Subject: Re: [PATCH 4/4] nfs: deadlock prevention for NFS

From: Trond Myklebust <[email-masked]>
Date: Fri, 25 Aug 2006 16:14:40 -0400

Grumble... If your patches are targetting NFS, could you please at the
very least Cc [email-masked] and/or myself.

On Fri, 2006-08-25 at 17:40 +0200, Peter Zijlstra wrote:
> Provide a proper a_ops->swapfile() implementation for NFS. This will
> set the NFS socket to SOCK_VMIO and put the socket reconnection under
> PF_MEMALLOC (I hope this is enough, otherwise more work needs to be done).
> 
> Signed-off-by: Peter Zijlstra <[email-masked]>
> ---
>  fs/nfs/file.c               |   21 ++++++++++++++++++++-
>  include/linux/sunrpc/xprt.h |    4 +++-
>  net/sunrpc/xprtsock.c       |   16 ++++++++++++++++
>  3 files changed, 39 insertions(+), 2 deletions(-)
> 
> Index: linux-2.6/fs/nfs/file.c
> ===================================================================
> --- linux-2.6.orig/fs/nfs/file.c
> +++ linux-2.6/fs/nfs/file.c
> @@ -27,6 +27,7 @@
>  #include <linux/slab.h>
>  #include <linux/pagemap.h>
>  #include <linux/smp_lock.h>
> +#include <net/sock.h>
>  
>  #include <asm/uaccess.h>
>  #include <asm/system.h>
> @@ -317,7 +318,25 @@ static int nfs_release_page(struct page 
>  
>  static int nfs_swapfile(struct address_space *mapping, int enable)
>  {
> -	return 0;
> +	int err = -EINVAL;
> +	struct rpc_clnt *client = NFS_CLIENT(mapping->host);
> +	struct sock *sk = client->cl_xprt->inet;
> +
> +	if (enable) {
> +		client->cl_xprt->swapper = 1;
> +		/*
> +		 * keep one extra sock reference so the reserve won't dip
> +		 * when the socket gets reconnected.
> +		 */
> +		sk_adjust_memalloc(1, 1);
> +		err = sk_set_vmio(sk);
> +	} else if (client->cl_xprt->swapper) {
> +		client->cl_xprt->swapper = 0;
> +		sk_adjust_memalloc(-1, -1);
> +		err = sk_clear_vmio(sk);
> +	}
> +
> +	return err;
>  }

This all belongs in net/sunrpc/xprtsock.c. The NFS code has no business
screwing around with the internals of the sunrpc transport.

>  const struct address_space_operations nfs_file_aops = {
> Index: linux-2.6/net/sunrpc/xprtsock.c
> ===================================================================
> --- linux-2.6.orig/net/sunrpc/xprtsock.c
> +++ linux-2.6/net/sunrpc/xprtsock.c
> @@ -1014,6 +1014,7 @@ static void xs_udp_connect_worker(void *
>  {
>  	struct rpc_xprt *xprt = (struct rpc_xprt *) args;
>  	struct socket *sock = xprt->sock;
> +	unsigned long pflags = current->flags;
>  	int err, status = -EIO;
>  
>  	if (xprt->shutdown || xprt->addr.sin_port == 0)
> @@ -1021,6 +1022,9 @@ static void xs_udp_connect_worker(void *
>  
>  	dprintk("RPC:      xs_udp_connect_worker for xprt %p\n", xprt);
>  
> +	if (xprt->swapper)
> +		current->flags |= PF_MEMALLOC;
> +
>  	/* Start by resetting any existing state */
>  	xs_close(xprt);
>  
> @@ -1054,6 +1058,9 @@ static void xs_udp_connect_worker(void *
>  		xprt->sock = sock;
>  		xprt->inet = sk;
>  
> +		if (xprt->swapper)
> +			sk_set_vmio(sk);
> +
>  		write_unlock_bh(&sk->sk_callback_lock);
>  	}
>  	xs_udp_do_set_buffer_size(xprt);
> @@ -1061,6 +1068,7 @@ static void xs_udp_connect_worker(void *
>  out:
>  	xprt_wake_pending_tasks(xprt, status);
>  	xprt_clear_connecting(xprt);
> +	current->flags = pflags;
>  }
>  
>  /*
> @@ -1097,11 +1105,15 @@ static void xs_tcp_connect_worker(void *
>  {
>  	struct rpc_xprt *xprt = (struct rpc_xprt *)args;
>  	struct socket *sock = xprt->sock;
> +	unsigned long pflags = current->flags;
>  	int err, status = -EIO;
>  
>  	if (xprt->shutdown || xprt->addr.sin_port == 0)
>  		goto out;
>  
> +	if (xprt->swapper)
> +		current->flags |= PF_MEMALLOC;
> +
>  	dprintk("RPC:      xs_tcp_connect_worker for xprt %p\n", xprt);
>  
>  	if (!xprt->sock) {
> @@ -1170,10 +1182,14 @@ static void xs_tcp_connect_worker(void *
>  				break;
>  		}
>  	}
> +
> +	if (xprt->swapper)
> +		sk_set_vmio(xprt->inet);
>  out:
>  	xprt_wake_pending_tasks(xprt, status);
>  out_clear:
>  	xprt_clear_connecting(xprt);
> +	current->flags = pflags;
>  }

How does this guarantee that the socket reconnection won't fail?

Also, what about the case of rpc_malloc()? Can't that cause rpciod to
deadlock when you add NFS swap into the equation?

Cheers,
  Trond