linux-kernel - Re: [PATCH net-next] virtio_net: Fix napi_skb_cache

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Message-ID: <ZpUHEszCj16rNoGy@gmail.com>
Date: Mon, 15 Jul 2024 04:25:06 -0700
From: Breno Leitao <leitao@...ian.org>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: Jason Wang <jasowang@...hat.com>,
	Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
	Eugenio Pérez <eperezma@...hat.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	rbc@...a.com, horms@...nel.org,
	"open list:VIRTIO CORE AND NET DRIVERS" <virtualization@...ts.linux.dev>,
	"open list:NETWORKING DRIVERS" <netdev@...r.kernel.org>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next] virtio_net: Fix napi_skb_cache_put warning

Hello Michael,

On Sun, Jul 14, 2024 at 03:38:42AM -0400, Michael S. Tsirkin wrote:
> On Fri, Jul 12, 2024 at 04:53:25AM -0700, Breno Leitao wrote:
> > After the commit bdacf3e34945 ("net: Use nested-BH locking for
> > napi_alloc_cache.") was merged, the following warning began to appear:
> > 
> > 	 WARNING: CPU: 5 PID: 1 at net/core/skbuff.c:1451 napi_skb_cache_put+0x82/0x4b0
> > 
> > 	  __warn+0x12f/0x340
> > 	  napi_skb_cache_put+0x82/0x4b0
> > 	  napi_skb_cache_put+0x82/0x4b0
> > 	  report_bug+0x165/0x370
> > 	  handle_bug+0x3d/0x80
> > 	  exc_invalid_op+0x1a/0x50
> > 	  asm_exc_invalid_op+0x1a/0x20
> > 	  __free_old_xmit+0x1c8/0x510
> > 	  napi_skb_cache_put+0x82/0x4b0
> > 	  __free_old_xmit+0x1c8/0x510
> > 	  __free_old_xmit+0x1c8/0x510
> > 	  __pfx___free_old_xmit+0x10/0x10
> > 
> > The issue arises because virtio is assuming it's running in NAPI context
> > even when it's not, such as in the netpoll case.
> > 
> > To resolve this, modify virtnet_poll_tx() to only set NAPI when budget
> > is available. Same for virtnet_poll_cleantx(), which always assumed that
> > it was in a NAPI context.
> > 
> > Fixes: df133f3f9625 ("virtio_net: bulk free tx skbs")
> > Suggested-by: Jakub Kicinski <kuba@...nel.org>
> > Signed-off-by: Breno Leitao <leitao@...ian.org>
> 
> Acked-by: Michael S. Tsirkin <mst@...hat.com>
> 
> though I'm not sure I understand the connection with bdacf3e34945.

The warning above appeared after bdacf3e34945 landed.