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[PATCH v4 9/9] vsock/virtio: Allocate nonlinear SKBs for handling large transmit buffers

Will Deacon posted 9 patches 2 months, 3 weeks ago
[PATCH v4 9/9] vsock/virtio: Allocate nonlinear SKBs for handling large transmit buffers
Posted by Will Deacon 2 months, 3 weeks ago 
When transmitting a vsock packet, virtio_transport_send_pkt_info() calls
virtio_transport_alloc_linear_skb() to allocate and fill SKBs with the
transmit data. Unfortunately, these are always linear allocations and
can therefore result in significant pressure on kmalloc() considering
that the maximum packet size (VIRTIO_VSOCK_MAX_PKT_BUF_SIZE +
VIRTIO_VSOCK_SKB_HEADROOM) is a little over 64KiB, resulting in a 128KiB
allocation for each packet.

Rework the vsock SKB allocation so that, for sizes with page order
greater than PAGE_ALLOC_COSTLY_ORDER, a nonlinear SKB is allocated
instead with the packet header in the SKB and the transmit data in the
fragments. Note that this affects both the vhost and virtio transports.

Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Will Deacon <will@kernel.org>
---
 net/vmw_vsock/virtio_transport_common.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
index c9eb7f7ac00d..fe92e5fa95b4 100644
--- a/net/vmw_vsock/virtio_transport_common.c
+++ b/net/vmw_vsock/virtio_transport_common.c
@@ -109,7 +109,8 @@ static int virtio_transport_fill_skb(struct sk_buff *skb,
 		return __zerocopy_sg_from_iter(info->msg, NULL, skb,
 					       &info->msg->msg_iter, len, NULL);
 
-	return memcpy_from_msg(skb_put(skb, len), info->msg, len);
+	virtio_vsock_skb_put(skb, len);
+	return skb_copy_datagram_from_iter(skb, 0, &info->msg->msg_iter, len);
 }
 
 static void virtio_transport_init_hdr(struct sk_buff *skb,
@@ -261,7 +262,7 @@ static struct sk_buff *virtio_transport_alloc_skb(struct virtio_vsock_pkt_info *
 	if (!zcopy)
 		skb_len += payload_len;
 
-	skb = virtio_vsock_alloc_linear_skb(skb_len, GFP_KERNEL);
+	skb = virtio_vsock_alloc_skb(skb_len, GFP_KERNEL);
 	if (!skb)
 		return NULL;
 
-- 
2.50.0.727.gbf7dc18ff4-goog
Re: [PATCH v4 9/9] vsock/virtio: Allocate nonlinear SKBs for handling large transmit buffers
Posted by Michael S. Tsirkin 1 month, 3 weeks ago 
On Thu, Jul 17, 2025 at 10:01:16AM +0100, Will Deacon wrote:
> When transmitting a vsock packet, virtio_transport_send_pkt_info() calls
> virtio_transport_alloc_linear_skb() to allocate and fill SKBs with the
> transmit data. Unfortunately, these are always linear allocations and
> can therefore result in significant pressure on kmalloc() considering
> that the maximum packet size (VIRTIO_VSOCK_MAX_PKT_BUF_SIZE +
> VIRTIO_VSOCK_SKB_HEADROOM) is a little over 64KiB, resulting in a 128KiB
> allocation for each packet.
> 
> Rework the vsock SKB allocation so that, for sizes with page order
> greater than PAGE_ALLOC_COSTLY_ORDER, a nonlinear SKB is allocated
> instead with the packet header in the SKB and the transmit data in the
> fragments. Note that this affects both the vhost and virtio transports.
> 
> Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
> Signed-off-by: Will Deacon <will@kernel.org>



So this caused a regression, see syzbot report:

https://lore.kernel.org/all/689a3d92.050a0220.7f033.00ff.GAE@google.com


I'm inclined to revert unless we have a fix quickly.


