From: Laurent Vivier lvivier@redhat.com
[ Upstream commit 24b2f5df86aaebbe7bac40304eaf5a146c02367c ]
The `tx_may_stop()` logic stops TX queues if free descriptors (`sq->vq->num_free`) fall below the threshold of (`MAX_SKB_FRAGS` + 2). If the total ring size (`ring_num`) is not strictly greater than this value, queues can become persistently stopped or stop after minimal use, severely degrading performance.
A single sk_buff transmission typically requires descriptors for: - The virtio_net_hdr (1 descriptor) - The sk_buff's linear data (head) (1 descriptor) - Paged fragments (up to MAX_SKB_FRAGS descriptors)
This patch enforces that the TX ring size ('ring_num') must be strictly greater than (MAX_SKB_FRAGS + 2). This ensures that the ring is always large enough to hold at least one maximally-fragmented packet plus at least one additional slot.
Reported-by: Lei Yang leiyang@redhat.com Signed-off-by: Laurent Vivier lvivier@redhat.com Reviewed-by: Xuan Zhuo xuanzhuo@linux.alibaba.com Acked-by: Jason Wang jasowang@redhat.com Link: https://patch.msgid.link/20250521092236.661410-4-lvivier@redhat.com Tested-by: Lei Yang leiyang@redhat.com Acked-by: Michael S. Tsirkin mst@redhat.com Signed-off-by: Paolo Abeni pabeni@redhat.com Signed-off-by: Sasha Levin sashal@kernel.org ---
**YES**
This commit should be backported to stable kernel trees. Here's my extensive analysis:
## 1. **Critical Bug Fix** The commit fixes a severe bug where TX queues can become permanently stopped, causing complete network transmission failure. Looking at the code change, it adds a crucial validation in `virtnet_tx_resize()`:
```c if (ring_num <= MAX_SKB_FRAGS + 2) { netdev_err(vi->dev, "tx size (%d) cannot be smaller than %d\n", ring_num, MAX_SKB_FRAGS + 2); return -EINVAL; } ```
This prevents a configuration that would make the network interface essentially unusable.
## 2. **Root Cause Analysis** The bug occurs because the `tx_may_stop()` logic (used throughout virtio_net for flow control) stops the TX queue when free descriptors fall below `MAX_SKB_FRAGS + 2`. If the total ring size is not strictly greater than this threshold, the queue can: - Stop after transmitting just one packet - Never have enough free slots to wake up again - Result in a permanently stalled TX queue
## 3. **User Impact** This affects real users who: - Use virtio_net in virtualized environments (very common in cloud/VM deployments) - Configure TX ring sizes via ethtool or other management tools - May unknowingly set ring sizes that trigger this bug
The commit message explicitly mentions a report from Lei Yang at Red Hat, indicating this was discovered through real-world usage.
## 4. **Minimal and Safe Fix** The fix is: - **Small**: Only 6 lines of code added - **Contained**: Only affects ring resize operations - **Safe**: Adds validation without changing existing behavior for valid configurations - **Clear**: Returns -EINVAL with a descriptive error message
## 5. **Meets Stable Criteria** According to stable kernel rules, this commit qualifies because it: - Fixes a bug that affects users (network stalls) - Is relatively small and contained - Has no major side effects - Doesn't introduce new features - Has been tested (by Lei Yang) and reviewed by maintainers (Jason Wang, Michael S. Tsirkin)
## 6. **Comparison with Similar Commits** Looking at the historical commits: - Similar commit #3 (virtio_net: ensure netdev_tx_reset_queue) was backported because it fixed TX queue handling issues - This commit is even more critical as it prevents complete TX failure - The fix is simpler than commit #3 (just validation vs. callback handling)
## 7. **No Architectural Changes** Unlike commits #1 and #2 which made architectural changes to ring sizing, this commit: - Only adds input validation - Doesn't change data structures - Doesn't modify existing workflows - Simply prevents an invalid configuration
The fact that experienced maintainers like Jason Wang and Michael S. Tsirkin acked this change further supports its importance for stable backporting. This is a textbook example of a fix that should go to stable: it addresses a real bug with minimal risk.
drivers/net/virtio_net.c | 6 ++++++ 1 file changed, 6 insertions(+)
diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c index e53ba600605a5..3054b2b4f6a0c 100644 --- a/drivers/net/virtio_net.c +++ b/drivers/net/virtio_net.c @@ -3481,6 +3481,12 @@ static int virtnet_tx_resize(struct virtnet_info *vi, struct send_queue *sq, { int qindex, err;
+ if (ring_num <= MAX_SKB_FRAGS + 2) { + netdev_err(vi->dev, "tx size (%d) cannot be smaller than %d\n", + ring_num, MAX_SKB_FRAGS + 2); + return -EINVAL; + } + qindex = sq - vi->sq;
virtnet_tx_pause(vi, sq);