[PATCH 0/4 v2] mm/swap: Add locking for pagevec

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

The swap code synchronizes its access to the (four) pagevec struct
(which is allocated per-CPU) by disabling preemption. This works and the
one struct needs to be accessed from interrupt context is protected by
disabling interrupts. This was manually audited and there is no lockdep
coverage for this.
There is one case where the per-CPU of a remote CPU needs to be accessed
and this is solved by started a worker on the remote CPU and waiting for
it to finish.

In v1 [0] it was attempted to add per-CPU spinlocks for the access to
struct. This would add lockdep coverage and access from a remote CPU so
the worker wouldn't be required.
It was argued about the cost of the uncontended spin_lock() and that the
benefit of avoiding the per-CPU worker to be rare because it is hardly
used.
A static key has been suggested which enables the per-CPU locking under
certain circumstances like in the NOHZ_FULL case and is implemented as
part of this series.

Sebastian