[PATCH 6.1.y] gve: fix Rx queue stall on alloc failure

[Eddie Phillips <eddiephillips@google.com>]

When the system is under extreme memory pressure, page allocations can
fail during the Rx buffer refill loop. If the number of buffers posted
to hardware falls below a critical low threshold and the refill loop
exits due to allocation failures, the queue can stall:

1. The device drops incoming packets because there are no descriptors.
2. Since no packets are processed, no Rx completions are generated.
3. Because no completions occur, NAPI is never scheduled, preventing
   the refill loop from running again even after memory is freed.

This results in a permanent queue stall.

Resolve this by introducing a starvation recovery timer for each Rx queue.
If the number of buffers posted to hardware falls below a critical low
threshold, start a timer to periodically reschedule NAPI. Once NAPI runs
and successfully refills the queue above the threshold, the timer is
not rescheduled.

The threshold is set to 32 because a single maximum-sized Receive Segment
Coalescing (RSC) packet can consume up to 19 descriptors in the Rx path.
Lower thresholds (such as 8 or 16) would be insufficient to process a
complete maximum-sized RSC packet, risking packet drops or unexpected
hardware behavior under memory pressure. Setting the threshold to 32
guarantees a safe margin to handle at least one full RSC packet.

Cc: stable@vger.kernel.org
Fixes: 9b8dd5e5ea48 ("gve: DQO: Add RX path")
Reviewed-by: Jordan Rhee <jordanrhee@google.com>
Signed-off-by: Eddie Phillips <eddiephillips@google.com>
Signed-off-by: Harshitha Ramamurthy <hramamurthy@google.com>
Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Link: https://patch.msgid.link/20260709211906.3322883-1-hramamurthy@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
(cherry picked from commit b65352a1bac64442ad95e64f385b40ccb9f1b0db)
---
 drivers/net/ethernet/google/gve/gve.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/net/ethernet/google/gve/gve.h b/drivers/net/ethernet/google/gve/gve.h
index c5e1312b9283..056f57166362 100644
--- a/drivers/net/ethernet/google/gve/gve.h
+++ b/drivers/net/ethernet/google/gve/gve.h
@@ -10,6 +10,7 @@
 #include <linux/dma-mapping.h>
 #include <linux/netdevice.h>
 #include <linux/pci.h>
+#include <linux/timer.h>
 #include <linux/u64_stats_sync.h>
 
 #include "gve_desc.h"
@@ -226,6 +227,7 @@ struct gve_rx_ring {
 	struct u64_stats_sync statss; /* sync stats for 32bit archs */
 
 	struct gve_rx_ctx ctx; /* Info for packet currently being processed in this ring. */
+  struct timer_list starvation_timer; /* for queue starvation recovery */
 };
 
 /* A TX desc ring entry */
-- 
2.55.0.571.g244d577d93-goog