It is expensive for a CPU to take an interrupt while holding the page
LRU lock, because other CPUs will pile up on the lock while the
interrupt runs.

Disabling interrupts while holding the lock reduces contention by an
additional 30% on 4-way.  This is when the only source of interrupts is
disk completion.  The improvement will be higher with more CPUs and it
will be higher if there is networking happening.

The maximum hold time of this lock is 17 microseconds on 500 MHx PIII,
which is well inside the kernel's maximum interrupt latency (which was
100 usecs when I last looked, a year ago).

This optimisation is not needed on uniprocessor, but the patch disables
IRQs while holding pagemap_lru_lock anyway, so it becomes an irq-safe
spinlock, and pages can be moved from the LRU in interrupt context.

pagemap_lru_lock has been renamed to _pagemap_lru_lock to pick up any
missed uses, and to reliably break any out-of-tree patches which may be
using the old semantics.