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Date: Thu, 26 Jul 2018 12:50:13 -0700
From: Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH v1 0/2] mm/kdump: exclude reserved pages in dumps
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To: David Hildenbrand <david@redhat.com>
Cc: Michal Hocko <mhocko@kernel.org>, Vlastimil Babka <vbabka@suse.cz>, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Baoquan He <bhe@redhat.com>, Dave Young <dyoung@redhat.com>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, Hari Bathini <hbathini@linux.vnet.ibm.com>, Huang Ying <ying.huang@intel.com>, "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>, =?ISO-8859-1?Q?Marc-Andr=E9?= Lureau <marcandre.lureau@redhat.com>, Matthew Wilcox <mawilcox@microsoft.com>, Miles Chen <miles.chen@mediatek.com>, Pavel Tatashin <pasha.tatashin@oracle.com>, Petr Tesarik <ptesarik@suse.cz>

On Thu, 26 Jul 2018 10:45:54 +0200 David Hildenbrand <david@redhat.com> wrote:

> > Does each user of PG_balloon check for PG_reserved? If this is the case
> > then yes this would be OK.
> > 
> 
> I can only spot one user of PageBalloon() at all (fs/proc/page.c) ,
> which makes me wonder if this bit is actually still relevant. I think
> the last "real" user was removed with
> 
> commit b1123ea6d3b3da25af5c8a9d843bd07ab63213f4
> Author: Minchan Kim <minchan@kernel.org>
> Date:   Tue Jul 26 15:23:09 2016 -0700
> 
>     mm: balloon: use general non-lru movable page feature
> 
>     Now, VM has a feature to migrate non-lru movable pages so balloon
>     doesn't need custom migration hooks in migrate.c and compaction.c.
> 
> 
> The only user of PG_balloon in general is
> "include/linux/balloon_compaction.h", used effectively only by
> virtio_balloon.
> 
> All such pages are allocated via balloon_page_alloc() and never set
> reserved.
> 
> So to me it looks like PG_balloon could be easily reused, especially to
> also exclude virtio-balloon pages from dumps.

Agree.  Maintaining a thingy for page-types.c which hardly anyone uses
(surely) isn't sufficient justification for consuming a page flag.  We
should check with the virtio developers first, but this does seem to be
begging to be reclaimed.