Michal Privoznik was using "free page reporting" in QEMU/virtio-balloon
with hugetlbfs and hit the warning below.  QEMU with free page hinting
uses fallocate(FALLOC_FL_PUNCH_HOLE) to discard pages that are reported
as free by a VM.  The reporting granularity is in pageblock granularity.
So when the guest reports 2M chunks, we fallocate(FALLOC_FL_PUNCH_HOLE)
one huge page in QEMU.

  WARNING: CPU: 7 PID: 6636 at mm/page_counter.c:57 page_counter_uncharge+0x4b/0x50
  Modules linked in: ...
  CPU: 7 PID: 6636 Comm: qemu-system-x86 Not tainted 5.9.0 #137
  Hardware name: Gigabyte Technology Co., Ltd. X570 AORUS PRO/X570 AORUS PRO, BIOS F21 07/31/2020
  RIP: 0010:page_counter_uncharge+0x4b/0x50
  ...
  Call Trace:
    hugetlb_cgroup_uncharge_file_region+0x4b/0x80
    region_del+0x1d3/0x300
    hugetlb_unreserve_pages+0x39/0xb0
    remove_inode_hugepages+0x1a8/0x3d0
    hugetlbfs_fallocate+0x3c4/0x5c0
    vfs_fallocate+0x146/0x290
    __x64_sys_fallocate+0x3e/0x70
    do_syscall_64+0x33/0x40
    entry_SYSCALL_64_after_hwframe+0x44/0xa9

Investigation of the issue uncovered bugs in hugetlb cgroup reservation
accounting.  This patch addresses the found issues.

Fixes: 075a61d0 ("hugetlb_cgroup: add accounting for shared mappings")
Reported-by: Michal Privoznik <mprivozn@redhat.com>
Co-developed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Tested-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Mina Almasry <almasrymina@google.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Cc: <stable@vger.kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Muchun Song <songmuchun@bytedance.com>
Cc: "Aneesh Kumar K . V" <aneesh.kumar@linux.vnet.ibm.com>
Cc: Tejun Heo <tj@kernel.org>
Link: https://lkml.kernel.org/r/20201021204426.36069-1-mike.kravetz@oracle.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>