net: track pfmemalloc drops via SKB_DROP_REASON_PFMEMALLOC

Add a new SKB drop reason (SKB_DROP_REASON_PFMEMALLOC) to track packets
dropped due to memory pressure. In production environments, we've observed
memory exhaustion reported by memory layer stack traces, but these drops
were not properly tracked in the SKB drop reason infrastructure.

While most network code paths now properly report pfmemalloc drops, some
protocol-specific socket implementations still use sk_filter() without
drop reason tracking:
- Bluetooth L2CAP sockets
- CAIF sockets
- IUCV sockets
- Netlink sockets
- SCTP sockets
- Unix domain sockets

These remaining cases represent less common paths and could be converted
in a follow-up patch if needed. The current implementation provides
significantly improved observability into memory pressure events in the
network stack, especially for key protocols like TCP and UDP, helping to
diagnose problems in production environments.

Reported-by: Matt Fleming <mfleming@cloudflare.com>
Signed-off-by: Jesper Dangaard Brouer <hawk@kernel.org>
Link: https://patch.msgid.link/175268316579.2407873.11634752355644843509.stgit@firesoul
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 12 insertions, 2 deletions

diff --git a/include/linux/filter.h b/include/linux/filter.h
index f5cf4d35d83e..4e82332afe03 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -1073,10 +1073,20 @@ bpf_jit_binary_lock_ro(struct bpf_binary_header *hdr)
 	return set_memory_rox((unsigned long)hdr, hdr->size >> PAGE_SHIFT);
 }
 
-int sk_filter_trim_cap(struct sock *sk, struct sk_buff *skb, unsigned int cap);
+int sk_filter_trim_cap(struct sock *sk, struct sk_buff *skb, unsigned int cap,
+		       enum skb_drop_reason *reason);
+
 static inline int sk_filter(struct sock *sk, struct sk_buff *skb)
 {
-	return sk_filter_trim_cap(sk, skb, 1);
+	enum skb_drop_reason ignore_reason;
+
+	return sk_filter_trim_cap(sk, skb, 1, &ignore_reason);
+}
+
+static inline int sk_filter_reason(struct sock *sk, struct sk_buff *skb,
+				   enum skb_drop_reason *reason)
+{
+	return sk_filter_trim_cap(sk, skb, 1, reason);
 }
 
 struct bpf_prog *bpf_prog_select_runtime(struct bpf_prog *fp, int *err);