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Date:   Wed, 26 Feb 2020 11:44:33 +0800
From:   Baoquan He <bhe@redhat.com>
To:     Michal Hocko <mhocko@kernel.org>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        akpm@linux-foundation.org, richardw.yang@linux.intel.com,
        david@redhat.com, osalvador@suse.de, dan.j.williams@intel.com,
        rppt@linux.ibm.com, robin.murphy@arm.com
Subject: Re: [PATCH v2 0/7] mm/hotplug: Only use subsection map in VMEMMAP
 case
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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.