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Maintain a cache of physically contiguous runs of pages for use as output buffers when software encryption is configured and in-place encryption is not possible. This makes allocation and free cheaper since in the common case we avoid touching the vm_page structures for the buffer, and fewer calls into UMA are needed. gallatin@ reports a ~10% absolute decrease in CPU usage with sendfile/KTLS on a Xeon after this change. It is possible that we will not be able to allocate these buffers if physical memory is fragmented. To avoid frequently calling into the physical memory allocator in this scenario, rate-limit allocation attempts after a failure. In the failure case we fall back to the old behaviour of allocating a page at a time. N.B.: this scheme could be simplified, either by simply using malloc() and looking up the PAs of the pages backing the buffer, or by falling back to page by page allocation and creating a mapping in the cache zone. This requires some way to save a mapping of an M_EXTPG page array in the mbuf, though. m_data is not really appropriate. The second approach may be possible by saving the mapping in the plinks union of the first vm_page structure of the array, but this would force a vm_page access when freeing an mbuf. Reviewed by: gallatin, jhb Tested by: gallatin Sponsored by: Ampere Computing Submitted by: Klara, Inc. Differential Revision: https://reviews.freebsd.org/D28556
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