mm: balloon: update comment about isolation/migration/compaction

Patch series "mm/kdump: allow to exclude pages that are logically
offline"

Right now, pages inflated as part of a balloon driver will be dumped by
dump tools like makedumpfile.  While XEN is able to check in the crash
kernel whether a certain pfn is actuall backed by memory in the
hypervisor (see xen_oldmem_pfn_is_ram) and optimize this case, dumps of
virtio-balloon, hv-balloon and VMWare balloon inflated memory will
essentially result in zero pages getting allocated by the hypervisor and
the dump getting filled with this data.

The allocation and reading of zero pages can directly be avoided if a
dumping tool could know which pages only contain stale information not
to be dumped.

Also for XEN, calling into the kernel and asking the hypervisor if a pfn
is backed can be avoided if the duming tool would skip such pages right
from the beginning.

Dumping tools have no idea whether a given page is part of a balloon
driver and shall not be dumped.  Esp.  PG_reserved cannot be used for
that purpose as all memory allocated during early boot is also
PG_reserved, see discussion at [1].  So some other way of indication is
required and a new page flag is frowned upon.

We have PG_balloon (MAPCOUNT value), which is essentially unused now.  I
suggest renaming it to something more generic (PG_offline) to mark pages
as logically offline.  This flag can than e.g.  also be used by
virtio-mem in the future to mark subsections as offline.  Or by other
code that wants to put pages logically offline (e.g.  later maybe
poisoned pages that shall no longer be used).

This series converts PG_balloon to PG_offline, allows dumping tools to
query the value to detect such pages and marks pages in the hv-balloon
and XEN balloon properly as PG_offline.  Note that virtio-balloon
already set pages to PG_balloon (and now PG_offline).

Please note that this is also helpful for a problem we were seeing under
Hyper-V: Dumping logically offline memory (pages kept fake offline while
onlining a section via online_page_callback) would under some condicions
result in a kernel panic when dumping them.

As I don't have access to neither XEN nor Hyper-V nor VMWare
installations, this was only tested with the virtio-balloon and pages
were properly skipped when dumping.  I'll also attach the makedumpfile
patch to this series.

[1] https://lkml.org/lkml/2018/7/20/566

This patch (of 8):

Commit b1123ea6d3b3 ("mm: balloon: use general non-lru movable page
feature") reworked balloon handling to make use of the general non-lru
movable page feature.  The big comment block in balloon_compaction.h
contains quite some outdated information.  Let's fix this.

Link: http://lkml.kernel.org/r/20181119101616.8901-2-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Alexander Duyck <alexander.h.duyck@linux.intel.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Baoquan He <bhe@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Christian Hansen <chansen3@cisco.com>
Cc: Dave Young <dyoung@redhat.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Juergen Gross <jgross@suse.com>
Cc: Julien Freche <jfreche@vmware.com>
Cc: Kairui Song <kasong@redhat.com>
Cc: Kazuhito Hagio <k-hagio@ab.jp.nec.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Konstantin Khlebnikov <koct9i@gmail.com>
Cc: "K. Y. Srinivasan" <kys@microsoft.com>
Cc: Len Brown <len.brown@intel.com>
Cc: Lianbo Jiang <lijiang@redhat.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Miles Chen <miles.chen@mediatek.com>
Cc: Nadav Amit <namit@vmware.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Omar Sandoval <osandov@fb.com>
Cc: Pankaj gupta <pagupta@redhat.com>
Cc: Pavel Machek <pavel@ucw.cz>
Cc: Pavel Tatashin <pasha.tatashin@oracle.com>
Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>
Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: Stephen Hemminger <sthemmin@microsoft.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Xavier Deguillard <xdeguillard@vmware.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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