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

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Date:   Thu, 20 Feb 2020 11:38:49 +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 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.

> Hence there's no need to operate
> subsection map in SPARSEMEM|!VMEMMAP case. In this patchset, change
> codes to make sub-section map and the relevant operation only available
> in VMEMMAP case.
> 
> And since sub-section hotplug added, the hot add/remove functionality
> have been broken in SPARSEMEM|!VMEMMAP case. Wei Yang and I, each of us
> make one patch to fix one of the failures. In this patchset, the patch
> 1/7 from me is used to fix the hot remove failure. Wei Yang's patch has
> been merged by Andrew.

Not sure I understand. Are there more issues to be fixed?
>  include/linux/mmzone.h |   2 +
>  mm/sparse.c            | 178 +++++++++++++++++++++++++++++------------
>  2 files changed, 127 insertions(+), 53 deletions(-)

Why do we need to add so much code to remove a functionality from one
memory model?
-- 
Michal Hocko
SUSE Labs