On Thu, Nov 12, 2015 at 06:41:27PM -0500, Johannes Weiner wrote:

There won't be a tcp control soft limit, so integrating the memcg code
into the global skmem limiting scheme complicates things
unnecessarily. Replace this with simple and clear charge and uncharge
calls--hidden behind a jump label--to account skb memory.

Note that this is not purely aesthetic: as a result of shoehorning the
per-memcg code into the same memory accounting functions that handle
the global level, the old code would compare the per-memcg consumption
against the smaller of the per-memcg limit and the global limit. This
allowed the total consumption of multiple sockets to exceed the global
limit, as long as the individual sockets stayed within bounds. After
this change, the code will always compare the per-memcg consumption to
the per-memcg limit, and the global consumption to the global limit,
and thus close this loophole.

Without a soft limit, the per-memcg memory pressure state in sockets
is generally questionable. However, we did it until now, so we
continue to enter it when the hard limit is hit, and packets are
dropped, to let other sockets in the cgroup know that they shouldn't
grow their transmit windows, either. However, keep it simple in the
new callback model and leave memory pressure lazily when the next
packet is accepted (as opposed to doing it synchroneously when packets
are processed). When packets are dropped, network performance will
already be in the toilet, so that should be a reasonable trade-off.

As described above, consumption is now checked on the per-memcg level
and the global level separately. Likewise, memory pressure states are
maintained on both the per-memcg level and the global level, and a
socket is considered under pressure when either level asserts as much.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>

It leaves the legacy functionality intact, while making the code look
much better.

Reviewed-by: Vladimir Davydov <redacted>

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