[PATCH net] gve: leave refcount headroom in the RX page bias

[Amaan Cheval <amaan.cheval@gmail.com>]

gve's GQI and DQO-QPL RX paths set every RX page's refcount to INT_MAX and
hand references to the stack by decrementing page_info->pagecnt_bias, to
avoid an atomic op per packet. This leaves the page no headroom: while a
fresh page's buffer sits in an skb its refcount is already INT_MAX, so a
single get_page() from the stack wraps it negative.

splice() of received TCP data into a pipe takes such a reference, and the
onward splice into a socket then fails: skb_splice_from_iter() sees
page_count() < 1, sendpage_ok() fails and the send returns -EIO (since
commit 2e910b95329c ("net: Add a function to splice pages into an
skbuff for MSG_SPLICE_PAGES"))

tee() of the pipe fails with -EFAULT via try_get_page(). On GQI the reuse
check gve_rx_can_recycle_buffer() also sees page_count() < pagecnt_bias,
WARNs and calls gve_schedule_reset(), so a task that merely pins a fresh
RX page (e.g. holds spliced data in an undrained tee() pipe) can reset
the NIC.

Use USHRT_MAX in a u16, as igb, igc, ixgbe, i40e and hns3 do: the bias only
has to make the per-packet atomic rare so a small value leaves page_count()
ample headroom below INT_MAX for the stack's references, and the u16 bounds
it. Refill by adding a fixed batch and setting the bias to it, which keeps
the rule `references = page_count() - bias` intact.

Reproduced on gVNIC in GQI-QPL mode (GCE n2-standard-8, 6.12), no kernel
changes needed, with relay.py -- a small splice(2) TCP proxy -- relaying a
download while the RX ring is refilled with fresh pages:

  ethtool -G eth0 rx 512 && ethtool -G eth0 rx 1024   # fresh RX pages
  ./relay.py $HOST 8080 &                             # splice()s both ways
  for i in $(seq 200); do curl -so /dev/null \
      --connect-to $HOST:80:127.0.0.1:8080 http://$HOST/file; done

Most downloads failed with EIO before the fix (tee() with EFAULT) and all
completed after it, including with the bias forced to 1. Built allmodconfig
W=1, no new warnings; no selftests run. relay.py is at the Link below.

This bug was found and fixed with the help of an AI coding assistant
(Claude Code) over an interactive session; it read the driver and
splice/pipe code and ran the tests above.

Link: https://github.com/amaanc/kernel-bias
Fixes: 9b8dd5e5ea48 ("gve: DQO: Add RX path")
Fixes: 58401b2a46e7 ("gve: Add rx buffer pagecnt bias")
Fixes: 82fd151d38d9 ("gve: Reduce alloc and copy costs in the GQ rx path")
Cc: stable@vger.kernel.org
Signed-off-by: Amaan Cheval <amaan.cheval@gmail.com>
---
 drivers/net/ethernet/google/gve/gve.h                |  2 +-
 .../net/ethernet/google/gve/gve_buffer_mgmt_dqo.c    |  4 ++--
 drivers/net/ethernet/google/gve/gve_rx.c             |  8 ++++----
 drivers/net/ethernet/google/gve/gve_utils.c          | 12 +++---------
 drivers/net/ethernet/google/gve/gve_utils.h          |  8 +++++++-
 5 files changed, 17 insertions(+), 17 deletions(-)

diff --git a/drivers/net/ethernet/google/gve/gve.h b/drivers/net/ethernet/google/gve/gve.h
index b7ce6e8632db..18d7edbf4b3b 100644
--- a/drivers/net/ethernet/google/gve/gve.h
+++ b/drivers/net/ethernet/google/gve/gve.h
@@ -119,7 +119,7 @@ struct gve_rx_slot_page_info {
 	void *page_address;
 	u32 page_offset; /* offset to write to in page */
 	unsigned int buf_size;
-	int pagecnt_bias; /* expected pagecnt if only the driver has a ref */
+	u16 pagecnt_bias; /* expected pagecnt if only the driver has a ref */
 	u16 pad; /* adjustment for rx padding */
 	u8 can_flip; /* tracks if the networking stack is using the page */
 };
diff --git a/drivers/net/ethernet/google/gve/gve_buffer_mgmt_dqo.c b/drivers/net/ethernet/google/gve/gve_buffer_mgmt_dqo.c
index 6880d1531ba5..571f851d6dcd 100644
--- a/drivers/net/ethernet/google/gve/gve_buffer_mgmt_dqo.c
+++ b/drivers/net/ethernet/google/gve/gve_buffer_mgmt_dqo.c
@@ -149,8 +149,8 @@ int gve_alloc_qpl_page_dqo(struct gve_rx_ring *rx,
 	buf_state->last_single_ref_offset = 0;
 
