Subject: Re: [PATCH net] netlink: do not enter direct reclaim from netlink_dump()

From: Greg <[email-masked]>
Date: Wed, 05 Oct 2016 12:54:43 -0700

On Thu, 2016-10-06 at 04:13 +0900, Eric Dumazet wrote:
> From: Eric Dumazet <[email-masked]>
> 
> Since linux-3.15, netlink_dump() can use up to 16384 bytes skb
> allocations.
> 
> Due to struct skb_shared_info ~320 bytes overhead, we end up using
> order-3 (on x86) page allocations, that might trigger direct reclaim and
> add stress.
> 
> The intent was really to attempt a large allocation but immediately
> fallback to a smaller one (order-1 on x86) in case of memory stress.
> 
> On recent kernels (linux-4.4), we can remove __GFP_DIRECT_RECLAIM to
> meet the goal. Old kernels would need to remove __GFP_WAIT
> 
> While we are at it, since we do an order-3 allocation, allow to use
> all the allocated bytes instead of 16384 to reduce syscalls during
> large dumps.
> 
> iproute2 already uses 32KB recvmsg() buffer sizes.
> 
> Alexei provided an initial patch downsizing to SKB_WITH_OVERHEAD(16384)
> 
> Fixes: 9063e21fb026 ("netlink: autosize skb lengthes")
> Signed-off-by: Eric Dumazet <[email-masked]>
> Reported-by: Alexei Starovoitov <[email-masked]>
> Cc: Greg Thelen <[email-masked]>
> ---
> Note: This will apply to net tree when it has synced with Linus tree.
> 
>  net/netlink/af_netlink.c |    7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
> index 627f898c05b96552318a881ce995ccc3342e1576..62bea4591054820eb516ef016214ee23fe89b6e9 100644
> --- a/net/netlink/af_netlink.c
> +++ b/net/netlink/af_netlink.c
> @@ -1832,7 +1832,7 @@ static int netlink_recvmsg(struct socket *sock, struct msghdr *msg, size_t len,
>  	/* Record the max length of recvmsg() calls for future allocations */
>  	nlk->max_recvmsg_len = max(nlk->max_recvmsg_len, len);
>  	nlk->max_recvmsg_len = min_t(size_t, nlk->max_recvmsg_len,
> -				     16384);
> +				     SKB_WITH_OVERHEAD(32768));
>  
>  	copied = data_skb->len;
>  	if (len < copied) {
> @@ -2083,8 +2083,9 @@ static int netlink_dump(struct sock *sk)
>  
>  	if (alloc_min_size < nlk->max_recvmsg_len) {
>  		alloc_size = nlk->max_recvmsg_len;
> -		skb = alloc_skb(alloc_size, GFP_KERNEL |
> -					    __GFP_NOWARN | __GFP_NORETRY);
> +		skb = alloc_skb(alloc_size,
> +				(GFP_KERNEL & ~__GFP_DIRECT_RECLAIM) |
> +				__GFP_NOWARN | __GFP_NORETRY);
>  	}
>  	if (!skb) {
>  		alloc_size = alloc_min_size;
> 
> 

This code has changed a lot since I first added it in 2011 but this
appears to be the right thing to do.  I guess the order of operations
for the bitwise '&' and the bitwise '~' are correct, I don't have my C
manual laying around.

Reviewed-by: Greg Rose <[email-masked]>