[PATCH bpf-next 2/6] ice: fix XDP Tx ring overrun

[Alexander Lobakin <alexandr.lobakin@intel.com>]

Sometimes, under heavy XDP Tx traffic, e.g. when using XDP traffic
generator (%BPF_F_TEST_XDP_LIVE_FRAMES), the machine can catch OOM due
to the driver not freeing all of the pages passed to it by
.ndo_xdp_xmit().
Turned out that during the development of the tagged commit, the check,
which ensures that we have a free descriptor to queue a frame, moved
into the branch happening only when a buffer has frags. Otherwise, we
only run a cleaning cycle, but don't check anything.
ATST, there can be situations when the driver gets new frames to send,
but there are no buffers that can be cleaned/completed and the ring has
no free slots. It's very rare, but still possible (> 6.5 Mpps per ring).
The driver then fills the next buffer/descriptor, effectively
overwriting the data, which still needs to be freed.

Restore the check after the cleaning routine to make sure there is a
slot to queue a new frame. When there are frags, there still will be a
separate check that we can place all of them, but if the ring is full,
there's no point in wasting any more time.

(minor: make `!ready_frames` unlikely since it happens ~1-2 times per
 billion of frames)

Fixes: 3246a10752a7 ("ice: Add support for XDP multi-buffer on Tx side")
Signed-off-by: Alexander Lobakin <alexandr.lobakin@intel.com>
---
 drivers/net/ethernet/intel/ice/ice_txrx_lib.c | 19 ++++++++++++-------
 1 file changed, 12 insertions(+), 7 deletions(-)

diff --git a/drivers/net/ethernet/intel/ice/ice_txrx_lib.c b/drivers/net/ethernet/intel/ice/ice_txrx_lib.c
index d1a7171e618b..784f2f9ebb2d 100644
--- a/drivers/net/ethernet/intel/ice/ice_txrx_lib.c
+++ b/drivers/net/ethernet/intel/ice/ice_txrx_lib.c
@@ -260,7 +260,7 @@ static u32 ice_clean_xdp_irq(struct ice_tx_ring *xdp_ring)
 			ready_frames = idx + cnt - ntc + 1;
 	}
 
-	if (!ready_frames)
+	if (unlikely(!ready_frames))
 		return 0;
 	ret = ready_frames;
 
@@ -322,17 +322,17 @@ int __ice_xmit_xdp_ring(struct xdp_buff *xdp, struct ice_tx_ring *xdp_ring)
 	u32 frag = 0;
 
 	free_space = ICE_DESC_UNUSED(xdp_ring);
-
-	if (ICE_DESC_UNUSED(xdp_ring) < ICE_RING_QUARTER(xdp_ring))
+	if (free_space < ICE_RING_QUARTER(xdp_ring))
 		free_space += ice_clean_xdp_irq(xdp_ring);
 
+	if (unlikely(!free_space))
+		goto busy;
+
 	if (unlikely(xdp_buff_has_frags(xdp))) {
 		sinfo = xdp_get_shared_info_from_buff(xdp);
 		nr_frags = sinfo->nr_frags;
-		if (free_space < nr_frags + 1) {
-			xdp_ring->ring_stats->tx_stats.tx_busy++;
-			return ICE_XDP_CONSUMED;
-		}
+		if (free_space < nr_frags + 1)
+			goto busy;
 	}
 
 	tx_desc = ICE_TX_DESC(xdp_ring, ntu);
@@ -396,6 +396,11 @@ int __ice_xmit_xdp_ring(struct xdp_buff *xdp, struct ice_tx_ring *xdp_ring)
 		ntu--;
 	}
 	return ICE_XDP_CONSUMED;
+
+busy:
+	xdp_ring->ring_stats->tx_stats.tx_busy++;
+
+	return ICE_XDP_CONSUMED;
 }
 
 /**
-- 
2.39.1