> ---
>  net/vmw_vsock/virtio_transport_common.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
> index c9eb7f7ac00d..fe92e5fa95b4 100644
> --- a/net/vmw_vsock/virtio_transport_common.c
> +++ b/net/vmw_vsock/virtio_transport_common.c
> @@ -109,7 +109,8 @@ static int virtio_transport_fill_skb(struct sk_buff *skb,
>  		return __zerocopy_sg_from_iter(info->msg, NULL, skb,
>  					       &info->msg->msg_iter, len, NULL);
>  
> -	return memcpy_from_msg(skb_put(skb, len), info->msg, len);
> +	virtio_vsock_skb_put(skb, len);
> +	return skb_copy_datagram_from_iter(skb, 0, &info->msg->msg_iter, len);
>  }
>  
>  static void virtio_transport_init_hdr(struct sk_buff *skb,
> @@ -261,7 +262,7 @@ static struct sk_buff *virtio_transport_alloc_skb(struct virtio_vsock_pkt_info *
>  	if (!zcopy)
>  		skb_len += payload_len;
>  
> -	skb = virtio_vsock_alloc_linear_skb(skb_len, GFP_KERNEL);
> +	skb = virtio_vsock_alloc_skb(skb_len, GFP_KERNEL);
>  	if (!skb)
>  		return NULL;
>  
> -- 
> 2.50.0.727.gbf7dc18ff4-goog
Re: [PATCH v4 9/9] vsock/virtio: Allocate nonlinear SKBs for handling large transmit buffers
Posted by Hillf Danton 1 month, 3 weeks ago 
On Wed, 13 Aug 2025 04:41:09 -0400 "Michael S. Tsirkin" wrote:
> On Thu, Jul 17, 2025 at 10:01:16AM +0100, Will Deacon wrote:
> > When transmitting a vsock packet, virtio_transport_send_pkt_info() calls
> > virtio_transport_alloc_linear_skb() to allocate and fill SKBs with the
> > transmit data. Unfortunately, these are always linear allocations and
> > can therefore result in significant pressure on kmalloc() considering
> > that the maximum packet size (VIRTIO_VSOCK_MAX_PKT_BUF_SIZE +
> > VIRTIO_VSOCK_SKB_HEADROOM) is a little over 64KiB, resulting in a 128KiB
> > allocation for each packet.
> > 
> > Rework the vsock SKB allocation so that, for sizes with page order
> > greater than PAGE_ALLOC_COSTLY_ORDER, a nonlinear SKB is allocated
> > instead with the packet header in the SKB and the transmit data in the
> > fragments. Note that this affects both the vhost and virtio transports.
> > 
> > Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
> > Signed-off-by: Will Deacon <will@kernel.org>
> 
> So this caused a regression, see syzbot report:
> 
> https://lore.kernel.org/all/689a3d92.050a0220.7f033.00ff.GAE@google.com
> 
> I'm inclined to revert unless we have a fix quickly.
> 
Because recomputing skb len survived the syzbot test [1], Will looks innocent.

[1] https://lore.kernel.org/lkml/689c8d08.050a0220.7f033.014a.GAE@google.com/
Re: [PATCH v4 9/9] vsock/virtio: Allocate nonlinear SKBs for handling large transmit buffers
Posted by Michael S. Tsirkin 1 month, 3 weeks ago 
On Wed, Aug 13, 2025 at 09:25:53PM +0800, Hillf Danton wrote:
> On Wed, 13 Aug 2025 04:41:09 -0400 "Michael S. Tsirkin" wrote:
> > On Thu, Jul 17, 2025 at 10:01:16AM +0100, Will Deacon wrote:
> > > When transmitting a vsock packet, virtio_transport_send_pkt_info() calls
> > > virtio_transport_alloc_linear_skb() to allocate and fill SKBs with the
> > > transmit data. Unfortunately, these are always linear allocations and
> > > can therefore result in significant pressure on kmalloc() considering
> > > that the maximum packet size (VIRTIO_VSOCK_MAX_PKT_BUF_SIZE +
> > > VIRTIO_VSOCK_SKB_HEADROOM) is a little over 64KiB, resulting in a 128KiB
> > > allocation for each packet.
> > > 
> > > Rework the vsock SKB allocation so that, for sizes with page order
> > > greater than PAGE_ALLOC_COSTLY_ORDER, a nonlinear SKB is allocated
> > > instead with the packet header in the SKB and the transmit data in the
> > > fragments. Note that this affects both the vhost and virtio transports.
> > > 
> > > Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
> > > Signed-off-by: Will Deacon <will@kernel.org>
> > 
> > So this caused a regression, see syzbot report:
> > 
> > https://lore.kernel.org/all/689a3d92.050a0220.7f033.00ff.GAE@google.com
> > 
> > I'm inclined to revert unless we have a fix quickly.
> > 
> Because recomputing skb len survived the syzbot test [1], Will looks innocent.
> 
> [1] https://lore.kernel.org/lkml/689c8d08.050a0220.7f033.014a.GAE@google.com/


I'm not sure I follow that patch though. Do you mind submitting
with an explanation in the commit log?


