1.. SPDX-License-Identifier: GPL-2.0
2
3=============
4Page Pool API
5=============
6
7The page_pool allocator is optimized for the XDP mode that uses one frame
8per-page, but it can fallback on the regular page allocator APIs.
9
10Basic use involves replacing alloc_pages() calls with the
11page_pool_alloc_pages() call.  Drivers should use page_pool_dev_alloc_pages()
12replacing dev_alloc_pages().
13
14API keeps track of in-flight pages, in order to let API user know
15when it is safe to free a page_pool object.  Thus, API users
16must call page_pool_put_page() to free the page, or attach
17the page to a page_pool-aware objects like skbs marked with
18skb_mark_for_recycle().
19
20API user must call page_pool_put_page() once on a page, as it
21will either recycle the page, or in case of refcnt > 1, it will
22release the DMA mapping and in-flight state accounting.
23
24Architecture 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
58API interface
59=============
60The number of pools created **must** match the number of hardware queues
61unless hardware restrictions make that impossible. This would otherwise beat the
62purpose of page pool, which is allocate pages fast from cache without locking.
63This lockless guarantee naturally comes from running under a NAPI softirq.
64The protection doesn't strictly have to be NAPI, any guarantee that allocating
65a 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_dev_alloc_pages(): Get a page from the page allocator or page_pool
91  caches.
92
93* page_pool_get_dma_addr(): Retrieve the stored DMA address.
94
95* page_pool_get_dma_dir(): Retrieve the stored DMA direction.
96
97* page_pool_put_page_bulk(): Tries to refill a number of pages into the
98  ptr_ring cache holding ptr_ring producer lock. If the ptr_ring is full,
99  page_pool_put_page_bulk() will release leftover pages to the page allocator.
100  page_pool_put_page_bulk() is suitable to be run inside the driver NAPI tx
101  completion loop for the XDP_REDIRECT use case.
102  Please note the caller must not use data area after running
103  page_pool_put_page_bulk(), as this function overwrites it.
104
105* page_pool_get_stats(): Retrieve statistics about the page_pool. This API
106  is only available if the kernel has been configured with
107  ``CONFIG_PAGE_POOL_STATS=y``. A pointer to a caller allocated ``struct
108  page_pool_stats`` structure is passed to this API which is filled in. The
109  caller can then report those stats to the user (perhaps via ethtool,
110  debugfs, etc.). See below for an example usage of this API.
111
112Stats API and structures
113------------------------
114If the kernel is configured with ``CONFIG_PAGE_POOL_STATS=y``, the API
115``page_pool_get_stats()`` and structures described below are available. It
116takes a  pointer to a ``struct page_pool`` and a pointer to a ``struct
117page_pool_stats`` allocated by the caller.
118
119The API will fill in the provided ``struct page_pool_stats`` with
120statistics about the page_pool.
121
122The stats structure has the following fields::
123
124    struct page_pool_stats {
125        struct page_pool_alloc_stats alloc_stats;
126        struct page_pool_recycle_stats recycle_stats;
127    };
128
129
130The ``struct page_pool_alloc_stats`` has the following fields:
131  * ``fast``: successful fast path allocations
132  * ``slow``: slow path order-0 allocations
133  * ``slow_high_order``: slow path high order allocations
134  * ``empty``: ptr ring is empty, so a slow path allocation was forced.
135  * ``refill``: an allocation which triggered a refill of the cache
136  * ``waive``: pages obtained from the ptr ring that cannot be added to
137    the cache due to a NUMA mismatch.
138
139The ``struct page_pool_recycle_stats`` has the following fields:
140  * ``cached``: recycling placed page in the page pool cache
141  * ``cache_full``: page pool cache was full
142  * ``ring``: page placed into the ptr ring
143  * ``ring_full``: page released from page pool because the ptr ring was full
144  * ``released_refcnt``: page released (and not recycled) because refcnt > 1
145
146Coding examples
147===============
148
149Registration
150------------
151
152.. code-block:: c
153
154    /* Page pool registration */
155    struct page_pool_params pp_params = { 0 };
156    struct xdp_rxq_info xdp_rxq;
157    int err;
158
159    pp_params.order = 0;
160    /* internal DMA mapping in page_pool */
161    pp_params.flags = PP_FLAG_DMA_MAP;
162    pp_params.pool_size = DESC_NUM;
163    pp_params.nid = NUMA_NO_NODE;
164    pp_params.dev = priv->dev;
165    pp_params.napi = napi; /* only if locking is tied to NAPI */
166    pp_params.dma_dir = xdp_prog ? DMA_BIDIRECTIONAL : DMA_FROM_DEVICE;
167    page_pool = page_pool_create(&pp_params);
168
169    err = xdp_rxq_info_reg(&xdp_rxq, ndev, 0);
170    if (err)
171        goto err_out;
172
173    err = xdp_rxq_info_reg_mem_model(&xdp_rxq, MEM_TYPE_PAGE_POOL, page_pool);
174    if (err)
175        goto err_out;
176
177NAPI poller
178-----------
179
180
181.. code-block:: c
182
183    /* NAPI Rx poller */
184    enum dma_data_direction dma_dir;
185
186    dma_dir = page_pool_get_dma_dir(dring->page_pool);
187    while (done < budget) {
188        if (some error)
189            page_pool_recycle_direct(page_pool, page);
190        if (packet_is_xdp) {
191            if XDP_DROP:
192                page_pool_recycle_direct(page_pool, page);
193        } else (packet_is_skb) {
194            skb_mark_for_recycle(skb);
195            new_page = page_pool_dev_alloc_pages(page_pool);
196        }
197    }
198
199Stats
200-----
201
202.. code-block:: c
203
204	#ifdef CONFIG_PAGE_POOL_STATS
205	/* retrieve stats */
206	struct page_pool_stats stats = { 0 };
207	if (page_pool_get_stats(page_pool, &stats)) {
208		/* perhaps the driver reports statistics with ethool */
209		ethtool_print_allocation_stats(&stats.alloc_stats);
210		ethtool_print_recycle_stats(&stats.recycle_stats);
211	}
212	#endif
213
214Driver unload
215-------------
216
217.. code-block:: c
218
219    /* Driver unload */
220    page_pool_put_full_page(page_pool, page, false);
221    xdp_rxq_info_unreg(&xdp_rxq);
222