virtio-mem: try to merge system ram resources

virtio-mem adds memory in memory block granularity, to be able to remove
it in the same granularity again later, and to grow slowly on demand.
This, however, results in quite a lot of resources when adding a lot of
memory.  Resources are effectively stored in a list-based tree.  Having a
lot of resources not only wastes memory, it also makes traversing that
tree more expensive, and makes /proc/iomem explode in size (e.g.,
requiring kexec-tools to manually merge resources later when e.g., trying
to create a kdump header).

Before this patch, we get (/proc/iomem) when hotplugging 2G via virtio-mem
on x86-64:
        [...]
        100000000-13fffffff : System RAM
        140000000-33fffffff : virtio0
          140000000-147ffffff : System RAM (virtio_mem)
          148000000-14fffffff : System RAM (virtio_mem)
          150000000-157ffffff : System RAM (virtio_mem)
          158000000-15fffffff : System RAM (virtio_mem)
          160000000-167ffffff : System RAM (virtio_mem)
          168000000-16fffffff : System RAM (virtio_mem)
          170000000-177ffffff : System RAM (virtio_mem)
          178000000-17fffffff : System RAM (virtio_mem)
          180000000-187ffffff : System RAM (virtio_mem)
          188000000-18fffffff : System RAM (virtio_mem)
          190000000-197ffffff : System RAM (virtio_mem)
          198000000-19fffffff : System RAM (virtio_mem)
          1a0000000-1a7ffffff : System RAM (virtio_mem)
          1a8000000-1afffffff : System RAM (virtio_mem)
          1b0000000-1b7ffffff : System RAM (virtio_mem)
          1b8000000-1bfffffff : System RAM (virtio_mem)
        3280000000-32ffffffff : PCI Bus 0000:00

With this patch, we get (/proc/iomem):
        [...]
        fffc0000-ffffffff : Reserved
        100000000-13fffffff : System RAM
        140000000-33fffffff : virtio0
          140000000-1bfffffff : System RAM (virtio_mem)
        3280000000-32ffffffff : PCI Bus 0000:00

Of course, with more hotplugged memory, it gets worse.  When unplugging
memory blocks again, try_remove_memory() (via offline_and_remove_memory())
will properly split the resource up again.

Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Wei Yang <richardw.yang@linux.intel.com>
Cc: Anton Blanchard <anton@ozlabs.org>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Dave Jiang <dave.jiang@intel.com>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Juergen Gross <jgross@suse.com>
Cc: Julien Grall <julien@xen.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: "K. Y. Srinivasan" <kys@microsoft.com>
Cc: Len Brown <lenb@kernel.org>
Cc: Leonardo Bras <leobras.c@gmail.com>
Cc: Libor Pechacek <lpechacek@suse.cz>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Nathan Lynch <nathanl@linux.ibm.com>
Cc: "Oliver O'Halloran" <oohall@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Pingfan Liu <kernelfans@gmail.com>
Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>
Cc: Roger Pau Monné <roger.pau@citrix.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: Stephen Hemminger <sthemmin@microsoft.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Vishal Verma <vishal.l.verma@intel.com>
Cc: Wei Liu <wei.liu@kernel.org>
Link: https://lkml.kernel.org/r/20200911103459.10306-7-david@redhat.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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