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Date:   Wed, 24 Jun 2020 11:52:36 +0800
From:   Baoquan He <bhe@redhat.com>
To:     Wei Yang <richard.weiyang@linux.alibaba.com>
Cc:     Dan Williams <dan.j.williams@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Oscar Salvador <osalvador@suse.de>,
        Linux MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        David Hildenbrand <david@redhat.com>
Subject: Re: [PATCH] mm/spase: never partially remove memmap for early section
Message-ID: <20200624035236.GI3346@MiWiFi-R3L-srv>
References: <20200623094258.6705-1-richard.weiyang@linux.alibaba.com>
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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
>