linux-kernel - Re: [PATCH v2 0/7] mm/hotplug: Only use subsection map in VMEMMAP case

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Date:   Tue, 25 Feb 2020 11:03:52 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Baoquan He <bhe@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, richardw.yang@...ux.intel.com,
        david@...hat.com, osalvador@...e.de, dan.j.williams@...el.com,
        rppt@...ux.ibm.com, robin.murphy@....com
Subject: Re: [PATCH v2 0/7] mm/hotplug: Only use subsection map in VMEMMAP
 case

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).
-- 
Michal Hocko
SUSE Labs