[PATCH v4 04/29] scsi: core: Support allocating a pseudo SCSI device

[Bart Van Assche <bvanassche@acm.org>]

From: Hannes Reinecke <hare@suse.de>

Allocate a pseudo SCSI device if 'nr_reserved_cmds' has been set. Pseudo
SCSI devices have the SCSI ID <max_id>:U64_MAX so they won't clash with
any devices the LLD might create. Pseudo SCSI devices are excluded from
scanning and will not show up in sysfs. Additionally, pseudo SCSI
devices are skipped by shost_for_each_device(). This prevents that the
SCSI error handler tries to submit a reset to a non-existent logical unit.

Do not allocate a budget map for pseudo SCSI devices since the
cmd_per_lun limit does not apply to pseudo SCSI devices.

Do not perform queue depth ramp up / ramp down for pseudo SCSI devices.

Pseudo SCSI devices will be used to send internal commands to a storage
device.

Cc: John Garry <john.g.garry@oracle.com>
Signed-off-by: Hannes Reinecke <hare@suse.de>
[ bvanassche: edited patch description / renamed host_sdev into
  pseudo_sdev / unexported scsi_get_host_dev() / modified error path in
  scsi_get_pseudo_dev() / skip pseudo devices in __scsi_iterate_devices()
  and also when calling sdev_init(), sdev_configure() and sdev_destroy().
  See also
  https://lore.kernel.org/linux-scsi/20211125151048.103910-2-hare@suse.de/ ]
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
---
 drivers/scsi/hosts.c       |  8 +++++
 drivers/scsi/scsi.c        |  9 +++--
 drivers/scsi/scsi_error.c  |  3 ++
 drivers/scsi/scsi_priv.h   |  2 ++
 drivers/scsi/scsi_scan.c   | 69 +++++++++++++++++++++++++++++++++++++-
 drivers/scsi/scsi_sysfs.c  |  5 ++-
 include/scsi/scsi_device.h | 16 +++++++++
 include/scsi/scsi_host.h   |  6 ++++
 8 files changed, 114 insertions(+), 4 deletions(-)

diff --git a/drivers/scsi/hosts.c b/drivers/scsi/hosts.c
index 9bb7f0114763..986586bf67dc 100644
--- a/drivers/scsi/hosts.c
+++ b/drivers/scsi/hosts.c
@@ -307,6 +307,14 @@ int scsi_add_host_with_dma(struct Scsi_Host *shost, struct device *dev,
 	if (error)
 		goto out_del_dev;
 
+	if (sht->nr_reserved_cmds) {
+		shost->pseudo_sdev = scsi_get_pseudo_dev(shost);
+		if (!shost->pseudo_sdev) {
+			error = -ENOMEM;
+			goto out_del_dev;
+		}
+	}
+
 	scsi_proc_host_add(shost);
 	scsi_autopm_put_host(shost);
 	return error;
diff --git a/drivers/scsi/scsi.c b/drivers/scsi/scsi.c
index ff6b0973d3b4..2d2a52c3ef49 100644
--- a/drivers/scsi/scsi.c
+++ b/drivers/scsi/scsi.c
@@ -257,6 +257,8 @@ EXPORT_SYMBOL(scsi_change_queue_depth);
  */
 int scsi_track_queue_full(struct scsi_device *sdev, int depth)
 {
+	if (scsi_device_is_pseudo_dev(sdev))
+		return 0;
 
 	/*
 	 * Don't let QUEUE_FULLs on the same
@@ -828,8 +830,11 @@ struct scsi_device *__scsi_iterate_devices(struct Scsi_Host *shost,
 	spin_lock_irqsave(shost->host_lock, flags);
 	while (list->next != &shost->__devices) {
 		next = list_entry(list->next, struct scsi_device, siblings);
-		/* skip devices that we can't get a reference to */
-		if (!scsi_device_get(next))
+		/*
+		 * Skip pseudo devices and also devices for which
+		 * scsi_device_get() fails.
+		 */
+		if (!scsi_device_is_pseudo_dev(next) && !scsi_device_get(next))
 			break;
 		next = NULL;
 		list = list->next;
diff --git a/drivers/scsi/scsi_error.c b/drivers/scsi/scsi_error.c
index 746ff6a1f309..540d82974529 100644
--- a/drivers/scsi/scsi_error.c
+++ b/drivers/scsi/scsi_error.c
@@ -749,6 +749,9 @@ static void scsi_handle_queue_ramp_up(struct scsi_device *sdev)
 	const struct scsi_host_template *sht = sdev->host->hostt;
 	struct scsi_device *tmp_sdev;
 
+	if (scsi_device_is_pseudo_dev(sdev))
+		return;
+
 	if (!sht->track_queue_depth ||
 	    sdev->queue_depth >= sdev->max_queue_depth)
 		return;
diff --git a/drivers/scsi/scsi_priv.h b/drivers/scsi/scsi_priv.h
index 5b2b19f5e8ec..da3bc87ac5a6 100644
--- a/drivers/scsi/scsi_priv.h
+++ b/drivers/scsi/scsi_priv.h
@@ -135,6 +135,8 @@ extern int scsi_complete_async_scans(void);
 extern int scsi_scan_host_selected(struct Scsi_Host *, unsigned int,
 				   unsigned int, u64, enum scsi_scan_mode);
 extern void scsi_forget_host(struct Scsi_Host *);
+struct scsi_device *scsi_get_pseudo_dev(struct Scsi_Host *);
+bool scsi_device_is_pseudo_dev(struct scsi_device *sdev);
 