-- 
MST
Re: [PATCH v4 9/9] vsock/virtio: Allocate nonlinear SKBs for handling large transmit buffers
Posted by Hillf Danton 1 month, 3 weeks ago 
On Fri, 15 Aug 2025 06:22:56 -0400 "Michael S. Tsirkin" wrote:
> On Wed, Aug 13, 2025 at 09:25:53PM +0800, Hillf Danton wrote:
> > On Wed, 13 Aug 2025 04:41:09 -0400 "Michael S. Tsirkin" wrote:
> > > On Thu, Jul 17, 2025 at 10:01:16AM +0100, Will Deacon wrote:
> > > > When transmitting a vsock packet, virtio_transport_send_pkt_info() calls
> > > > virtio_transport_alloc_linear_skb() to allocate and fill SKBs with the
> > > > transmit data. Unfortunately, these are always linear allocations and
> > > > can therefore result in significant pressure on kmalloc() considering
> > > > that the maximum packet size (VIRTIO_VSOCK_MAX_PKT_BUF_SIZE +
> > > > VIRTIO_VSOCK_SKB_HEADROOM) is a little over 64KiB, resulting in a 128KiB
> > > > allocation for each packet.
> > > > 
> > > > Rework the vsock SKB allocation so that, for sizes with page order
> > > > greater than PAGE_ALLOC_COSTLY_ORDER, a nonlinear SKB is allocated
> > > > instead with the packet header in the SKB and the transmit data in the
> > > > fragments. Note that this affects both the vhost and virtio transports.
> > > > 
> > > > Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
> > > > Signed-off-by: Will Deacon <will@kernel.org>
> > > 
> > > So this caused a regression, see syzbot report:
> > > 
> > > https://lore.kernel.org/all/689a3d92.050a0220.7f033.00ff.GAE@google.com
> > > 
> > > I'm inclined to revert unless we have a fix quickly.
> > > 
> > Because recomputing skb len survived the syzbot test [1], Will looks innocent.
> > 
> > [1] https://lore.kernel.org/lkml/689c8d08.050a0220.7f033.014a.GAE@google.com/
> 
> I'm not sure I follow that patch though. Do you mind submitting
> with an explanation in the commit log?
> 
It is a simple debug patch to test if Will's work is good at least in the
syzbot scenario, but stil a couple miles away from a patch with the SOB tag.
Re: [PATCH v4 9/9] vsock/virtio: Allocate nonlinear SKBs for handling large transmit buffers
Posted by Michael S. Tsirkin 1 month, 3 weeks ago 
On Fri, Aug 15, 2025 at 08:07:46PM +0800, Hillf Danton wrote:
> On Fri, 15 Aug 2025 06:22:56 -0400 "Michael S. Tsirkin" wrote:
> > On Wed, Aug 13, 2025 at 09:25:53PM +0800, Hillf Danton wrote:
> > > On Wed, 13 Aug 2025 04:41:09 -0400 "Michael S. Tsirkin" wrote:
> > > > On Thu, Jul 17, 2025 at 10:01:16AM +0100, Will Deacon wrote:
> > > > > When transmitting a vsock packet, virtio_transport_send_pkt_info() calls
> > > > > virtio_transport_alloc_linear_skb() to allocate and fill SKBs with the
> > > > > transmit data. Unfortunately, these are always linear allocations and
> > > > > can therefore result in significant pressure on kmalloc() considering
> > > > > that the maximum packet size (VIRTIO_VSOCK_MAX_PKT_BUF_SIZE +
> > > > > VIRTIO_VSOCK_SKB_HEADROOM) is a little over 64KiB, resulting in a 128KiB
> > > > > allocation for each packet.
> > > > > 
> > > > > Rework the vsock SKB allocation so that, for sizes with page order
> > > > > greater than PAGE_ALLOC_COSTLY_ORDER, a nonlinear SKB is allocated
> > > > > instead with the packet header in the SKB and the transmit data in the
> > > > > fragments. Note that this affects both the vhost and virtio transports.
> > > > > 
> > > > > Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
> > > > > Signed-off-by: Will Deacon <will@kernel.org>
> > > > 
> > > > So this caused a regression, see syzbot report:
> > > > 
> > > > https://lore.kernel.org/all/689a3d92.050a0220.7f033.00ff.GAE@google.com
> > > > 
> > > > I'm inclined to revert unless we have a fix quickly.
> > > > 
> > > Because recomputing skb len survived the syzbot test [1], Will looks innocent.
> > > 
> > > [1] https://lore.kernel.org/lkml/689c8d08.050a0220.7f033.014a.GAE@google.com/
> > 
> > I'm not sure I follow that patch though. Do you mind submitting
> > with an explanation in the commit log?
> > 
> It is a simple debug patch to test if Will's work is good at least in the
> syzbot scenario, but stil a couple miles away from a patch with the SOB tag.

Oh that makes sense then.

Patchew 2.1-devel-b67e4c2

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