by delegating the work to the slow path.
Some kernel memory, like pipe buffers, is pageable. We must not enable
interrupts, and consequently, preemption, while in critical section in
the fast copyout path, because we use pcpu buffers. If page fault
occurs while copying from the pcpu copyout_buf to kernel memory, abort
fast path and delegate work to the slow implementation.
In collaboration with: pho, tijl
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
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