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

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Message-ID: <20180413134047.GR17484@dhcp22.suse.cz>
Date:   Fri, 13 Apr 2018 15:40:47 +0200
From:   Michal Hocko <mhocko@...nel.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>,
        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 13-04-18 15:16:26, 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.

Well, theoretically possible but this would require a lot of auditing
because the hotplug and per section assumption is quite a spread one.

> 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.

But you cannot use a page flag for that, I am afraid. Page flags are
extremely scarce resource. I haven't looked at the rest of the series
but _if_ we have a bit spare which I am not really sure about then you
should prove there are no other ways around this.
 
> Signed-off-by: David Hildenbrand <david@...hat.com>
-- 
Michal Hocko
SUSE Labs