mm: memcontrol: deprecate swapaccounting=0 mode

mm: memcontrol: deprecate swapaccounting=0 mode

The swapaccounting= commandline option already does very little today.  To
close a trivial containment failure case, the swap ownership tracking part
of the swap controller has recently become mandatory (see commit
2d1c498072de ("mm: memcontrol: make swap tracking an integral part of
memory control") for details), which makes up the majority of the work
during swapout, swapin, and the swap slot map.

The only thing left under this flag is the page_counter operations and the
visibility of the swap control files in the first place, which are rather
meager savings.  There also aren't many scenarios, if any, where
controlling the memory of a cgroup while allowing it unlimited access to a
global swap space is a workable resource isolation strategy.

On the other hand, there have been several bugs and confusion around the
many possible swap controller states (cgroup1 vs cgroup2 behavior, memory
accounting without swap accounting, memcg runtime disabled).

This puts the maintenance overhead of retaining the toggle above its
practical benefits.  Deprecate it.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Johannes Weiner <[email protected]>
Suggested-by: Shakeel Butt <[email protected]>
Reviewed-by: Shakeel Butt <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Roman Gushchin <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>