linux-kernel - Re: [PATCH RFC 2/8] mm: introduce PG

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Message-ID: <20180413171120.GA1245@bombadil.infradead.org>
Date:   Fri, 13 Apr 2018 10:11:20 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-mm@...ck.org, Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Huang Ying <ying.huang@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Pavel Tatashin <pasha.tatashin@...cle.com>,
        Miles Chen <miles.chen@...iatek.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Rik van Riel <riel@...hat.com>,
        James Hogan <jhogan@...nel.org>,
        "Levin, Alexander (Sasha Levin)" <alexander.levin@...izon.com>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC 2/8] mm: introduce PG_offline

On Fri, Apr 13, 2018 at 03:16:26PM +0200, David Hildenbrand wrote:
> online_pages()/offline_pages() theoretically allows us to work on
> sub-section sizes. This is especially relevant in the context of
> virtualization. It e.g. allows us to add/remove memory to Linux in a VM in
> 4MB chunks.
> 
> While the whole section is marked as online/offline, we have to know
> the state of each page. E.g. to not read memory that is not online
> during kexec() or to properly mark a section as offline as soon as all
> contained pages are offline.

Can you not use PG_reserved for this purpose?

> + * PG_offline indicates that a page is offline and the backing storage
> + * might already have been removed (virtualization). Don't touch!

 * PG_reserved is set for special pages, which can never be swapped out. Some
 * of them might not even exist...

They seem pretty congruent to me.