Revert "net: skb: introduce and use a single page frag cache"

After the previous commit is finally safe to revert commit dbae2b062824
("net: skb: introduce and use a single page frag cache"): do it here.

The intended goal of such change was to counter a performance regression
introduced by commit 3226b158e67c ("net: avoid 32 x truesize
under-estimation for tiny skbs").

Unfortunately, the blamed commit introduces another regression for the
virtio_net driver. Such a driver calls napi_alloc_skb() with a tiny
size, so that the whole head frag could fit a 512-byte block.

The single page frag cache uses a 1K fragment for such allocation, and
the additional overhead, under small UDP packets flood, makes the page
allocator a bottleneck.

Thanks to commit bf9f1baa279f ("net: add dedicated kmem_cache for
typical/small skb->head"), this revert does not re-introduce the
original regression. Actually, in the relevant test on top of this
revert, I measure a small but noticeable positive delta, just above
noise level.

The revert itself required some additional mangling due to recent updates
in the affected code.

Suggested-by: Eric Dumazet <[email protected]>
Fixes: dbae2b062824 ("net: skb: introduce and use a single page frag cache")
Reviewed-by: Eric Dumazet <[email protected]>
Signed-off-by: Paolo Abeni <[email protected]>

1 files changed, 17 insertions, 0 deletions

diff --git a/net/core/dev.c b/net/core/dev.c
index 72459dd02f38..1b252e9459fd 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -6991,6 +6991,23 @@ netif_napi_dev_list_add(struct net_device *dev, struct napi_struct *napi)
 	list_add_rcu(&napi->dev_list, higher); /* adds after higher */
 }
 
+/* Double check that napi_get_frags() allocates skbs with
+ * skb->head being backed by slab, not a page fragment.
+ * This is to make sure bug fixed in 3226b158e67c
+ * ("net: avoid 32 x truesize under-estimation for tiny skbs")
+ * does not accidentally come back.
+ */
+static void napi_get_frags_check(struct napi_struct *napi)
+{
+	struct sk_buff *skb;
+
+	local_bh_disable();
+	skb = napi_get_frags(napi);
+	WARN_ON_ONCE(skb && skb->head_frag);
+	napi_free_frags(napi);
+	local_bh_enable();
+}
+
 void netif_napi_add_weight_locked(struct net_device *dev,
 				  struct napi_struct *napi,
 				  int (*poll)(struct napi_struct *, int),