linux-kernel - [PATCH 4.4 133/192] [PATCH 4.4 02/16] cxlflash: Fix to avoid virtual LUN failover failure

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Message-Id: <20160912152204.611841193@linuxfoundation.org>
Date:   Mon, 12 Sep 2016 19:00:42 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        "Matthew R. Ochs" <mrochs@...ux.vnet.ibm.com>,
        Manoj Kumar <manoj@...ux.vnet.ibm.com>,
        Uma Krishnan <ukrishn@...ux.vnet.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Sasha Levin <alexander.levin@...izon.com>
Subject: [PATCH 4.4 133/192] [PATCH 4.4 02/16] cxlflash: Fix to avoid virtual LUN failover failure

4.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: "Matthew R. Ochs" <mrochs@...ux.vnet.ibm.com>

[ Upstream commit d5e26bb1d812ba74f29b6bcbc88c3dbfb3eed824 ]

Applications which use virtual LUN's that are backed by a physical LUN
over both adapter ports may experience an I/O failure in the event of a
link loss (e.g. cable pull).

Virtual LUNs may be accessed through one or both ports of the adapter.
This access is encoded in the translation entries that comprise the
virtual LUN and used by the AFU for load-balancing I/O and handling
failover scenarios. In a link loss scenario, even though the AFU is able
to maintain connectivity to the LUN, it is up to the application to
retry the failed I/O. When applications are unaware of the virtual LUN's
underlying topology, they are unable to make a sound decision of when to
retry an I/O and therefore are forced to make their reaction to a failed
I/O absolute. The result is either a failure to retry I/O or increased
latency for scenarios where a retry is pointless.

To remedy this scenario, provide feedback back to the application on
virtual LUN creation as to which ports the LUN may be accessed. LUN's
spanning both ports are candidates for a retry in a presence of an I/O
failure.

Signed-off-by: Matthew R. Ochs <mrochs@...ux.vnet.ibm.com>
Acked-by: Manoj Kumar <manoj@...ux.vnet.ibm.com>
Reviewed-by: Uma Krishnan <ukrishn@...ux.vnet.ibm.com>
Signed-off-by: Martin K. Petersen <martin.petersen@...cle.com>
Signed-off-by: Sasha Levin <alexander.levin@...izon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 drivers/scsi/cxlflash/vlun.c       |    2 ++
 include/uapi/scsi/cxlflash_ioctl.h |   10 ++++++++++
 2 files changed, 12 insertions(+)

--- a/drivers/scsi/cxlflash/vlun.c
+++ b/drivers/scsi/cxlflash/vlun.c
@@ -1008,6 +1008,8 @@ int cxlflash_disk_virtual_open(struct sc
 	virt->last_lba = last_lba;
 	virt->rsrc_handle = rsrc_handle;

+	if (lli->port_sel == BOTH_PORTS)
+		virt->hdr.return_flags |= DK_CXLFLASH_ALL_PORTS_ACTIVE;
 out:
 	if (likely(ctxi))
 		put_context(ctxi);
--- a/include/uapi/scsi/cxlflash_ioctl.h
+++ b/include/uapi/scsi/cxlflash_ioctl.h
@@ -31,6 +31,16 @@ struct dk_cxlflash_hdr {
 };

 /*
+ * Return flag definitions available to all ioctls
+ *
+ * Similar to the input flags, these are grown from the bottom-up with the
+ * intention that ioctl-specific return flag definitions would grow from the
+ * top-down, allowing the two sets to co-exist. While not required/enforced
+ * at this time, this provides future flexibility.
+ */
+#define DK_CXLFLASH_ALL_PORTS_ACTIVE	0x0000000000000001ULL
+
+/*
  * Notes:
  * -----
  * The 'context_id' field of all ioctl structures contains the context