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diff --git a/Documentation/networking/page_pool.rst b/Documentation/networking/page_pool.rst new file mode 100644 index 000000000..5db8c263b --- /dev/null +++ b/Documentation/networking/page_pool.rst @@ -0,0 +1,223 @@ +.. SPDX-License-Identifier: GPL-2.0 + +============= +Page Pool API +============= + +The page_pool allocator is optimized for the XDP mode that uses one frame +per-page, but it can fallback on the regular page allocator APIs. + +Basic use involves replacing alloc_pages() calls with the +page_pool_alloc_pages() call. Drivers should use page_pool_dev_alloc_pages() +replacing dev_alloc_pages(). + +API keeps track of inflight pages, in order to let API user know +when it is safe to free a page_pool object. Thus, API users +must run page_pool_release_page() when a page is leaving the page_pool or +call page_pool_put_page() where appropriate in order to maintain correct +accounting. + +API user must call page_pool_put_page() once on a page, as it +will either recycle the page, or in case of refcnt > 1, it will +release the DMA mapping and inflight state accounting. + +Architecture overview +===================== + +.. code-block:: none + + +------------------+ + | Driver | + +------------------+ + ^ + | + | + | + v + +--------------------------------------------+ + | request memory | + +--------------------------------------------+ + ^ ^ + | | + | Pool empty | Pool has entries + | | + v v + +-----------------------+ +------------------------+ + | alloc (and map) pages | | get page from cache | + +-----------------------+ +------------------------+ + ^ ^ + | | + | cache available | No entries, refill + | | from ptr-ring + | | + v v + +-----------------+ +------------------+ + | Fast cache | | ptr-ring cache | + +-----------------+ +------------------+ + +API interface +============= +The number of pools created **must** match the number of hardware queues +unless hardware restrictions make that impossible. This would otherwise beat the +purpose of page pool, which is allocate pages fast from cache without locking. +This lockless guarantee naturally comes from running under a NAPI softirq. +The protection doesn't strictly have to be NAPI, any guarantee that allocating +a page will cause no race conditions is enough. + +* page_pool_create(): Create a pool. + * flags: PP_FLAG_DMA_MAP, PP_FLAG_DMA_SYNC_DEV + * order: 2^order pages on allocation + * pool_size: size of the ptr_ring + * nid: preferred NUMA node for allocation + * dev: struct device. Used on DMA operations + * dma_dir: DMA direction + * max_len: max DMA sync memory size + * offset: DMA address offset + +* page_pool_put_page(): The outcome of this depends on the page refcnt. If the + driver bumps the refcnt > 1 this will unmap the page. If the page refcnt is 1 + the allocator owns the page and will try to recycle it in one of the pool + caches. If PP_FLAG_DMA_SYNC_DEV is set, the page will be synced for_device + using dma_sync_single_range_for_device(). + +* page_pool_put_full_page(): Similar to page_pool_put_page(), but will DMA sync + for the entire memory area configured in area pool->max_len. + +* page_pool_recycle_direct(): Similar to page_pool_put_full_page() but caller + must guarantee safe context (e.g NAPI), since it will recycle the page + directly into the pool fast cache. + +* page_pool_release_page(): Unmap the page (if mapped) and account for it on + inflight counters. + +* page_pool_dev_alloc_pages(): Get a page from the page allocator or page_pool + caches. + +* page_pool_get_dma_addr(): Retrieve the stored DMA address. + +* page_pool_get_dma_dir(): Retrieve the stored DMA direction. + +* page_pool_put_page_bulk(): Tries to refill a number of pages into the + ptr_ring cache holding ptr_ring producer lock. If the ptr_ring is full, + page_pool_put_page_bulk() will