[PATCH net-next] net: page_pool: Add documentation for page_pool API

[PATCH net-next] net: page_pool: Add documentation for page_pool API

[Ilias Apalodimas]

Add documentation explaining the basic functionality and design
principles of the API

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
---
 Documentation/networking/page_pool.rst | 159 +++++++++++++++++++++++++
 1 file changed, 159 insertions(+)
 create mode 100644 Documentation/networking/page_pool.rst

diff --git a/Documentation/networking/page_pool.rst b/Documentation/networking/page_pool.rst
new file mode 100644
index 000000000000..098d339ef272
--- /dev/null
+++ b/Documentation/networking/page_pool.rst
@@ -0,0 +1,159 @@
+.. 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 involve 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:      order^n 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.
+
+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);
+        }
+    }
+
+Driver unload
+-------------
+
+.. code-block:: c
+
+    /* Driver unload */
+    page_pool_put_full_page(page_pool, page, false);
+    xdp_rxq_info_unreg(&xdp_rxq);
-- 
2.25.1

Re: [PATCH net-next] net: page_pool: Add documentation for page_pool API

[Randy Dunlap]

Hi again Ilias,

On 2/20/20 10:25 AM, Ilias Apalodimas wrote:
> Add documentation explaining the basic functionality and design
> principles of the API
> 
> Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
> ---
>  Documentation/networking/page_pool.rst | 159 +++++++++++++++++++++++++
>  1 file changed, 159 insertions(+)
>  create mode 100644 Documentation/networking/page_pool.rst
> 
> diff --git a/Documentation/networking/page_pool.rst b/Documentation/networking/page_pool.rst
> new file mode 100644
> index 000000000000..098d339ef272
> --- /dev/null
> +++ b/Documentation/networking/page_pool.rst
> @@ -0,0 +1,159 @@
> +.. 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 involve replacing alloc_pages() calls with the

             involves

> +page_pool_alloc_pages() call.  Drivers should use page_pool_dev_alloc_pages()
> +replacing dev_alloc_pages().
> +
...

> +
> +Architecture overview
> +=====================
> +
> +.. code-block:: none
> +
...

> +
> +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:      order^n pages on allocation

what is "n" above?
My quick reading of mm/page_alloc.c suggests that order is the power of 2
that should be used for the memory 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
> +
...

> +
> +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;

so 0^n?

> +    /* 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
> +-----------

thanks.
-- 
~Randy

Re: [PATCH net-next] net: page_pool: Add documentation for page_pool API

[Ilias Apalodimas]

Hi Randy, 
On Thu, Feb 20, 2020 at 04:14:00PM -0800, Randy Dunlap wrote:
> Hi again Ilias,
> 
> On 2/20/20 10:25 AM, Ilias Apalodimas wrote:
> > Add documentation explaining the basic functionality and design
> > principles of the API
> > 
> > Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
> > ---
> >  Documentation/networking/page_pool.rst | 159 +++++++++++++++++++++++++
> >  1 file changed, 159 insertions(+)
> >  create mode 100644 Documentation/networking/page_pool.rst
> > 
> > diff --git a/Documentation/networking/page_pool.rst b/Documentation/networking/page_pool.rst
> > new file mode 100644
> > index 000000000000..098d339ef272
> > --- /dev/null
> > +++ b/Documentation/networking/page_pool.rst
> > @@ -0,0 +1,159 @@
> > +.. 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 involve replacing alloc_pages() calls with the
> 
>              involves
> 

Ok

> > +page_pool_alloc_pages() call.  Drivers should use page_pool_dev_alloc_pages()
> > +replacing dev_alloc_pages().
> > +
> ...
> 
> > +
> > +Architecture overview
> > +=====================
> > +
> > +.. code-block:: none
> > +
> ...
> 
> > +
> > +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:      order^n pages on allocation
> 
> what is "n" above?
> My quick reading of mm/page_alloc.c suggests that order is the power of 2
> that should be used for the memory allocation... ???

Yes this must change to 2^order

> 
> > +    * 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
> > +
> ...
> 
> > +
> > +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;
> 
> so 0^n?

See above!

> 
> > +    /* 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
> > +-----------
> 
> thanks.

Thanks again for taking the time

> -- 
> ~Randy
> 

Cheers
/Ilias