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@xxxxxxxxxxxxxxx (v4.6)
Signed-off-by: NeilBrown <neilb@xxxxxxx>
---
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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