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Message-ID: <aKTjACALTDMrzuxJ@mini-arch>
Date: Tue, 19 Aug 2025 13:48:00 -0700
From: Stanislav Fomichev <stfomichev@...il.com>
To: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
Cc: bpf@...r.kernel.org, ast@...nel.org, daniel@...earbox.net,
andrii@...nel.org, netdev@...r.kernel.org,
magnus.karlsson@...el.com, aleksander.lobakin@...el.com,
Eryk Kubanski <e.kubanski@...tner.samsung.com>
Subject: Re: [PATCH v5 bpf] xsk: fix immature cq descriptor production
On 08/19, Maciej Fijalkowski wrote:
> Eryk reported an issue that I have put under Closes: tag, related to
> umem addrs being prematurely produced onto pool's completion queue.
> Let us make the skb's destructor responsible for producing all addrs
> that given skb used.
>
> Introduce struct xsk_addrs which will carry descriptor count with array
> of addresses taken from processed descriptors that will be carried via
> skb_shared_info::destructor_arg. This way we can refer to it within
> xsk_destruct_skb(). In order to mitigate the overhead that will be
> coming from memory allocations, let us introduce kmem_cache of
> xsk_addrs. There will be a single kmem_cache for xsk generic xmit on the
> system.
>
> Commit from fixes tag introduced the buggy behavior, it was not broken
> from day 1, but rather when xsk multi-buffer got introduced.
>
> Fixes: b7f72a30e9ac ("xsk: introduce wrappers and helpers for supporting multi-buffer in Tx path")
> Reported-by: Eryk Kubanski <e.kubanski@...tner.samsung.com>
> Closes: https://lore.kernel.org/netdev/20250530103456.53564-1-e.kubanski@partner.samsung.com/
> Acked-by: Magnus Karlsson <magnus.karlsson@...el.com>
> Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
> ---
>
> v1:
> https://lore.kernel.org/bpf/20250702101648.1942562-1-maciej.fijalkowski@intel.com/
> v2:
> https://lore.kernel.org/bpf/20250705135512.1963216-1-maciej.fijalkowski@intel.com/
> v3:
> https://lore.kernel.org/bpf/20250806154127.2161434-1-maciej.fijalkowski@intel.com/
> v4:
> https://lore.kernel.org/bpf/20250813171210.2205259-1-maciej.fijalkowski@intel.com/
>
> v1->v2:
> * store addrs in array carried via destructor_arg instead having them
> stored in skb headroom; cleaner and less hacky approach;
> v2->v3:
> * use kmem_cache for xsk_addrs allocation (Stan/Olek)
> * set err when xsk_addrs allocation fails (Dan)
> * change xsk_addrs layout to avoid holes
> * free xsk_addrs on error path
> * rebase
> v3->v4:
> * have kmem_cache as percpu vars
> * don't drop unnecessary braces (unrelated) (Stan)
> * use idx + i in xskq_prod_write_addr (Stan)
> * alloc kmem_cache on bind (Stan)
> * keep num_descs as first member in xsk_addrs (Magnus)
> * add ack from Magnus
> v4->v5:
> * have a single kmem_cache per xsk subsystem (Stan)
>
> ---
> net/xdp/xsk.c | 91 +++++++++++++++++++++++++++++++++++++--------
> net/xdp/xsk_queue.h | 12 ++++++
> 2 files changed, 87 insertions(+), 16 deletions(-)
>
> diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
> index 9c3acecc14b1..012991de9df2 100644
> --- a/net/xdp/xsk.c
> +++ b/net/xdp/xsk.c
> @@ -36,6 +36,13 @@
