Documentation/networking/page_pool.rst

   1 .. SPDX-License-Identifier: GPL-2.0
   2
   3 =============
   4 Page Pool API
   5 =============
   6
   7 The page_pool allocator is optimized for the XDP mode that uses one frame
   8 per-page, but it can fallback on the regular page allocator APIs.
   9
  10 Basic use involves replacing alloc_pages() calls with the
  11 page_pool_alloc_pages() call.  Drivers should use page_pool_dev_alloc_pages()
  12 replacing dev_alloc_pages().
  13
  14 API keeps track of inflight pages, in order to let API user know
  15 when it is safe to free a page_pool object.  Thus, API users
  16 must run page_pool_release_page() when a page is leaving the page_pool or
  17 call page_pool_put_page() where appropriate in order to maintain correct
  18 accounting.
  19
  20 API user must call page_pool_put_page() once on a page, as it
  21 will either recycle the page, or in case of refcnt > 1, it will
  22 release the DMA mapping and inflight state accounting.
  23
  24 Architecture overview
  25 =====================
  26
  27 .. code-block:: none
  28
  29     +------------------+
  30     |       Driver     |
  31     +------------------+
  32             ^
  33             |
  34             |
  35             |
  36             v
  37     +--------------------------------------------+
  38     |                request memory              |
  39     +--------------------------------------------+
  40         ^                                  ^
  41         |                                  |
  42         | Pool empty                       | Pool has entries
  43         |                                  |
  44         v                                  v
  45     +-----------------------+     +------------------------+
  46     | alloc (and map) pages |     |  get page from cache   |
  47     +-----------------------+     +------------------------+
  48                                     ^                    ^
  49                                     |                    |
  50                                     | cache available    | No entries, refill
  51                                     |                    | from ptr-ring
  52                                     |                    |
  53                                     v                    v
  54                           +-----------------+     +------------------+
  55                           |   Fast cache    |     |  ptr-ring cache  |
  56                           +-----------------+     +------------------+
  57
  58 API interface
  59 =============
  60 The number of pools created **must** match the number of hardware queues
  61 unless hardware restrictions make that impossible. This would otherwise beat the
  62 purpose of page pool, which is allocate pages fast from cache without locking.
  63 This lockless guarantee naturally comes from running under a NAPI softirq.
  64 The protection doesn't strictly have to be NAPI, any guarantee that allocating
  65 a page will cause no race conditions is enough.
  66
  67 * page_pool_create(): Create a pool.
  68     * flags:      PP_FLAG_DMA_MAP, PP_FLAG_DMA_SYNC_DEV
  69     * order:      2^order pages on allocation
  70     * pool_size:  size of the ptr_ring
  71     * nid:        preferred NUMA node for allocation
  72     * dev:        struct device. Used on DMA operations
  73     * dma_dir:    DMA direction
  74     * max_len:    max DMA sync memory size
  75     * offset:     DMA address offset
  76
  77 * page_pool_put_page(): The outcome of this depends on the page refcnt. If the
  78   driver bumps the refcnt > 1 this will unmap the page. If the page refcnt is 1
  79   the allocator owns the page and will try to recycle it in one of the pool
  80   caches. If PP_FLAG_DMA_SYNC_DEV is set, the page will be synced for_device
  81   using dma_sync_single_range_for_device().
  82
  83 * page_pool_put_full_page(): Similar to page_pool_put_page(), but will DMA sync
  84   for the entire memory area configured in area pool->max_len.
  85
  86 * page_pool_recycle_direct(): Similar to page_pool_put_full_page() but caller
  87   must guarantee safe context (e.g NAPI), since it will recycle the page
  88   directly into the pool fast cache.
  89
  90 * page_pool_release_page(): Unmap the page (if mapped) and account for it on
  91   inflight counters.
  92
  93 * page_pool_dev_alloc_pages(): Get a page from the page allocator or page_pool
  94   caches.
  95
  96 * page_pool_get_dma_addr(): Retrieve the stored DMA address.
  97
  98 * page_pool_get_dma_dir(): Retrieve the stored DMA direction.
  99
 100 * page_pool_put_page_bulk(): Tries to refill a number of pages into the
 101   ptr_ring cache holding ptr_ring producer lock. If the ptr_ring is full,
 102   page_pool_put_page_bulk() will release leftover pages to the page allocator.
 103   page_pool_put_page_bulk() is suitable to be run inside the driver NAPI tx
 104   completion loop for the XDP_REDIRECT use case.
 105   Please note the caller must not use data area after running
 106   page_pool_put_page_bulk(), as this function overwrites it.
 107
 108 Coding examples
 109 ===============
 110
 111 Registration
 112 ------------
 113
 114 .. code-block:: c
 115
 116     /* Page pool registration */
 117     struct page_pool_params pp_params = { 0 };
 118     struct xdp_rxq_info xdp_rxq;
 119     int err;
 120
 121     pp_params.order = 0;
 122     /* internal DMA mapping in page_pool */
 123     pp_params.flags = PP_FLAG_DMA_MAP;
 124     pp_params.pool_size = DESC_NUM;
 125     pp_params.nid = NUMA_NO_NODE;
 126     pp_params.dev = priv->dev;
 127     pp_params.dma_dir = xdp_prog ? DMA_BIDIRECTIONAL : DMA_FROM_DEVICE;
 128     page_pool = page_pool_create(&pp_params);
 129
 130     err = xdp_rxq_info_reg(&xdp_rxq, ndev, 0);
 131     if (err)
 132         goto err_out;
 133
 134     err = xdp_rxq_info_reg_mem_model(&xdp_rxq, MEM_TYPE_PAGE_POOL, page_pool);
 135     if (err)
 136         goto err_out;
 137
 138 NAPI poller
 139 -----------
 140
 141
 142 .. code-block:: c
 143
 144     /* NAPI Rx poller */
 145     enum dma_data_direction dma_dir;
 146
 147     dma_dir = page_pool_get_dma_dir(dring->page_pool);
 148     while (done < budget) {
 149         if (some error)
 150             page_pool_recycle_direct(page_pool, page);
 151         if (packet_is_xdp) {
 152             if XDP_DROP:
 153                 page_pool_recycle_direct(page_pool, page);
 154         } else (packet_is_skb) {
 155             page_pool_release_page(page_pool, page);
 156             new_page = page_pool_dev_alloc_pages(page_pool);
 157         }
 158     }
 159
 160 Driver unload
 161 -------------
 162
 163 .. code-block:: c
 164
 165     /* Driver unload */
 166     page_pool_put_full_page(page_pool, page, false);
 167     xdp_rxq_info_unreg(&xdp_rxq);