[PATCH] SUNRPC: backchannel RPC request must reference XPRT

[NeilBrown <neilb@suse.de>]

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The backchannel RPC requests - that are queued waiting
for the reply to be sent by the "NFSv4 callback" thread -
have a pointer to the xprt, but it is not reference counted.
It is possible for the xprt to be freed while there are
still queued requests.

I think this has been a problem since
Commit fb7a0b9addbd ("nfs41: New backchannel helper routines")
when the code was introduced, but I suspect it became more of
a problem after
Commit 80b14d5e61ca ("SUNRPC: Add a structure to track multiple transports")
(or there abouts).
Before this second patch, the nfs client would hold a reference to
the xprt to keep it alive.  After multipath was introduced,
a client holds a reference to a swtich, and the switch can have multiple
xprts which can be added and removed.

I'm not sure of all the causal issues, but this patch has
fixed a customer problem were an NFSv4.1 client would run out
of memory with tens of thousands of backchannel rpc requests
queued for an xprt that had been freed.  This was a 64K-page
machine so each rpc_rqst consumed more than 128K of memory.

Fixes: 80b14d5e61ca ("SUNRPC: Add a structure to track multiple transports")
cc: stable@vger.kernel.org (v4.6)
Signed-off-by: NeilBrown <neilb@suse.de>
---
 net/sunrpc/backchannel_rqst.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/sunrpc/backchannel_rqst.c b/net/sunrpc/backchannel_rqst.c
index 339e8c077c2d..c95ca39688b6 100644
--- a/net/sunrpc/backchannel_rqst.c
+++ b/net/sunrpc/backchannel_rqst.c
@@ -61,6 +61,7 @@ static void xprt_free_allocation(struct rpc_rqst *req)
 	free_page((unsigned long)xbufp->head[0].iov_base);
 	xbufp = &req->rq_snd_buf;
 	free_page((unsigned long)xbufp->head[0].iov_base);
+	xprt_put(req->rq_xprt);
 	kfree(req);
 }
 
@@ -85,7 +86,7 @@ struct rpc_rqst *xprt_alloc_bc_req(struct rpc_xprt *xprt, gfp_t gfp_flags)
 	if (req == NULL)
 		return NULL;
 
-	req->rq_xprt = xprt;
+	req->rq_xprt = xprt_get(xprt);
 	INIT_LIST_HEAD(&req->rq_bc_list);
 
 	/* Preallocate one XDR receive buffer */
-- 
2.23.0


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