
> > ...
> > }
> >>
>
> --
> Wei Yang
> Help you, Help me
>

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