mm: thp: Account pte-mapped anonymous THP usage

Add accounting for pte-mapped anonymous transparent hugepages at various
locations. This visibility will aid in debugging and tuning performance
for the "small order" thp extension that will be added in a subsequent
commit, where hugepages can be allocated which are large (greater than
order-0) but smaller than PMD_ORDER. This new accounting follows a
similar pattern to the existing NR_ANON_THPS, which measures pmd-mapped
anonymous transparent hugepages.

We account pte-mapped anonymous thp mappings per-page, where the page is
mapped at least once via PTE and the page belongs to a large folio. So
when a page belonging to a large folio is PTE-mapped for the first time,
then we add 1 to NR_ANON_THPS_PTEMAPPED. And when a page belonging to a
large folio is PTE-unmapped for the last time, then we remove 1 from
NR_ANON_THPS_PTEMAPPED.

/proc/meminfo:
  Introduce new "AnonHugePteMap" field, which reports the amount of
  memory (in KiB) mapped from large folios globally (similar to
  AnonHugePages field).

/proc/vmstat:
  Introduce new "nr_anon_thp_pte" field, which reports the amount of
  memory (in pages) mapped from large folios globally (similar to
  nr_anon_transparent_hugepages field).

/sys/devices/system/node/nodeX/meminfo
  Introduce new "AnonHugePteMap" field, which reports the amount of
  memory (in KiB) mapped from large folios per-node (similar to
  AnonHugePages field).

show_mem (panic logger):
  Introduce new "anon_thp_pte" field, which reports the amount of memory
  (in KiB) mapped from large folios per-node (similar to anon_thp
  field).

memory.stat (cgroup v1 and v2):
  Introduce new "anon_thp_pte" field, which reports the amount of memory
  (in bytes) mapped from large folios in the memcg (similar to rss_huge
  (v1) / anon_thp (v2) fields).

/proc/<pid>/smaps & /proc/<pid>/smaps_rollup:
  Introduce new "AnonHugePteMap" field, which reports the amount of
  memory (in KiB) mapped from large folios within the vma/process
  (similar to AnonHugePages field).

NOTE on charge migration: The new NR_ANON_THPS_PTEMAPPED charge is NOT
moved between cgroups, even when the (v1)
memory.move_charge_at_immigrate feature is enabled. That feature is
marked deprecated and the current code does not attempt to move the
NR_ANON_MAPPED charge for large PTE-mapped folios anyway (see comment in
mem_cgroup_move_charge_pte_range()). If this code was enhanced to allow
moving the NR_ANON_MAPPED charge for large PTE-mapped folios, we would
also need to add support for moving the new NR_ANON_THPS_PTEMAPPED
charge. This would likely get quite fiddly. Given the deprecation of
memory.move_charge_at_immigrate, I assume it is not valuable to
implement.

NOTE on naming: Given the new small order anonymous thp feature will be
exposed to user space as an extension to thp, I've opted to call the new
counters after thp also (as aposed to "large"/"large folio"/etc.), so
"huge" no longer strictly means PMD - one could argue hugetlb already
breaks this rule anyway. I also did not want to risk breaking back
compat by renaming/redefining the existing counters (which would have
resulted in more consistent and clearer names). So the existing
NR_ANON_THPS counters remain and continue to only refer to PMD-mapped
THPs. And I've added new counters, which only refer to PTE-mapped THPs.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>