Re: [PATCH 11/14] nfsd: don't use sv_nrthreads in connection limiting calculations.

[Jeff Layton <jlayton@kernel.org>]

On Mon, 2024-07-15 at 17:14 +1000, NeilBrown wrote:
> The heuristic for limiting the number of incoming connections to nfsd
> currently uses sv_nrthreads - allowing more connections if more threads
> were configured.
> 
> A future patch will allow number of threads to grow dynamically so that
> there is no need to configure sv_nrthreads.  So we need a different
> solution for limiting connections.
> 
> It isn't clear what problem is solved by limiting connections (as
> mentioned in a code comment) but the most likely problem is a connection
> storm - many connections that are not doing productive work.  These will
> be closed after about 6 minutes already but it might help to slow down a
> storm.
> 
> This patch add a per-connection flag XPT_PEER_VALID which indicates
> that the peer has presented a filehandle for which it has some sort of
> access.  i.e the peer is known to be trusted in some way.  We now only
> count connections which have NOT be determined to be valid.  There
> should be relative few of these at any given time.
> 
> If the number of non-validated peer exceed as limit - currently 64 - we
> close the oldest non-validated peer to avoid having too many of these
> useless connections.
> 
> Signed-off-by: NeilBrown <neilb@suse.de>
> ---
>  fs/nfsd/netns.h                 |  4 ++--
>  fs/nfsd/nfsfh.c                 |  8 ++++++++
>  include/linux/sunrpc/svc.h      |  2 +-
>  include/linux/sunrpc/svc_xprt.h |  4 ++++
>  net/sunrpc/svc_xprt.c           | 33 +++++++++++++++++----------------
>  5 files changed, 32 insertions(+), 19 deletions(-)
> 
> diff --git a/fs/nfsd/netns.h b/fs/nfsd/netns.h
> index 238fc4e56e53..0d2ac15a5003 100644
> --- a/fs/nfsd/netns.h
> +++ b/fs/nfsd/netns.h
> @@ -128,8 +128,8 @@ struct nfsd_net {
>  	unsigned char writeverf[8];
>  
>  	/*
> -	 * Max number of connections this nfsd container will allow. Defaults
> -	 * to '0' which is means that it bases this on the number of threads.
> +	 * Max number of non-validated connections this nfsd container
> +	 * will allow.  Defaults to '0' gets mapped to 64.
>  	 */
>  	unsigned int max_connections;
>  
> diff --git a/fs/nfsd/nfsfh.c b/fs/nfsd/nfsfh.c
> index 0b75305fb5f5..08742bf8de02 100644
> --- a/fs/nfsd/nfsfh.c
> +++ b/fs/nfsd/nfsfh.c
> @@ -391,6 +391,14 @@ fh_verify(struct svc_rqst *rqstp, struct svc_fh *fhp, umode_t type, int access)
>  		goto out;
>  
>  skip_pseudoflavor_check:
> +	if (test_bit(XPT_TEMP, &rqstp->rq_xprt->xpt_flags) &&
> +	    !test_and_set_bit(XPT_PEER_VALID, &rqstp->rq_xprt->xpt_flags)) {
> +		struct svc_serv *serv = rqstp->rq_server;
> +		spin_lock(&serv->sv_lock);
> +		serv->sv_tmpcnt -= 1;
> +		spin_unlock(&serv->sv_lock);
> +	}
> +

This is the only place you set XPT_PEER_VALID, but this change affects
more services than just nfsd. What about lockd? Do we need a similar
change there?

