Re: [PATCH net] vhost/vsock: trim nonlinear SKBs to declared payload length

[sashiko-bot@kernel.org]

> vhost_vsock_alloc_skb() sizes its skb from the total guest descriptor
> length, while virtio_vsock_hdr.len independently declares the payload
> length.  Since commit ab9aa2f3afc2 ("vhost/vsock: Allocate nonlinear SKBs
> for handling large receive buffers"), large descriptors use skb fragments.
> 
> virtio_vsock_skb_put() currently changes only skb->len for a nonlinear
> skb, leaving skb->data_len and all fragments attached.  The zero-payload
> fast path skips the helper entirely.  A guest can therefore retain the
> full descriptor allocation while receive credit accounts no payload.
> 
> On Linux v7.2, 455 zero-payload skbs retained 30,255,680 bytes on a
> 256 KiB receive buffer while rx_bytes and buf_used remained zero.  A
> full-payload control retained 265,984 bytes in four skbs.  The existing
> SKB_TRUESIZE(0) queue budget caps skb count but does not account for these
> descriptor-sized fragments.
> [ ... ]
> 
> Fixes: ab9aa2f3afc2 ("vhost/vsock: Allocate nonlinear SKBs for handling large receive buffers")
> Cc: stable@vger.kernel.org
> Assisted-by: LLM
> Signed-off-by: Daehyeon Ko <4ncienth@gmail.com>

Sashiko has reviewed this patch and found no issues. It looks great!

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