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Date:   Tue, 17 May 2022 16:10:20 +0800
From:   Muchun Song <songmuchun@bytedance.com>
To:     Oscar Salvador <osalvador@suse.de>
Cc:     corbet@lwn.net, mike.kravetz@oracle.com, akpm@linux-foundation.org,
        mcgrof@kernel.org, keescook@chromium.org, yzaikin@google.com,
        david@redhat.com, masahiroy@kernel.org, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        duanxiongchun@bytedance.com, smuchun@gmail.com
Subject: Re: [PATCH v12 4/7] mm: hotplug: introduce
 SECTION_CANNOT_OPTIMIZE_VMEMMAP
Message-ID: <YoNYbLWvUDzJomWu@FVFYT0MHHV2J.usts.net>
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On Tue, May 17, 2022 at 09:52:36AM +0200, Oscar Salvador wrote:
> On Mon, May 16, 2022 at 08:03:49PM +0800, Muchun Song wrote:
> > On Mon, May 16, 2022 at 12:38:46PM +0200, Oscar Salvador wrote:
> > > On Mon, May 16, 2022 at 06:22:08PM +0800, Muchun Song wrote:
> > > > --- a/mm/sparse.c
> > > > +++ b/mm/sparse.c
> > > > @@ -913,6 +913,13 @@ int __meminit sparse_add_section(int nid, unsigned long start_pfn,
> > > >  	ms = __nr_to_section(section_nr);
> > > >  	set_section_nid(section_nr, nid);
> > > >  	__section_mark_present(ms, section_nr);
> > > > +	/*
> > > > +	 * Mark whole section as non-optimizable once there is a subsection
> > > > +	 * whose vmemmap pages are allocated from alternative allocator. The
> > > > +	 * early section is always optimizable.
> > > > +	 */
> > > > +	if (!early_section(ms) && altmap)
> > > > +		section_mark_cannot_optimize_vmemmap(ms);
> > > 
> > > Because no one expects those sections to be removed?
> > > IIRC, early_section + altmap only happened in case of sub-section pmem
> > > scenario?
> > 
> > Right. The commit ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
> > has more information.
> > 
> > > I guess my question is: can we really have early_sections coming
> > > from alternative allocator?
> > >
> > 
> > We can't. The early section does not consider partially being
> > populated currently.
> 
> Then, IIUC, we can forget about the early_section() check?
>

Sorry for the confusing. I mean early_section() should be checked.
I could find a comment in section_activate, that says:

	/*
	 * The early init code does not consider partially populated
	 * initial sections, it simply assumes that memory will never be
	 * referenced.  If we hot-add memory into such a section then we
	 * do not need to populate the memmap and can simply reuse what
	 * is already there.
	 */
	if (nr_pages < PAGES_PER_SECTION && early_section(ms))
		return pfn_to_page(pfn);
 
We can see that we could hot-add a sub-section within a early section.
So I think early_section + altmap could happened in this case, then
we could not drop that check. Right?

Thanks.