> #define TX_BATCH_SIZE 32
> #define MAX_PER_SOCKET_BUDGET 32
>
> +struct xsk_addrs {
> + u32 num_descs;
> + u64 addrs[MAX_SKB_FRAGS + 1];
> +};
> +
> +static struct kmem_cache *xsk_tx_generic_cache;
> +
> void xsk_set_rx_need_wakeup(struct xsk_buff_pool *pool)
> {
> if (pool->cached_need_wakeup & XDP_WAKEUP_RX)
> @@ -532,25 +539,39 @@ static int xsk_wakeup(struct xdp_sock *xs, u8 flags)
> return dev->netdev_ops->ndo_xsk_wakeup(dev, xs->queue_id, flags);
> }
>
> -static int xsk_cq_reserve_addr_locked(struct xsk_buff_pool *pool, u64 addr)
> +static int xsk_cq_reserve_locked(struct xsk_buff_pool *pool)
> {
> unsigned long flags;
> int ret;
>
> spin_lock_irqsave(&pool->cq_lock, flags);
> - ret = xskq_prod_reserve_addr(pool->cq, addr);
> + ret = xskq_prod_reserve(pool->cq);
> spin_unlock_irqrestore(&pool->cq_lock, flags);
>
> return ret;
> }
>
> -static void xsk_cq_submit_locked(struct xsk_buff_pool *pool, u32 n)
> +static void xsk_cq_submit_addr_locked(struct xdp_sock *xs,
> + struct sk_buff *skb)
> {
> + struct xsk_buff_pool *pool = xs->pool;
> + struct xsk_addrs *xsk_addrs;
> unsigned long flags;
> + u32 num_desc, i;
> + u32 idx;
> +
> + xsk_addrs = (struct xsk_addrs *)skb_shinfo(skb)->destructor_arg;
> + num_desc = xsk_addrs->num_descs;
>
> spin_lock_irqsave(&pool->cq_lock, flags);
> - xskq_prod_submit_n(pool->cq, n);
> + idx = xskq_get_prod(pool->cq);
> +
> + for (i = 0; i < num_desc; i++)
> + xskq_prod_write_addr(pool->cq, idx + i, xsk_addrs->addrs[i]);
> + xskq_prod_submit_n(pool->cq, num_desc);
> +
> spin_unlock_irqrestore(&pool->cq_lock, flags);
> + kmem_cache_free(xsk_tx_generic_cache, xsk_addrs);
> }
>
> static void xsk_cq_cancel_locked(struct xsk_buff_pool *pool, u32 n)
> @@ -562,11 +583,6 @@ static void xsk_cq_cancel_locked(struct xsk_buff_pool *pool, u32 n)
> spin_unlock_irqrestore(&pool->cq_lock, flags);
> }
>
> -static u32 xsk_get_num_desc(struct sk_buff *skb)
> -{
> - return skb ? (long)skb_shinfo(skb)->destructor_arg : 0;
> -}
> -
> static void xsk_destruct_skb(struct sk_buff *skb)
> {
> struct xsk_tx_metadata_compl *compl = &skb_shinfo(skb)->xsk_meta;
> @@ -576,21 +592,37 @@ static void xsk_destruct_skb(struct sk_buff *skb)
> *compl->tx_timestamp = ktime_get_tai_fast_ns();
> }
>
> - xsk_cq_submit_locked(xdp_sk(skb->sk)->pool, xsk_get_num_desc(skb));
> + xsk_cq_submit_addr_locked(xdp_sk(skb->sk), skb);
> sock_wfree(skb);
> }
>
> -static void xsk_set_destructor_arg(struct sk_buff *skb)
> +static u32 xsk_get_num_desc(struct sk_buff *skb)
> {
> - long num = xsk_get_num_desc(xdp_sk(skb->sk)->skb) + 1;
> + struct xsk_addrs *addrs;
>
> - skb_shinfo(skb)->destructor_arg = (void *)num;
> + addrs = (struct xsk_addrs *)skb_shinfo(skb)->destructor_arg;
> + return addrs->num_descs;
> +}
> +
> +static void xsk_set_destructor_arg(struct sk_buff *skb, struct xsk_addrs *addrs)
> +{
> + skb_shinfo(skb)->destructor_arg = (void *)addrs;
> +}
> +
> +static void xsk_inc_skb_descs(struct sk_buff *skb)
> +{
> + struct xsk_addrs *addrs;
> +
> + addrs = (struct xsk_addrs *)skb_shinfo(skb)->destructor_arg;
> + addrs->num_descs++;
> }
>
