Re: [PATCH net-next v2] netlink: Trim skb to alloc size to avoid MSG_TRUNC

[David Miller <davem@davemloft.net>]

From: Ronen Arad <ronen.arad@intel.com>
Date: Thu, 15 Oct 2015 01:55:17 -0700

> netlink_dump() allocates skb based on the calculated min_dump_alloc or
> a per socket max_recvmsg_len.
> min_alloc_size is maximum space required for any single netdev
> attributes as calculated by rtnl_calcit().
> max_recvmsg_len tracks the user provided buffer to netlink_recvmsg.
> It is capped at 16KiB.
> The intention is to avoid small allocations and to minimize the number
> of calls required to obtain dump information for all net devices.
> 
> netlink_dump packs as many small messages as could fit within an skb
> that was sized for the largest single netdev information. The actual
> space available within an skb is larger than what is requested. It could
> be much larger and up to near 2x with align to next power of 2 approach.
> 
> Allowing netlink_dump to use all the space available within the
> allocated skb increases the buffer size a user has to provide to avoid
> truncaion (i.e. MSG_TRUNG flag set).
> 
> It was observed that with many VLANs configured on at least one netdev,
> a larger buffer of near 64KiB was necessary to avoid "Message truncated"
> error in "ip link" or "bridge [-c[ompressvlans]] vlan show" when
> min_alloc_size was only little over 32KiB.
> 
> This patch trims skb to allocated size in order to allow the user to
> avoid truncation with more reasonable buffer size.
> 
> Signed-off-by: Ronen Arad <ronen.arad@intel.com>

Getting spurious MSG_TRUNC is surely a bug, so I am going to apply
this to 'net' as it is a genuine bug fix.

Thanks.