Received-SPF: pass (google.com: best guess record for domain of linux-nfs-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Content-Type: text/plain;
        charset=us-ascii
Mime-Version: 1.0 (Mac OS X Mail 12.4 \(3445.104.11\))
Subject: Re: [PATCH] SUNRPC: backchannel RPC request must reference XPRT
From:   Chuck Lever <chuck.lever@oracle.com>
In-Reply-To: <711ebfa5340c6e29ff640e855db5ad8e41a09a60.camel@hammerspace.com>
Date:   Tue, 15 Oct 2019 17:47:09 -0400
Cc:     Bruce Fields <bfields@fieldses.org>, Neil Brown <neilb@suse.de>,
        Anna Schumaker <anna.schumaker@netapp.com>,
        Linux NFS Mailing List <linux-nfs@vger.kernel.org>
Content-Transfer-Encoding: quoted-printable
Message-Id: <94FD3327-AEE7-4160-87E4-4E7569FB8D18@oracle.com>
References: <87tv8iqz3b.fsf@notabene.neil.brown.name>
 <20191011165603.GD19318@fieldses.org>
 <87imoqrjb8.fsf@notabene.neil.brown.name>
 <711ebfa5340c6e29ff640e855db5ad8e41a09a60.camel@hammerspace.com>
To:     Trond Myklebust <trondmy@hammerspace.com>
Sender: linux-nfs-owner@vger.kernel.org
Precedence: bulk


> On Oct 15, 2019, at 5:16 PM, Trond Myklebust <trondmy@hammerspace.com> =
wrote:
>=20
> Hi Neil,
>=20
> On Tue, 2019-10-15 at 10:36 +1100, NeilBrown wrote:
>> 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.
>>=20
>> 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.
>>=20
>> 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.
>>=20
>> 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(-)
>>=20
>> 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 =3D &req->rq_snd_buf;
>> 	free_page((unsigned long)xbufp->head[0].iov_base);
>> +	xprt_put(req->rq_xprt);
>> 	kfree(req);
>> }
>=20
> Would it perhaps make better sense to move the xprt_get() to
> xprt_lookup_bc_request() and to release it in xprt_free_bc_rqst()?

/me wonders if the same problem exists for the RPC/RDMA backchannel....


> Otherwise as far as I can tell, we will have freed slots on the xprt-
>> bc_pa_list that hold a reference to the transport itself, meaning =
that
> the latter never gets released.
>=20
>>=20
>> @@ -85,7 +86,7 @@ struct rpc_rqst *xprt_alloc_bc_req(struct rpc_xprt
>> *xprt, gfp_t gfp_flags)
>> 	if (req =3D=3D NULL)
>> 		return NULL;
>>=20
>> -	req->rq_xprt =3D xprt;
>> +	req->rq_xprt =3D xprt_get(xprt);
>> 	INIT_LIST_HEAD(&req->rq_bc_list);
>>=20
>> 	/* Preallocate one XDR receive buffer */
> --=20
> Trond Myklebust
> Linux NFS client maintainer, Hammerspace
> trond.myklebust@hammerspace.com

--
Chuck Lever