The page_pool is a generic API for drivers that have a need for a pool of recycling pages used for streaming DMA.


The page_pool is primarily motivated by two things (1) performance and (2) changing the memory model for drivers.

Drivers have developed performance workarounds when the speed of the page allocator and the DMA APIs became too slow for their HW needs. The page pool solves them on a general level providing performance gains and benefits that local driver recycling hacks cannot realize.

A fundamental property is that pages are returned to the page_pool. This property allow a certain class of Optimization principle.

Memory model

Once drivers are converted to using page_pool API, then it will become easier to change the underlying memory model backing the driver with pages (without changing the driver).

One prime use-case is NIC zero-copy RX into userspace. As DaveM describes in his Google-plus post, the mapping and unmapping operations in the address space of the process has a cost that cancels out most of the gains of such zero-copy schemes.

This mapping cost can be solved the same way as the keeping DMA mapped trick. By keeping the pages VM-mapped to userspace. This is a layer that can be added later to the page_pool. It will likely be beneficial to also consider using huge-pages (as backing) to reduce the TLB-stress.


Advantages of a recycling page pool as bullet points:

  1. Faster than going through page-allocator. Given a specialized allocator require less checks, and can piggyback on driver’s resource protection (for alloc-side).
  2. DMA IOMMU mapping cost is removed by keeping pages mapped.
  3. Makes DMA pages writable by predictable DMA unmap point. (UPDATE kernel v4.10: This can also be acheived via Alexander Duyck’s changes to the DMA API, namely using DMA_ATTR_SKIP_CPU_SYNC, which skips DMA sync as a part the unmap, but requires driver to carefully DMA sync needed memory)
  4. OOM protection at device level, as having a feedback-loop knows number of outstanding pages.
  5. Flexible memory model allowing zero-copy RX, solving memory early demux (does depend on HW filters into RX queues)
  6. Less fragmentation of the page buddy algorithm, when driver maintains a steady-state working-set.