sunrpc: keep a count of swapfiles associated with the rpc_clnt

Jerome reported seeing a warning pop when working with a swapfile on
NFS. The nfs_swap_activate can end up calling sk_set_memalloc while
holding the rcu_read_lock and that function can sleep.

To fix that, we need to take a reference to the xprt while holding the
rcu_read_lock, set the socket up for swapping and then drop that
reference. But, xprt_put is not exported and having NFS deal with the
underlying xprt is a bit of layering violation anyway.

Fix this by adding a set of activate/deactivate functions that take a
rpc_clnt pointer instead of an rpc_xprt, and have nfs_swap_activate and
nfs_swap_deactivate call those.

Also, add a per-rpc_clnt atomic counter to keep track of the number of
active swapfiles associated with it. When the counter does a 0->1
transition, we enable swapping on the xprt, when we do a 1->0 transition
we disable swapping on it.

This also allows us to be a bit more selective with the RPC_TASK_SWAPPER
flag. If non-swapper and swapper clnts are sharing a xprt, then we only
need to flag the tasks from the swapper clnt with that flag.

Acked-by: Mel Gorman <mgorman@suse.de>
Reported-by: Jerome Marchand <jmarchan@redhat.com>
Signed-off-by: Jeff Layton <jeff.layton@primarydata.com>
Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>

1 files changed, 16 insertions, 0 deletions

diff --git a/include/linux/sunrpc/sched.h b/include/linux/sunrpc/sched.h
index 2aa68976a482..d703f0ef37d8 100644
--- a/include/linux/sunrpc/sched.h
+++ b/include/linux/sunrpc/sched.h
@@ -268,4 +268,20 @@ static inline void rpc_assign_waitqueue_name(struct rpc_wait_queue *q,
 }
 #endif
 
+#if IS_ENABLED(CONFIG_SUNRPC_SWAP)
+int rpc_clnt_swap_activate(struct rpc_clnt *clnt);
+void rpc_clnt_swap_deactivate(struct rpc_clnt *clnt);
+#else
+static inline int
+rpc_clnt_swap_activate(struct rpc_clnt *clnt)
+{
+	return -EINVAL;
+}
+
+static inline void
+rpc_clnt_swap_deactivate(struct rpc_clnt *clnt)
+{
+}
+#endif /* CONFIG_SUNRPC_SWAP */
+
 #endif /* _LINUX_SUNRPC_SCHED_H_ */