>  	/* Finally, check access permissions. */
>  	error = nfsd_permission(rqstp, exp, dentry, access);
>  out:
> diff --git a/include/linux/sunrpc/svc.h b/include/linux/sunrpc/svc.h
> index 99e9345d829e..0b414af448e0 100644
> --- a/include/linux/sunrpc/svc.h
> +++ b/include/linux/sunrpc/svc.h
> @@ -79,7 +79,7 @@ struct svc_serv {
>  	unsigned int		sv_xdrsize;	/* XDR buffer size */
>  	struct list_head	sv_permsocks;	/* all permanent sockets */
>  	struct list_head	sv_tempsocks;	/* all temporary sockets */
> -	int			sv_tmpcnt;	/* count of temporary sockets */
> +	int			sv_tmpcnt;	/* count of temporary "valid" sockets */
>  	struct timer_list	sv_temptimer;	/* timer for aging temporary sockets */
>  
>  	char *			sv_name;	/* service name */
> diff --git a/include/linux/sunrpc/svc_xprt.h b/include/linux/sunrpc/svc_xprt.h
> index 0981e35a9fed..92565133b3b6 100644
> --- a/include/linux/sunrpc/svc_xprt.h
> +++ b/include/linux/sunrpc/svc_xprt.h
> @@ -99,6 +99,10 @@ enum {
>  	XPT_HANDSHAKE,		/* xprt requests a handshake */
>  	XPT_TLS_SESSION,	/* transport-layer security established */
>  	XPT_PEER_AUTH,		/* peer has been authenticated */
> +	XPT_PEER_VALID,		/* peer has presented a filehandle that
> +				 * it has access to.  It is NOT counted
> +				 * in ->sv_tmpcnt.
> +				 */
>  };
>  
>  static inline void unregister_xpt_user(struct svc_xprt *xpt, struct svc_xpt_user *u)
> diff --git a/net/sunrpc/svc_xprt.c b/net/sunrpc/svc_xprt.c
> index 53ebc719ff5a..a9215e1a2f38 100644
> --- a/net/sunrpc/svc_xprt.c
> +++ b/net/sunrpc/svc_xprt.c
> @@ -606,7 +606,8 @@ int svc_port_is_privileged(struct sockaddr *sin)
>  }
>  
>  /*
> - * Make sure that we don't have too many active connections. If we have,
> + * Make sure that we don't have too many connections that have not yet
> + * demonstrated that they have access the the NFS server. If we have,
>   * something must be dropped. It's not clear what will happen if we allow
>   * "too many" connections, but when dealing with network-facing software,
>   * we have to code defensively. Here we do that by imposing hard limits.
> @@ -625,27 +626,26 @@ int svc_port_is_privileged(struct sockaddr *sin)
>   */
>  static void svc_check_conn_limits(struct svc_serv *serv)
>  {
> -	unsigned int limit = serv->sv_maxconn ? serv->sv_maxconn :
> -				(serv->sv_nrthreads+3) * 20;
> +	unsigned int limit = serv->sv_maxconn ? serv->sv_maxconn : 64;
>  
>  	if (serv->sv_tmpcnt > limit) {
> -		struct svc_xprt *xprt = NULL;
> +		struct svc_xprt *xprt = NULL, *xprti;
>  		spin_lock_bh(&serv->sv_lock);
>  		if (!list_empty(&serv->sv_tempsocks)) {
> -			/* Try to help the admin */
> -			net_notice_ratelimited("%s: too many open connections, consider increasing the %s\n",
> -					       serv->sv_name, serv->sv_maxconn ?
> -					       "max number of connections" :
> -					       "number of threads");
>  			/*
>  			 * Always select the oldest connection. It's not fair,
> -			 * but so is life
> +			 * but nor is life.
>  			 */
> -			xprt = list_entry(serv->sv_tempsocks.prev,
> -					  struct svc_xprt,
> -					  xpt_list);
> -			set_bit(XPT_CLOSE, &xprt->xpt_flags);
> -			svc_xprt_get(xprt);
> +			list_for_each_entry_reverse(xprti, &serv->sv_tempsocks,
> +						    xpt_list)
> +			{
> +				if (!test_bit(XPT_PEER_VALID, &xprti->xpt_flags)) {
> +					xprt = xprti;
> +					set_bit(XPT_CLOSE, &xprt->xpt_flags);
> +					svc_xprt_get(xprt);
> +					break;
> +				}
> +			}
>  		}
>  		spin_unlock_bh(&serv->sv_lock);
>  
> @@ -1039,7 +1039,8 @@ static void svc_delete_xprt(struct svc_xprt *xprt)
>  
>  	spin_lock_bh(&serv->sv_lock);
>  	list_del_init(&xprt->xpt_list);
> -	if (test_bit(XPT_TEMP, &xprt->xpt_flags))
> +	if (test_bit(XPT_TEMP, &xprt->xpt_flags) &&
> +	    !test_bit(XPT_PEER_VALID, &xprt->xpt_flags))
>  		serv->sv_tmpcnt--;
>  	spin_unlock_bh(&serv->sv_lock);
>  

-- 
Jeff Layton <jlayton@kernel.org>