to_vmem_altmap() needs to return valid results until
arch_remove_memory() completes. It also needs to be valid for any pfn
in a section regardless of whether that pfn maps to data. This escape
was a result of a bug in the unit test.
The signature of this bug is that free_pagetable() fails to retrieve a
vmem_altmap and goes off into the weeds:
BUG: unable to handle kernel NULL pointer dereference at (null)
IP: [<ffffffff811d2629>] get_pfnblock_flags_mask+0x49/0x60
[..]
Call Trace:
[<ffffffff811d3477>] free_hot_cold_page+0x97/0x1d0
[<ffffffff811d367a>] __free_pages+0x2a/0x40
[<ffffffff8191e669>] free_pagetable+0x8c/0xd4
[<ffffffff8191ef4e>] remove_pagetable+0x37a/0x808
[<ffffffff8191b210>] vmemmap_free+0x10/0x20
Fixes: 4b94ffdc4163 ("x86, mm: introduce vmem_altmap to augment vmemmap_populate()")
Cc: Andrew Morton <akpm@linux-foundation.org>
Reported-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
eb7d78c9e7