release leftover pages to the page allocator. + page_pool_put_page_bulk() is suitable to be run inside the driver NAPI tx + completion loop for the XDP_REDIRECT use case. + Please note the caller must not use data area after running + page_pool_put_page_bulk(), as this function overwrites it. + +* page_pool_get_stats(): Retrieve statistics about the page_pool. This API + is only available if the kernel has been configured with + ``CONFIG_PAGE_POOL_STATS=y``. A pointer to a caller allocated ``struct + page_pool_stats`` structure is passed to this API which is filled in. The + caller can then report those stats to the user (perhaps via ethtool, + debugfs, etc.). See below for an example usage of this API. + +Stats API and structures +------------------------ +If the kernel is configured with ``CONFIG_PAGE_POOL_STATS=y``, the API +``page_pool_get_stats()`` and structures described below are available. It +takes a pointer to a ``struct page_pool`` and a pointer to a ``struct +page_pool_stats`` allocated by the caller. + +The API will fill in the provided ``struct page_pool_stats`` with +statistics about the page_pool. + +The stats structure has the following fields:: + + struct page_pool_stats { + struct page_pool_alloc_stats alloc_stats; + struct page_pool_recycle_stats recycle_stats; + }; + + +The ``struct page_pool_alloc_stats`` has the following fields: + * ``fast``: successful fast path allocations + * ``slow``: slow path order-0 allocations + * ``slow_high_order``: slow path high order allocations + * ``empty``: ptr ring is empty, so a slow path allocation was forced. + * ``refill``: an allocation which triggered a refill of the cache + * ``waive``: pages obtained from the ptr ring that cannot be added to + the cache due to a NUMA mismatch. + +The ``struct page_pool_recycle_stats`` has the following fields: + * ``cached``: recycling placed page in the page pool cache + * ``cache_full``: page pool cache was full + * ``ring``: page placed into the ptr ring + * ``ring_full``: page released from page pool because the ptr ring was full + * ``released_refcnt``: page released (and not recycled) because refcnt > 1 + +Coding examples +=============== + +Registration +------------ + +.. code-block:: c + + /* Page pool registration */ + struct page_pool_params pp_params = { 0 }; + struct xdp_rxq_info xdp_rxq; + int err; + + pp_params.order = 0; + /* internal DMA mapping in page_pool */ + pp_params.flags = PP_FLAG_DMA_MAP; + pp_params.pool_size = DESC_NUM; + pp_params.nid = NUMA_NO_NODE; + pp_params.dev = priv->dev; + pp_params.dma_dir = xdp_prog ? DMA_BIDIRECTIONAL : DMA_FROM_DEVICE; + page_pool = page_pool_create(&pp_params); + + err = xdp_rxq_info_reg(&xdp_rxq, ndev, 0); + if (err) + goto err_out; + + err = xdp_rxq_info_reg_mem_model(&xdp_rxq, MEM_TYPE_PAGE_POOL, page_pool); + if (err) + goto err_out; + +NAPI poller +----------- + + +.. code-block:: c + + /* NAPI Rx poller */ + enum dma_data_direction dma_dir; + + dma_dir = page_pool_get_dma_dir(dring->page_pool); + while (done < budget) { + if (some error) + page_pool_recycle_direct(page_pool, page); + if (packet_is_xdp) { + if XDP_DROP: + page_pool_recycle_direct(page_pool, page); + } else (packet_is_skb) { + page_pool_release_page(page_pool, page); + new_page = page_pool_dev_alloc_pages(page_pool); + } + } + +Stats +----- + +.. code-block:: c + + #ifdef CONFIG_PAGE_POOL_STATS + /* retrieve stats */ + struct page_pool_stats stats = { 0 }; + if (page_pool_get_stats(page_pool, &stats)) { + /* perhaps the driver reports statistics with ethool */ + ethtool_print_allocation_stats(&stats.alloc_stats); + ethtool_print_recycle_stats(&stats.recycle_stats); + } + #endif + +Driver unload +------------- + +.. code-block:: c + + /* Driver unload */ + page_pool_put_full_page(page_pool, page, false); + xdp_rxq_info_unreg(&xdp_rxq); |