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Diffstat (limited to 'Documentation/networking/page_pool.rst')
| -rw-r--r-- | Documentation/networking/page_pool.rst | 149 |
1 files changed, 58 insertions, 91 deletions
diff --git a/Documentation/networking/page_pool.rst b/Documentation/networking/page_pool.rst index 873efd97f822..215ebc92752c 100644 --- a/Documentation/networking/page_pool.rst +++ b/Documentation/networking/page_pool.rst @@ -4,22 +4,8 @@ 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 in-flight 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 in-flight state accounting. +.. kernel-doc:: include/net/page_pool/helpers.h + :doc: page_pool allocator Architecture overview ===================== @@ -64,87 +50,68 @@ 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 - in-flight 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. +.. kernel-doc:: net/core/page_pool.c + :identifiers: page_pool_create + +.. kernel-doc:: include/net/page_pool/types.h + :identifiers: struct page_pool_params + +.. kernel-doc:: include/net/page_pool/helpers.h + :identifiers: page_pool_put_page page_pool_put_full_page + page_pool_recycle_direct page_pool_dev_alloc_pages + page_pool_get_dma_addr page_pool_get_dma_dir + +.. kernel-doc:: net/core/page_pool.c + :identifiers: page_pool_put_page_bulk page_pool_get_stats + +DMA sync +-------- +Driver is always responsible for syncing the pages for the CPU. +Drivers may choose to take care of syncing for the device as well +or set the ``PP_FLAG_DMA_SYNC_DEV`` flag to request that pages +allocated from the page pool are already synced for the device. + +If ``PP_FLAG_DMA_SYNC_DEV`` is set, the driver must inform the core what portion +of the buffer has to be synced. This allows the core to avoid syncing the entire +page when the drivers knows that the device only accessed a portion of the page. + +Most drivers will reserve headroom in front of the frame. This part +of the buffer is not touched by the device, so to avoid syncing +it drivers can set the ``offset`` field in struct page_pool_params +appropriately. + +For pages recycled on the XDP xmit and skb paths the page pool will +use the ``max_len`` member of struct page_pool_params to decide how +much of the page needs to be synced (starting at ``offset``). +When directly freeing pages in the driver (page_pool_put_page()) +the ``dma_sync_size`` argument specifies how much of the buffer needs +to be synced. + +If in doubt set ``offset`` to 0, ``max_len`` to ``PAGE_SIZE`` and +pass -1 as ``dma_sync_size``. That combination of arguments is always +correct. + +Note that the syncing parameters are for the entire page. +This is important to remember when using fragments (``PP_FLAG_PAGE_FRAG``), +where allocated buffers may be smaller than a full page. +Unless the driver author really understands page pool internals +it's recommended to always use ``offset = 0``, ``max_len = PAGE_SIZE`` +with fragmented page pools. 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. +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 +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 +.. kernel-doc:: include/net/page_pool/types.h + :identifiers: struct page_pool_recycle_stats + struct page_pool_alloc_stats + struct page_pool_stats Coding examples =============== @@ -194,7 +161,7 @@ NAPI poller if XDP_DROP: page_pool_recycle_direct(page_pool, page); } else (packet_is_skb) { - page_pool_release_page(page_pool, page); + skb_mark_for_recycle(skb); new_page = page_pool_dev_alloc_pages(page_pool); } } |