> static void xsk_consume_skb(struct sk_buff *skb)
> {
> struct xdp_sock *xs = xdp_sk(skb->sk);
>
> + kmem_cache_free(xsk_tx_generic_cache,
> + (struct xsk_addrs *)skb_shinfo(skb)->destructor_arg);
> skb->destructor = sock_wfree;
> xsk_cq_cancel_locked(xs->pool, xsk_get_num_desc(skb));
> /* Free skb without triggering the perf drop trace */
> @@ -609,6 +641,7 @@ static struct sk_buff *xsk_build_skb_zerocopy(struct xdp_sock *xs,
> {
> struct xsk_buff_pool *pool = xs->pool;
> u32 hr, len, ts, offset, copy, copied;
> + struct xsk_addrs *addrs = NULL;
> struct sk_buff *skb = xs->skb;
> struct page *page;
> void *buffer;
> @@ -623,6 +656,12 @@ static struct sk_buff *xsk_build_skb_zerocopy(struct xdp_sock *xs,
> return ERR_PTR(err);
>
> skb_reserve(skb, hr);
> +
> + addrs = kmem_cache_zalloc(xsk_tx_generic_cache, GFP_KERNEL);
> + if (!addrs)
> + return ERR_PTR(-ENOMEM);
Do we need to kfree the skb that we allocated on line 621 above? (maybe
not because I always get confused by the mb/overflow handling here)
> +
> + xsk_set_destructor_arg(skb, addrs);
> }
>
> addr = desc->addr;
> @@ -662,6 +701,7 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
> {
> struct xsk_tx_metadata *meta = NULL;
> struct net_device *dev = xs->dev;
> + struct xsk_addrs *addrs = NULL;
> struct sk_buff *skb = xs->skb;
> bool first_frag = false;
> int err;
> @@ -694,6 +734,15 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
> err = skb_store_bits(skb, 0, buffer, len);
> if (unlikely(err))
> goto free_err;
> +
> + addrs = kmem_cache_zalloc(xsk_tx_generic_cache, GFP_KERNEL);
> + if (!addrs) {
> + err = -ENOMEM;
> + goto free_err;
> + }
> +
> + xsk_set_destructor_arg(skb, addrs);
> +
> } else {
> int nr_frags = skb_shinfo(skb)->nr_frags;
> struct page *page;
> @@ -759,7 +808,9 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
> skb->mark = READ_ONCE(xs->sk.sk_mark);
> skb->destructor = xsk_destruct_skb;
> xsk_tx_metadata_to_compl(meta, &skb_shinfo(skb)->xsk_meta);
> - xsk_set_destructor_arg(skb);
> +
> + addrs = (struct xsk_addrs *)skb_shinfo(skb)->destructor_arg;
> + addrs->addrs[addrs->num_descs++] = desc->addr;
>
> return skb;
>
> @@ -769,7 +820,7 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
>
> if (err == -EOVERFLOW) {
> /* Drop the packet */
> - xsk_set_destructor_arg(xs->skb);
> + xsk_inc_skb_descs(xs->skb);
> xsk_drop_skb(xs->skb);
> xskq_cons_release(xs->tx);
> } else {
> @@ -812,7 +863,7 @@ static int __xsk_generic_xmit(struct sock *sk)
> * if there is space in it. This avoids having to implement
> * any buffering in the Tx path.
> */
> - err = xsk_cq_reserve_addr_locked(xs->pool, desc.addr);
> + err = xsk_cq_reserve_locked(xs->pool);
> if (err) {
> err = -EAGAIN;
> goto out;
> @@ -1815,6 +1866,14 @@ static int __init xsk_init(void)
> if (err)
> goto out_pernet;
>
> + xsk_tx_generic_cache = kmem_cache_create("xsk_generic_xmit_cache",
> + sizeof(struct xsk_addrs), 0,
> + SLAB_HWCACHE_ALIGN, NULL);
> + if (!xsk_tx_generic_cache) {
> + err = -ENOMEM;
> + goto out_pernet;
This probably needs an unregister_netdevice_notifier call?
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