powernv/memtrace: don't abuse memory hot(un)plug infrastructure for memory allocations

Let's use alloc_contig_pages() for allocating memory and remove the
linear mapping manually via arch_remove_linear_mapping(). Mark all pages
PG_offline, such that they will definitely not get touched - e.g.,
when hibernating. When freeing memory, try to revert what we did.

The original idea was discussed in:
 https://lkml.kernel.org/r/48340e96-7e6b-736f-9e23-d3111b915b6e@redhat.com

This is similar to CONFIG_DEBUG_PAGEALLOC handling on other
architectures, whereby only single pages are unmapped from the linear
mapping. Let's mimic what memory hot(un)plug would do with the linear
mapping.

We now need MEMORY_HOTPLUG and CONTIG_ALLOC as dependencies. Add a TODO
that we want to use __GFP_ZERO for clearing once alloc_contig_pages()
understands that.

Tested with in QEMU/TCG with 10 GiB of main memory:
  [root@localhost ~]# echo 0x40000000 > /sys/kernel/debug/powerpc/memtrace/enable
  [  105.903043][ T1080] memtrace: Allocated trace memory on node 0 at 0x0000000080000000
  [root@localhost ~]# echo 0x40000000 > /sys/kernel/debug/powerpc/memtrace/enable
  [  145.042493][ T1080] radix-mmu: Mapped 0x0000000080000000-0x00000000c0000000 with 64.0 KiB pages
  [  145.049019][ T1080] memtrace: Freed trace memory back on node 0
  [  145.333960][ T1080] memtrace: Allocated trace memory on node 0 at 0x0000000080000000
  [root@localhost ~]# echo 0x80000000 > /sys/kernel/debug/powerpc/memtrace/enable
  [  213.606916][ T1080] radix-mmu: Mapped 0x0000000080000000-0x00000000c0000000 with 64.0 KiB pages
  [  213.613855][ T1080] memtrace: Freed trace memory back on node 0
  [  214.185094][ T1080] memtrace: Allocated trace memory on node 0 at 0x0000000080000000
  [root@localhost ~]# echo 0x100000000 > /sys/kernel/debug/powerpc/memtrace/enable
  [  234.874872][ T1080] radix-mmu: Mapped 0x0000000080000000-0x0000000100000000 with 64.0 KiB pages
  [  234.886974][ T1080] memtrace: Freed trace memory back on node 0
  [  234.890153][ T1080] memtrace: Failed to allocate trace memory on node 0
  [root@localhost ~]# echo 0x40000000 > /sys/kernel/debug/powerpc/memtrace/enable
  [  259.490196][ T1080] memtrace: Allocated trace memory on node 0 at 0x0000000080000000

I also made sure allocated memory is properly zeroed.

Note 1: We currently won't be allocating from ZONE_MOVABLE - because our
	pages are not movable. However, as we don't run with any memory
	hot(un)plug mechanism around, we could make an exception to
	increase the chance of allocations succeeding.

Note 2: PG_reserved isn't sufficient. E.g., kernel_page_present() used
	along PG_reserved in hibernation code will always return "true"
	on powerpc, resulting in the pages getting touched. It's too
	generic - e.g., indicates boot allocations.

Note 3: For now, we keep using memory_block_size_bytes() as minimum
	granularity.

Suggested-by: Michal Hocko <mhocko@kernel.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201111145322.15793-9-david@redhat.com

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