On Fri, Jan 08, 2021 at 07:58:13AM -0800, Shakeel Butt wrote:

This patch adds swapcache stat for the cgroup v2. The swapcache
represents the memory that is accounted against both the memory and the
swap limit of the cgroup. The main motivation behind exposing the
swapcache stat is for enabling users to gracefully migrate from cgroup
v1's memsw counter to cgroup v2's memory and swap counters.

Cgroup v1's memsw limit allows users to limit the memory+swap usage of a
workload but without control on the exact proportion of memory and swap.
Cgroup v2 provides separate limits for memory and swap which enables
more control on the exact usage of memory and swap individually for the
workload.

With some little subtleties, the v1's memsw limit can be switched with
the sum of the v2's memory and swap limits. However the alternative for
memsw usage is not yet available in cgroup v2. Exposing per-cgroup
swapcache stat enables that alternative. Adding the memory usage and
swap usage and subtracting the swapcache will approximate the memsw
usage. This will help in the transparent migration of the workloads
depending on memsw usage and limit to v2' memory and swap counters.

The reasons these applications are still interested in this approximate
memsw usage are: (1) these applications are not really interested in two
separate memory and swap usage metrics. A single usage metric is more
simple to use and reason about for them.

(2) The memsw usage metric hides the underlying system's swap setup from
the applications. Applications with multiple instances running in a
datacenter with heterogeneous systems (some have swap and some don't)
will keep seeing a consistent view of their usage.

Signed-off-by: Shakeel Butt <redacted>
Acked-by: Michal Hocko <mhocko@suse.com>

Reviewed-by: Roman Gushchin <redacted>

Thanks!