linux-kernel - Re: [PATCH v12 4/7] mm: hotplug: introduce SECTION_CANNOT_OPTIMIZE

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Message-ID: <YoNURLcKtffT+5BH@localhost.localdomain>
Date:   Tue, 17 May 2022 09:52:36 +0200
From:   Oscar Salvador <osalvador@...e.de>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     corbet@....net, mike.kravetz@...cle.com, akpm@...ux-foundation.org,
        mcgrof@...nel.org, keescook@...omium.org, yzaikin@...gle.com,
        david@...hat.com, masahiroy@...nel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        duanxiongchun@...edance.com, smuchun@...il.com
Subject: Re: [PATCH v12 4/7] mm: hotplug: introduce
 SECTION_CANNOT_OPTIMIZE_VMEMMAP

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?


-- 
Oscar Salvador
SUSE Labs