[PATCH v2 net] skbuff: back tiny skbs with kmalloc() in __netdev_alloc_skb() too

[PATCH v2 net] skbuff: back tiny skbs with kmalloc() in __netdev_alloc_skb() too

[Alexander Lobakin]

Commit 3226b158e67c ("net: avoid 32 x truesize under-estimation for
tiny skbs") ensured that skbs with data size lower than 1025 bytes
will be kmalloc'ed to avoid excessive page cache fragmentation and
memory consumption.
However, the fix adressed only __napi_alloc_skb() (primarily for
virtio_net and napi_get_frags()), but the issue can still be achieved
through __netdev_alloc_skb(), which is still used by several drivers.
Drivers often allocate a tiny skb for headers and place the rest of
the frame to frags (so-called copybreak).
Mirror the condition to __netdev_alloc_skb() to handle this case too.

Since v1 [0]:
 - fix "Fixes:" tag;
 - refine commit message (mention copybreak usecase).

[0] https://lore.kernel.org/netdev/20210114235423.232737-1-alobakin@pm.me

Fixes: a1c7fff7e18f ("net: netdev_alloc_skb() use build_skb()")
Signed-off-by: Alexander Lobakin <alobakin@pm.me>
---
 net/core/skbuff.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index c1a6f262636a..785daff48030 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -437,7 +437,11 @@ struct sk_buff *__netdev_alloc_skb(struct net_device *dev, unsigned int len,
 
 	len += NET_SKB_PAD;
 
-	if ((len > SKB_WITH_OVERHEAD(PAGE_SIZE)) ||
+	/* If requested length is either too small or too big,
+	 * we use kmalloc() for skb->head allocation.
+	 */
+	if (len <= SKB_WITH_OVERHEAD(1024) ||
+	    len > SKB_WITH_OVERHEAD(PAGE_SIZE) ||
 	    (gfp_mask & (__GFP_DIRECT_RECLAIM | GFP_DMA))) {
 		skb = __alloc_skb(len, gfp_mask, SKB_ALLOC_RX, NUMA_NO_NODE);
 		if (!skb)
-- 
2.30.0

Re: [PATCH v2 net] skbuff: back tiny skbs with kmalloc() in __netdev_alloc_skb() too

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net.git (refs/heads/master):

On Fri, 15 Jan 2021 15:04:40 +0000 you wrote:
> Commit 3226b158e67c ("net: avoid 32 x truesize under-estimation for
> tiny skbs") ensured that skbs with data size lower than 1025 bytes
> will be kmalloc'ed to avoid excessive page cache fragmentation and
> memory consumption.
> However, the fix adressed only __napi_alloc_skb() (primarily for
> virtio_net and napi_get_frags()), but the issue can still be achieved
> through __netdev_alloc_skb(), which is still used by several drivers.
> Drivers often allocate a tiny skb for headers and place the rest of
> the frame to frags (so-called copybreak).
> Mirror the condition to __netdev_alloc_skb() to handle this case too.
> 
> [...]

Here is the summary with links:
  - [v2,net] skbuff: back tiny skbs with kmalloc() in __netdev_alloc_skb() too
    https://git.kernel.org/netdev/net/c/66c556025d68

You are awesome, thank you!
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