Re: [PATCH net-next v1 08/11] xdp: tracking page_pool resources and safe removal

[Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>]

On Thu, Jun 13, 2019 at 08:28:42PM +0200, Jesper Dangaard Brouer wrote:
Hi, Jesper

>This patch is needed before we can allow drivers to use page_pool for
>DMA-mappings. Today with page_pool and XDP return API, it is possible to
>remove the page_pool object (from rhashtable), while there are still
>in-flight packet-pages. This is safely handled via RCU and failed lookups in
>__xdp_return() fallback to call put_page(), when page_pool object is gone.
>In-case page is still DMA mapped, this will result in page note getting
>correctly DMA unmapped.
>
>To solve this, the page_pool is extended with tracking in-flight pages. And
>XDP disconnect system queries page_pool and waits, via workqueue, for all
>in-flight pages to be returned.
>
>To avoid killing performance when tracking in-flight pages, the implement
>use two (unsigned) counters, that in placed on different cache-lines, and
>can be used to deduct in-flight packets. This is done by mapping the
>unsigned "sequence" counters onto signed Two's complement arithmetic
>operations. This is e.g. used by kernel's time_after macros, described in
>kernel commit 1ba3aab3033b and 5a581b367b5, and also explained in RFC1982.
>
>The trick is these two incrementing counters only need to be read and
>compared, when checking if it's safe to free the page_pool structure. Which
>will only happen when driver have disconnected RX/alloc side. Thus, on a
>non-fast-path.
>
>It is chosen that page_pool tracking is also enabled for the non-DMA
>use-case, as this can be used for statistics later.
>
>After this patch, using page_pool requires more strict resource "release",
>e.g. via page_pool_release_page() that was introduced in this patchset, and
>previous patches implement/fix this more strict requirement.
>
>Drivers no-longer call page_pool_destroy(). Drivers already call
>xdp_rxq_info_unreg() which call xdp_rxq_info_unreg_mem_model(), which will
>attempt to disconnect the mem id, and if attempt fails schedule the
>disconnect for later via delayed workqueue.
>
>Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
>---
> drivers/net/ethernet/mellanox/mlx5/core/en_main.c |    3 -
> include/net/page_pool.h                           |   41 ++++++++++---
> net/core/page_pool.c                              |   62 +++++++++++++++-----
> net/core/xdp.c                                    |   65 +++++++++++++++++++--
> 4 files changed, 136 insertions(+), 35 deletions(-)
>
>diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en_main.c b/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
>index 2f647be292b6..6c9d4d7defbc 100644
>--- a/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
>+++ b/drivers/net/ethernet/mellanox/mlx5/core/en_main.c

[...]

>--- a/net/core/xdp.c
>+++ b/net/core/xdp.c
>@@ -38,6 +38,7 @@ struct xdp_mem_allocator {
> 	};
> 	struct rhash_head node;
> 	struct rcu_head rcu;
>+	struct delayed_work defer_wq;
> };
>
> static u32 xdp_mem_id_hashfn(const void *data, u32 len, u32 seed)
>@@ -79,13 +80,13 @@ static void __xdp_mem_allocator_rcu_free(struct rcu_head *rcu)
>
> 	xa = container_of(rcu, struct xdp_mem_allocator, rcu);
>
>+	/* Allocator have indicated safe to remove before this is called */
>+	if (xa->mem.type == MEM_TYPE_PAGE_POOL)
>+		page_pool_free(xa->page_pool);
>+

What would you recommend to do for the following situation:

Same receive queue is shared between 2 network devices. The receive ring is
filled by pages from page_pool, but you don't know the actual port (ndev)
filling this ring, because a device is recognized only after packet is received.

The API is so that xdp rxq is bind to network device, each frame has reference
on it, so rxq ndev must be static. That means each netdev has it's own rxq
instance even no need in it. Thus, after your changes, page must be returned to
the pool it was taken from, or released from old pool and recycled in new one
somehow.

And that is inconvenience at least. It's hard to move pages between pools w/o
performance penalty. No way to use common pool either, as unreg_rxq now drops
the pool and 2 rxqa can't reference same pool.

-- 
Regards,
Ivan Khoronzhuk