On 02/25/20 at 11:03am, Michal Hocko wrote: > On Fri 21-02-20 22:28:47, Baoquan He wrote: > > On 02/20/20 at 11:38am, Michal Hocko wrote: > > > On Thu 20-02-20 12:33:09, Baoquan He wrote: > > > > Memory sub-section hotplug was added to fix the issue that nvdimm could > > > > be mapped at non-section aligned starting address. A subsection map is > > > > added into struct mem_section_usage to implement it. However, sub-section > > > > is only supported in VMEMMAP case. > > > > > > Why? Is there any fundamental reason or just a lack of implementation? > > > VMEMMAP should be really only an implementation detail unless I am > > > missing something subtle. > > > > Thanks for checking. > > > > VMEMMAP is one of two ways to convert a PFN to the corresponding > > 'struct page' in SPARSE model. I mentioned them as VMEMMAP case, or > > !VMEMMAP case because we called them like this previously when reviewed > > patches, hope it won't cause confusion. > > > > Currently, config ZONE_DEVICE depends on SPARSEMEM_VMEMMAP. The > > subsection_map is added to struct mem_section_usage to track which sub > > section is present, VMEMMAP fills those bits which corresponding > > sub-sections are present, while !VMEMMAP, namely classic SPARSE, fills > > the whole map always. > > > > As we know, VMEMMAP builds page table to map a cluster of 'struct page' > > into the corresponding area of 'vmemmap'. Subsection hotplug can be > > supported naturally, w/o any change, just map needed region related to > > sub-sections on demand. For !VMEMMAP, it allocates memmap with > > alloc_pages() or vmalloc, thing is a little complicated, e.g the mixed > > section, boot memory occupies the starting area, later pmem hot added to > > the rear part. > > > > About !VMEMMAP which doesn't support sub-section hotplog, Dan said > > it's more because the effort and maintenance burden outweighs the > > benefit. And the current 64 bit ARCHes all enable > > SPARSEMEM_VMEMMAP_ENABLE by default. > > OK, if this is the primary argument then make sure to document it in the > changelog (cover letter). Will add it when repost.
