Re: [PATCH v1 0/2] mm/kdump: exclude reserved pages in dumps

[Andrew Morton <akpm@linux-foundation.org>]

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.