Re: [PATCH 06/19] nfsd: set PF_FSTRANS for nfsd threads.

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Apr 16, 2014 at 02:03:36PM +1000, NeilBrown wrote:
> If a localhost mount is present, then it is easy to deadlock NFS by
> nfsd entering direct reclaim and calling nfs_release_page() which
> requires nfsd to perform an fsync() (which it cannot do because it is
> reclaiming memory).
> 
> By setting PF_FSTRANS we stop the memory allocator from ever
> attempting any FS operation would could deadlock.
> 
> We need this flag set for any thread which is handling a request from
> the local host, but we also need to always have it for at least 1 or 2
> threads so that we don't end up with all threads blocked in allocation.
> 
> When we set PF_FSTRANS we also tell lockdep that we are handling
> reclaim so that it can detect deadlocks for us.
> 
> Signed-off-by: NeilBrown <neilb@suse.de>
> ---
>  fs/nfsd/nfssvc.c           |   18 ++++++++++++++++++
>  include/linux/sunrpc/svc.h |    1 +
>  net/sunrpc/svc.c           |    6 ++++++
>  3 files changed, 25 insertions(+)
> 
> diff --git a/fs/nfsd/nfssvc.c b/fs/nfsd/nfssvc.c
> index 9a4a5f9e7468..6af8bc2daf7d 100644
> --- a/fs/nfsd/nfssvc.c
> +++ b/fs/nfsd/nfssvc.c
> @@ -565,6 +565,8 @@ nfsd(void *vrqstp)
>  	struct svc_xprt *perm_sock = list_entry(rqstp->rq_server->sv_permsocks.next, typeof(struct svc_xprt), xpt_list);
>  	struct net *net = perm_sock->xpt_net;
>  	int err;
> +	unsigned int pflags = 0;
> +	gfp_t reclaim_state = 0;
>  
>  	/* Lock module and set up kernel thread */
>  	mutex_lock(&nfsd_mutex);
> @@ -611,14 +613,30 @@ nfsd(void *vrqstp)
>  			;
>  		if (err == -EINTR)
>  			break;
> +		if (rqstp->rq_local && !current_test_flags(PF_FSTRANS)) {
> +			current_set_flags_nested(&pflags, PF_FSTRANS);
> +			atomic_inc(&rqstp->rq_pool->sp_nr_fstrans);
> +			reclaim_state = lockdep_set_current_reclaim_state(GFP_KERNEL);
> +		}
>  		validate_process_creds();
>  		svc_process(rqstp);
>  		validate_process_creds();
> +		if (current_test_flags(PF_FSTRANS) &&
> +		    atomic_dec_if_positive(&rqstp->rq_pool->sp_nr_fstrans) >= 0) {

Yuck, that function has horrible semantics :/ Not your fault though.

> +			current_restore_flags_nested(&pflags, PF_FSTRANS);
> +			lockdep_restore_current_reclaim_state(reclaim_state);
> +		}


Not really knowing much about NFS anymore (all I did know seems to have
bitrotten) the above looks ok I suppose. Refcounting that task state in
something !task_struct feels weird though.

>  	}
>  
>  	/* Clear signals before calling svc_exit_thread() */
>  	flush_signals(current);
>  
> +	if (current_test_flags(PF_FSTRANS)) {
> +		current_restore_flags_nested(&pflags, PF_FSTRANS);
> +		lockdep_restore_current_reclaim_state(reclaim_state);
> +		atomic_dec(&rqstp->rq_pool->sp_nr_fstrans);
> +	}
> +
>  	mutex_lock(&nfsd_mutex);
>  	nfsdstats.th_cnt --;
>  
> diff --git a/include/linux/sunrpc/svc.h b/include/linux/sunrpc/svc.h
> index a0dbbd1e00e9..4b274aba51dd 100644
> --- a/include/linux/sunrpc/svc.h
> +++ b/include/linux/sunrpc/svc.h
> @@ -48,6 +48,7 @@ struct svc_pool {
>  	struct list_head	sp_threads;	/* idle server threads */
>  	struct list_head	sp_sockets;	/* pending sockets */
>  	unsigned int		sp_nrthreads;	/* # of threads in pool */
> +	atomic_t		sp_nr_fstrans;	/* # threads with PF_FSTRANS */
>  	struct list_head	sp_all_threads;	/* all server threads */
>  	struct svc_pool_stats	sp_stats;	/* statistics on pool operation */
>  	int			sp_task_pending;/* has pending task */
> diff --git a/net/sunrpc/svc.c b/net/sunrpc/svc.c
> index 5de6801cd924..8b13f35b6cbb 100644
> --- a/net/sunrpc/svc.c
> +++ b/net/sunrpc/svc.c
> @@ -477,6 +477,12 @@ __svc_create(struct svc_program *prog, unsigned int bufsize, int npools,
>  		INIT_LIST_HEAD(&pool->sp_threads);
>  		INIT_LIST_HEAD(&pool->sp_sockets);
>  		INIT_LIST_HEAD(&pool->sp_all_threads);
> +		/* The number of threads with PF_FSTRANS set
> +		 * should never be reduced below 2, except when
> +		 * threads exit.  So we use atomic_dec_if_positive()
> +		 * on this value.
> +		 */
> +		atomic_set(&pool->sp_nr_fstrans, -2);

Its not my code, but that's not the recommended multi-line comment
style.

>  		spin_lock_init(&pool->sp_lock);
>  	}
>  
> 
> 

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