net-procfs: use xarray iterator to implement /proc/net/dev

In commit 759ab1e ("net: store netdevs in an xarray")
Jakub added net->dev_by_index to map ifindex to netdevices.

We can get rid of the old hash table (net->dev_index_head),
one patch at a time, if performance is acceptable.

This patch removes unpleasant code to something more readable.

As a bonus, /proc/net/dev gets netdevices sorted by their ifindex.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20240207165318.3814525-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/net-procfs.c

@@ -6,66 +6,32 @@
 
 #include "dev.h"
 
-#define BUCKET_SPACE (32 - NETDEV_HASHBITS - 1)
-
-#define get_bucket(x) ((x) >> BUCKET_SPACE)
-#define get_offset(x) ((x) & ((1 << BUCKET_SPACE) - 1))
-#define set_bucket_offset(b, o) ((b) << BUCKET_SPACE | (o))
-
-static inline struct net_device *dev_from_same_bucket(struct seq_file *seq, loff_t *pos)
+static void *dev_seq_from_index(struct seq_file *seq, loff_t *pos)
 {
-	struct net *net = seq_file_net(seq);
+	unsigned long ifindex = *pos;
 	struct net_device *dev;
-	struct hlist_head *h;
-	unsigned int count = 0, offset = get_offset(*pos);
 
-	h = &net->dev_index_head[get_bucket(*pos)];
-	hlist_for_each_entry_rcu(dev, h, index_hlist) {
-		if (++count == offset)
-			return dev;
+	for_each_netdev_dump(seq_file_net(seq), dev, ifindex) {
+		*pos = dev->ifindex;
+		return dev;
 	}
-
-	return NULL;
-}
-
-static inline struct net_device *dev_from_bucket(struct seq_file *seq, loff_t *pos)
-{
-	struct net_device *dev;
-	unsigned int bucket;
-
-	do {
-		dev = dev_from_same_bucket(seq, pos);
-		if (dev)
-			return dev;
-
-		bucket = get_bucket(*pos) + 1;
-		*pos = set_bucket_offset(bucket, 1);
-	} while (bucket < NETDEV_HASHENTRIES);
-
 	return NULL;
 }
 
-/*
- *	This is invoked by the /proc filesystem handler to display a device
- *	in detail.
- */
 static void *dev_seq_start(struct seq_file *seq, loff_t *pos)
 	__acquires(RCU)
 {
 	rcu_read_lock();
 	if (!*pos)
 		return SEQ_START_TOKEN;
 
-	if (get_bucket(*pos) >= NETDEV_HASHENTRIES)
-		return NULL;
-
-	return dev_from_bucket(seq, pos);
+	return dev_seq_from_index(seq, pos);
 }
 
 static void *dev_seq_next(struct seq_file *seq, void *v, loff_t *pos)
 {
 	++*pos;
-	return dev_from_bucket(seq, pos);
+	return dev_seq_from_index(seq, pos);
 }
 
 static void dev_seq_stop(struct seq_file *seq, void *v)