[PATCH net v4 2/2] net/mlx5e: Prevent stale XSK buffer release on MPWQE refill retry

With AF_XDP on a striding RQ, mlx5e defers releasing XSK buffers until
an MPWQE is refilled. If XSK allocation then returns -ENOMEM,
actual_wq_head is not advanced and a later NAPI poll retries the same
WQE.

mlx5e_free_rx_mpwqe() leaves each released slot marked as releasable. On
retry it can therefore call xsk_buff_free() again through stale pointers
after the frames have returned to the XSK pool and been reallocated.

Set all skip_release_bitmap bits in the common error path of
mlx5e_xsk_alloc_rx_mpwqe(). This matches mlx5e_alloc_rx_mpwqe(). A
successful allocation already clears the bitmap after replacing every
buffer, so retries become idempotent without changing the success path.

Fault injection forced three consecutive failures for one selected MPWQE.
Both an early allocation failure and a partial 8-of-16-buffer unwind
released the original 16 XSK buffers only once. Each error left a full
bitmap, the following NAPI retry skipped the release, and a later
successful allocation cleared it. A 20-second AF_XDP zero-copy pressure
run exercised 1,575,262 buffer allocation failures without invalid
descriptors, WQE errors, or kernel warnings.

Fixes: 4c2a13236807 ("net/mlx5e: RX, Defer page release in striding rq for better recycling")
Cc: [email protected]
Signed-off-by: Jerome Tollet <[email protected]>
---
 drivers/net/ethernet/mellanox/mlx5/core/en/xsk/rx.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en/xsk/rx.c b/drivers/net/ethernet/mellanox/mlx5/core/en/xsk/rx.c
index 4f984f6a2cb9..55ec6387ab28 100644
--- a/drivers/net/ethernet/mellanox/mlx5/core/en/xsk/rx.c
+++ b/drivers/net/ethernet/mellanox/mlx5/core/en/xsk/rx.c
@@ -3,6 +3,7 @@
 
 #include "rx.h"
 #include "en/xdp.h"
+#include <linux/bitmap.h>
 #include <net/xdp_sock_drv.h>
 #include <linux/filter.h>
 
@@ -156,6 +157,7 @@ err_reuse_batch:
 		xsk_buff_free(xsk_buffs[batch]);
 
 err:
+	bitmap_fill(wi->skip_release_bitmap, rq->mpwqe.pages_per_wqe);
 	rq->stats->buff_alloc_err++;
 	return -ENOMEM;
 }
-- 
2.55.0