netdev - [PATCH net] Revert "virtio_net: replace netdev_alloc_skb_ip_align() with napi_alloc

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Date:   Tue, 12 Jan 2021 21:12:07 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Paolo Abeni <pabeni@...hat.com>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Greg Thelen <gthelen@...gle.com>
Subject: [PATCH net] Revert "virtio_net: replace netdev_alloc_skb_ip_align() with napi_alloc_skb()"

From: Eric Dumazet <edumazet@...gle.com>

This reverts commit c67f5db82027ba6d2ea4ac9176bc45996a03ae6a.

While using page fragments instead of a kmalloc backed skb->head might give
a small performance improvement in some cases, there is a huge risk of
memory use under estimation.

GOOD_COPY_LEN is 128 bytes. This means that we need a small amount
of memory to hold the headers and struct skb_shared_info

Yet, napi_alloc_skb() might use a whole 32KB page (or 64KB on PowerPc)
for long lived incoming TCP packets.

We have been tracking OOM issues on GKE hosts hitting tcp_mem limits
but consuming far more memory for TCP buffers than instructed in tcp_mem[2]

Even if we force napi_alloc_skb() to only use order-0 pages, the issue
would still be there on arches with PAGE_SIZE >= 32768

Using alloc_skb() and thus standard kmallloc() for skb->head allocations
will get the benefit of letting other objects in each page being independently
used by other skbs, regardless of the lifetime.

Note that a similar problem exists for skbs allocated from napi_get_frags(),
this is handled in a separate patch.

I would like to thank Greg Thelen for his precious help on this matter,
analysing crash dumps is always a time consuming task.

Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Cc: Paolo Abeni <pabeni@...hat.com>
Cc: Michael S. Tsirkin <mst@...hat.com>
Cc: Greg Thelen <gthelen@...gle.com>
---
 drivers/net/virtio_net.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 508408fbe78fbd8658dc226834b5b1b334b8b011..5886504c1acacf3f6148127b5c1cc7f6a906b827 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -386,7 +386,7 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,
 	p = page_address(page) + offset;

 	/* copy small packet so we can reuse these pages for small data */
-	skb = napi_alloc_skb(&rq->napi, GOOD_COPY_LEN);
+	skb = netdev_alloc_skb_ip_align(vi->dev, GOOD_COPY_LEN);
 	if (unlikely(!skb))
 		return NULL;

-- 
2.30.0.284.gd98b1dd5eaa7-goog