 	/* The page already has 1 ref. */
-	page_ref_add(buf_state->page_info.page, INT_MAX - 1);
-	buf_state->page_info.pagecnt_bias = INT_MAX;
+	page_ref_add(buf_state->page_info.page, GVE_PAGECNT_BIAS - 1);
+	buf_state->page_info.pagecnt_bias = GVE_PAGECNT_BIAS;
 
 	return 0;
 }
diff --git a/drivers/net/ethernet/google/gve/gve_rx.c b/drivers/net/ethernet/google/gve/gve_rx.c
index 81ea800e66e9..6533fff06155 100644
--- a/drivers/net/ethernet/google/gve/gve_rx.c
+++ b/drivers/net/ethernet/google/gve/gve_rx.c
@@ -152,8 +152,8 @@ static void gve_setup_rx_buffer(struct gve_rx_ring *rx,
 	page_info->buf_size = rx->packet_buffer_size;
 	*slot_addr = cpu_to_be64(addr);
 	/* The page already has 1 ref */
-	page_ref_add(page, INT_MAX - 1);
-	page_info->pagecnt_bias = INT_MAX;
+	page_ref_add(page, GVE_PAGECNT_BIAS - 1);
+	page_info->pagecnt_bias = GVE_PAGECNT_BIAS;
 }
 
 static int gve_rx_alloc_buffer(struct gve_priv *priv, struct device *dev,
@@ -230,8 +230,8 @@ static int gve_rx_prefill_pages(struct gve_rx_ring *rx,
 			rx->qpl_copy_pool[j].buf_size = rx->packet_buffer_size;
 
 			/* The page already has 1 ref. */
-			page_ref_add(page, INT_MAX - 1);
-			rx->qpl_copy_pool[j].pagecnt_bias = INT_MAX;
+			page_ref_add(page, GVE_PAGECNT_BIAS - 1);
+			rx->qpl_copy_pool[j].pagecnt_bias = GVE_PAGECNT_BIAS;
 		}
 	}
 
diff --git a/drivers/net/ethernet/google/gve/gve_utils.c b/drivers/net/ethernet/google/gve/gve_utils.c
index caed38cd5f2b..cb6fba3122d5 100644
--- a/drivers/net/ethernet/google/gve/gve_utils.c
+++ b/drivers/net/ethernet/google/gve/gve_utils.c
@@ -93,15 +93,9 @@ void gve_dec_pagecnt_bias(struct gve_rx_slot_page_info *page_info)
 {
 	page_info->pagecnt_bias--;
 	if (page_info->pagecnt_bias == 0) {
-		int pagecount = page_count(page_info->page);
-
-		/* If we have run out of bias - set it back up to INT_MAX
-		 * minus the existing refs.
-		 */
-		page_info->pagecnt_bias = INT_MAX - pagecount;
-
-		/* Set pagecount back up to max. */
-		page_ref_add(page_info->page, INT_MAX - pagecount);
+		/* Out of bias: take another batch of references. */
+		page_info->pagecnt_bias = GVE_PAGECNT_BIAS;
+		page_ref_add(page_info->page, GVE_PAGECNT_BIAS);
 	}
 }
 
diff --git a/drivers/net/ethernet/google/gve/gve_utils.h b/drivers/net/ethernet/google/gve/gve_utils.h
index bf2e9a0adb36..316f97607351 100644
--- a/drivers/net/ethernet/google/gve/gve_utils.h
+++ b/drivers/net/ethernet/google/gve/gve_utils.h
@@ -25,7 +25,13 @@ struct sk_buff *gve_rx_copy_data(struct net_device *dev, struct napi_struct *nap
 struct sk_buff *gve_rx_copy(struct net_device *dev, struct napi_struct *napi,
 			    struct gve_rx_slot_page_info *page_info, u16 len);
 
-/* Decrement pagecnt_bias. Set it back to INT_MAX if it reached zero. */
+/* Up-front references gve holds on an RX page, so handing a buffer to the
+ * stack is a pagecnt_bias decrement, not a get_page(). Keep it in a u16 so
+ * the 32-bit page refcount keeps the headroom every page needs below INT_MAX.
+ */
+#define GVE_PAGECNT_BIAS USHRT_MAX
+
+/* Decrement pagecnt_bias. Top it up by GVE_PAGECNT_BIAS if it reached zero. */
 void gve_dec_pagecnt_bias(struct gve_rx_slot_page_info *page_info);
 
 void gve_add_napi(struct gve_priv *priv, int ntfy_idx,

base-commit: d8a4f4c59da52b7c31ae337243b8473a19a3f819
-- 
2.47.3