Patch series "memcg: initiate deprecation of v1 features", v2.

Start the deprecation process of the memcg v1 features which we discussed
during LSFMMBPF 2024 [1].  For now add the warnings to collect the
information on how the current users are using these features.  Next we
will work on providing better alternatives in v2 (if needed) and fully
deprecate these features.

Link: https://lwn.net/Articles/974575 [1]


This patch (of 4):

Memcg v1 provides opt-in TCP memory accounting feature.  However it is
mostly unused due to its performance impact on the network traffic.  In
v2, the TCP memory is accounted in the regular memory usage and is
transparent to the users but they can observe the TCP memory usage through
memcg stats.

Let's initiate the deprecation process of memcg v1's tcp accounting
functionality and add warnings to gather if there are any users and if
there are, collect how they are using it and plan to provide them better
alternative in v2.

Link: https://lkml.kernel.org/r/20240814220021.3208384-1-shakeel.butt@linux.dev
Link: https://lkml.kernel.org/r/20240814220021.3208384-2-shakeel.butt@linux.devSigned-off-by: Shakeel Butt <shakeel.butt@linux.dev>
Reviewed-by: T.J. Mercier <tjmercier@google.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Muchun Song <muchun.song@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>