Re: [PATCH v12 4/7] mm: hotplug: introduce SECTION_CANNOT_OPTIMIZE_VMEMMAP

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
Date: Mon, 16 May 2022 20:03:49 +0800	[thread overview]
Message-ID: <YoI9pdLzB/V9Kw1y@FVFYT0MHHV2J.usts.net> (raw)
In-Reply-To: <YoIptjs65NgT1/ed@localhost.localdomain>

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.

> I think this should be spelled out more.

I think you mean add some comments here to describe the case
of early_section + altmap, right?

Thanks.