[PATCH net] eth: fbnic: remove the debugging trick of super high page bias

[PATCH net] eth: fbnic: remove the debugging trick of super high page bias

[Jakub Kicinski]

Alex added page bias of LONG_MAX, which is admittedly quite
a clever way of catching overflows of the pp ref count.
The page pool code was "optimized" to leave the ref at 1
for freed pages so it can't catch basic bugs by itself any more.
(Something we should probably address under DEBUG_NET...)

Unfortunately for fbnic since commit f7dc3248dcfb ("skbuff: Optimization
of SKB coalescing for page pool") core _may_ actually take two extra
pp refcounts, if one of them is returned before driver gives up the bias
the ret < 0 check in page_pool_unref_netmem() will trigger.

While at it add a FBNIC_ to the name of the driver constant.

Fixes: 0cb4c0a13723 ("eth: fbnic: Implement Rx queue alloc/start/stop/free")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
---
 drivers/net/ethernet/meta/fbnic/fbnic_txrx.h | 6 ++----
 drivers/net/ethernet/meta/fbnic/fbnic_txrx.c | 4 ++--
 2 files changed, 4 insertions(+), 6 deletions(-)

diff --git a/drivers/net/ethernet/meta/fbnic/fbnic_txrx.h b/drivers/net/ethernet/meta/fbnic/fbnic_txrx.h
index 2e361d6f03ff..34693596e5eb 100644
--- a/drivers/net/ethernet/meta/fbnic/fbnic_txrx.h
+++ b/drivers/net/ethernet/meta/fbnic/fbnic_txrx.h
@@ -91,10 +91,8 @@ struct fbnic_queue_stats {
 	struct u64_stats_sync syncp;
 };
 
-/* Pagecnt bias is long max to reserve the last bit to catch overflow
- * cases where if we overcharge the bias it will flip over to be negative.
- */
-#define PAGECNT_BIAS_MAX	LONG_MAX
+#define FBNIC_PAGECNT_BIAS_MAX	PAGE_SIZE
+
 struct fbnic_rx_buf {
 	struct page *page;
 	long pagecnt_bias;
diff --git a/drivers/net/ethernet/meta/fbnic/fbnic_txrx.c b/drivers/net/ethernet/meta/fbnic/fbnic_txrx.c
index ac11389a764c..f9543d03485f 100644
--- a/drivers/net/ethernet/meta/fbnic/fbnic_txrx.c
+++ b/drivers/net/ethernet/meta/fbnic/fbnic_txrx.c
@@ -661,8 +661,8 @@ static void fbnic_page_pool_init(struct fbnic_ring *ring, unsigned int idx,
 {
 	struct fbnic_rx_buf *rx_buf = &ring->rx_buf[idx];
 
-	page_pool_fragment_page(page, PAGECNT_BIAS_MAX);
-	rx_buf->pagecnt_bias = PAGECNT_BIAS_MAX;
+	page_pool_fragment_page(page, FBNIC_PAGECNT_BIAS_MAX);
+	rx_buf->pagecnt_bias = FBNIC_PAGECNT_BIAS_MAX;
 	rx_buf->page = page;
 }
 
-- 
2.50.1

Re: [PATCH net] eth: fbnic: remove the debugging trick of super high page bias

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Fri,  1 Aug 2025 10:07:54 -0700 you wrote:
> Alex added page bias of LONG_MAX, which is admittedly quite
> a clever way of catching overflows of the pp ref count.
> The page pool code was "optimized" to leave the ref at 1
> for freed pages so it can't catch basic bugs by itself any more.
> (Something we should probably address under DEBUG_NET...)
> 
> Unfortunately for fbnic since commit f7dc3248dcfb ("skbuff: Optimization
> of SKB coalescing for page pool") core _may_ actually take two extra
> pp refcounts, if one of them is returned before driver gives up the bias
> the ret < 0 check in page_pool_unref_netmem() will trigger.
> 
> [...]

Here is the summary with links:
  - [net] eth: fbnic: remove the debugging trick of super high page bias
    https://git.kernel.org/netdev/net/c/e407fceeaf1b

You are awesome, thank you!
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