[PATCH net 5/5] net/mlx5e: RX, Fix XDP multi-buf frag counting for legacy RQ

[Tariq Toukan]

From: Dragos Tatulea <dtatulea@xxxxxxxxxx>

XDP multi-buf programs can modify the layout of the XDP buffer when the
program calls bpf_xdp_pull_data() or bpf_xdp_adjust_tail(). The
referenced commit in the fixes tag corrected the assumption in the mlx5
driver that the XDP buffer layout doesn't change during a program
execution. However, this fix introduced another issue: the dropped
fragments still need to be counted on the driver side to avoid page
fragment reference counting issues.

Such issue can be observed with the
test_xdp_native_adjst_tail_shrnk_data selftest when using a payload of
3600 and shrinking by 256 bytes (an upcoming selftest patch): the last
fragment gets released by the XDP code but doesn't get tracked by the
driver. This results in a negative pp_ref_count during page release and
the following splat:

  WARNING: include/net/page_pool/helpers.h:297 at mlx5e_page_release_fragmented.isra.0+0x4a/0x50 [mlx5_core], CPU#12: ip/3137
  Modules linked in: [...]
  CPU: 12 UID: 0 PID: 3137 Comm: ip Not tainted 6.19.0-rc3+ #12 NONE
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/2014
  RIP: 0010:mlx5e_page_release_fragmented.isra.0+0x4a/0x50 [mlx5_core]
  [...]
  Call Trace:
   <TASK>
   mlx5e_dealloc_rx_wqe+0xcb/0x1a0 [mlx5_core]
   mlx5e_free_rx_descs+0x7f/0x110 [mlx5_core]
   mlx5e_close_rq+0x50/0x60 [mlx5_core]
   mlx5e_close_queues+0x36/0x2c0 [mlx5_core]
   mlx5e_close_channel+0x1c/0x50 [mlx5_core]
   mlx5e_close_channels+0x45/0x80 [mlx5_core]
   mlx5e_safe_switch_params+0x1a5/0x230 [mlx5_core]
   mlx5e_change_mtu+0xf3/0x2f0 [mlx5_core]
   netif_set_mtu_ext+0xf1/0x230
   do_setlink.isra.0+0x219/0x1180
   rtnl_newlink+0x79f/0xb60
   rtnetlink_rcv_msg+0x213/0x3a0
   netlink_rcv_skb+0x48/0xf0
   netlink_unicast+0x24a/0x350
   netlink_sendmsg+0x1ee/0x410
   __sock_sendmsg+0x38/0x60
   ____sys_sendmsg+0x232/0x280
   ___sys_sendmsg+0x78/0xb0
   __sys_sendmsg+0x5f/0xb0
   [...]
   do_syscall_64+0x57/0xc50

This patch fixes the issue by doing page frag counting on all the
original XDP buffer fragments for all relevant XDP actions (XDP_TX ,
XDP_REDIRECT and XDP_PASS). This is basically reverting to the original
counting before the commit in the fixes tag.

As frag_page is still pointing to the original tail, the nr_frags
parameter to xdp_update_skb_frags_info() needs to be calculated
in a different way to reflect the new nr_frags.

Fixes: afd5ba577c10 ("net/mlx5e: RX, Fix generating skb from non-linear xdp_buff for legacy RQ")
Signed-off-by: Dragos Tatulea <dtatulea@xxxxxxxxxx>
Cc: Amery Hung <ameryhung@xxxxxxxxx>
Signed-off-by: Tariq Toukan <tariqt@xxxxxxxxxx>
---
 drivers/net/ethernet/mellanox/mlx5/core/en_rx.c | 11 ++++-------
 1 file changed, 4 insertions(+), 7 deletions(-)

diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c b/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
index 40e53a612989..268e20884757 100644
--- a/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
+++ b/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
@@ -1589,6 +1589,7 @@ mlx5e_skb_from_cqe_nonlinear(struct mlx5e_rq *rq, struct mlx5e_wqe_frag_info *wi
 	struct skb_shared_info *sinfo;
 	u32 frag_consumed_bytes;
 	struct bpf_prog *prog;
+	u8 nr_frags_free = 0;
 	struct sk_buff *skb;
 	dma_addr_t addr;
 	u32 truesize;
@@ -1631,15 +1632,13 @@ mlx5e_skb_from_cqe_nonlinear(struct mlx5e_rq *rq, struct mlx5e_wqe_frag_info *wi
 
 	prog = rcu_dereference(rq->xdp_prog);
 	if (prog) {
-		u8 nr_frags_free, old_nr_frags = sinfo->nr_frags;
+		u8 old_nr_frags = sinfo->nr_frags;
 
 		if (mlx5e_xdp_handle(rq, prog, mxbuf)) {
 			if (__test_and_clear_bit(MLX5E_RQ_FLAG_XDP_XMIT,
 						 rq->flags)) {
 				struct mlx5e_wqe_frag_info *pwi;
 
-				wi -= old_nr_frags - sinfo->nr_frags;
-
 				for (pwi = head_wi; pwi < wi; pwi++)
 					pwi->frag_page->frags++;
 			}
@@ -1647,10 +1646,8 @@ mlx5e_skb_from_cqe_nonlinear(struct mlx5e_rq *rq, struct mlx5e_wqe_frag_info *wi
 		}
 
 		nr_frags_free = old_nr_frags - sinfo->nr_frags;
-		if (unlikely(nr_frags_free)) {
-			wi -= nr_frags_free;
+		if (unlikely(nr_frags_free))
 			truesize -= nr_frags_free * frag_info->frag_stride;
-		}
 	}
 
 	skb = mlx5e_build_linear_skb(
@@ -1666,7 +1663,7 @@ mlx5e_skb_from_cqe_nonlinear(struct mlx5e_rq *rq, struct mlx5e_wqe_frag_info *wi
 
 	if (xdp_buff_has_frags(&mxbuf->xdp)) {
 		/* sinfo->nr_frags is reset by build_skb, calculate again. */
-		xdp_update_skb_frags_info(skb, wi - head_wi - 1,
+		xdp_update_skb_frags_info(skb, wi - head_wi - nr_frags_free - 1,
 					  sinfo->xdp_frags_size, truesize,
 					  xdp_buff_get_skb_flags(&mxbuf->xdp));
 
-- 
2.44.0