 /* scsi_sysctl.c */
 #ifdef CONFIG_SYSCTL
diff --git a/drivers/scsi/scsi_scan.c b/drivers/scsi/scsi_scan.c
index de039efef290..a3523f964bc1 100644
--- a/drivers/scsi/scsi_scan.c
+++ b/drivers/scsi/scsi_scan.c
@@ -225,6 +225,8 @@ static int scsi_realloc_sdev_budget_map(struct scsi_device *sdev,
 	int ret;
 	struct sbitmap sb_backup;
 
+	WARN_ON_ONCE(scsi_device_is_pseudo_dev(sdev));
+
 	depth = min_t(unsigned int, depth, scsi_device_max_queue_depth(sdev));
 
 	/*
@@ -349,6 +351,9 @@ static struct scsi_device *scsi_alloc_sdev(struct scsi_target *starget,
 
 	scsi_sysfs_device_initialize(sdev);
 
+	if (scsi_device_is_pseudo_dev(sdev))
+		return sdev;
+
 	depth = sdev->host->cmd_per_lun ?: 1;
 
 	/*
@@ -1070,6 +1075,9 @@ static int scsi_add_lun(struct scsi_device *sdev, unsigned char *inq_result,
 
 	sdev->sdev_bflags = *bflags;
 
+	if (scsi_device_is_pseudo_dev(sdev))
+		return SCSI_SCAN_LUN_PRESENT;
+
 	/*
 	 * No need to freeze the queue as it isn't reachable to anyone else yet.
 	 */
@@ -1213,6 +1221,12 @@ static int scsi_probe_and_add_lun(struct scsi_target *starget,
 	if (!sdev)
 		goto out;
 
+	if (scsi_device_is_pseudo_dev(sdev)) {
+		if (bflagsp)
+			*bflagsp = BLIST_NOLUN;
+		return SCSI_SCAN_LUN_PRESENT;
+	}
+
 	result = kmalloc(result_len, GFP_KERNEL);
 	if (!result)
 		goto out_free_sdev;
@@ -2084,12 +2098,65 @@ void scsi_forget_host(struct Scsi_Host *shost)
  restart:
 	spin_lock_irqsave(shost->host_lock, flags);
 	list_for_each_entry(sdev, &shost->__devices, siblings) {
-		if (sdev->sdev_state == SDEV_DEL)
+		if (scsi_device_is_pseudo_dev(sdev) ||
+		    sdev->sdev_state == SDEV_DEL)
 			continue;
 		spin_unlock_irqrestore(shost->host_lock, flags);
 		__scsi_remove_device(sdev);
 		goto restart;
 	}
 	spin_unlock_irqrestore(shost->host_lock, flags);
+
+	/*
+	 * Remove the pseudo device last since it may be needed during removal
+	 * of other SCSI devices.
+	 */
+	if (shost->pseudo_sdev)
+		__scsi_remove_device(shost->pseudo_sdev);
 }
 
+/**
+ * scsi_get_pseudo_dev() - Attach a pseudo SCSI device to a SCSI host
+ * @shost: Host that needs a pseudo SCSI device
+ *
+ * Lock status: None assumed.
+ *
+ * Returns:     The scsi_device or NULL
+ *
+ * Notes:
+ *	Attach a single scsi_device to the Scsi_Host. The primary aim for this
+ *	device is to serve as a container from which SCSI commands can be
+ *	allocated. Each SCSI command will carry a command tag allocated by the
+ *	block layer. These SCSI commands can be used by the LLDD to send
+ *	internal or passthrough commands without having to manage tag allocation
+ *	inside the LLDD.
+ */
+struct scsi_device *scsi_get_pseudo_dev(struct Scsi_Host *shost)
+{
+	struct scsi_device *sdev = NULL;
+	struct scsi_target *starget;
+
+	guard(mutex)(&shost->scan_mutex);
+
+	if (!scsi_host_scan_allowed(shost))
+		goto out;
+
+	starget = scsi_alloc_target(&shost->shost_gendev, 0, shost->max_id);
+	if (!starget)
+		goto out;
+
+	sdev = scsi_alloc_sdev(starget, U64_MAX, NULL);
+	if (!sdev) {
+		scsi_target_reap(starget);
+		goto put_target;
+	}
+
+	sdev->borken = 0;
+
+put_target:
+	/* See also the get_device(dev) call in scsi_alloc_target(). */
+	put_device(&starget->dev);
+
+out:
+	return sdev;
+}
diff --git a/drivers/scsi/scsi_sysfs.c b/drivers/scsi/scsi_sysfs.c
index 169af7d47ce7..22f76a1ca23b 100644
--- a/drivers/scsi/scsi_sysfs.c
+++ b/drivers/scsi/scsi_sysfs.c
@@ -1406,6 +1406,9 @@ int scsi_sysfs_add_sdev(struct scsi_device *sdev)
 	int error;
 	struct scsi_target *starget = sdev->sdev_target;
 
+	if (WARN_ON_ONCE(scsi_device_is_pseudo_dev(sdev)))
+		return -EINVAL;
+
 	error = scsi_target_add(starget);
 	if (error)
 		return error;
@@ -1513,7 +1516,7 @@ void __scsi_remove_device(struct scsi_device *sdev)
 	kref_put(&sdev->host->tagset_refcnt, scsi_mq_free_tags);
 	cancel_work_sync(&sdev->requeue_work);
 
-	if (sdev->host->hostt->sdev_destroy)
+	if (!scsi_device_is_pseudo_dev(sdev) && sdev->host->hostt->sdev_destroy)
 		sdev->host->hostt->sdev_destroy(sdev);
 	transport_destroy_device(dev);
 
diff --git a/include/scsi/scsi_device.h b/include/scsi/scsi_device.h
index 6d6500148c4b..3846f5dfc51c 100644
--- a/include/scsi/scsi_device.h
+++ b/include/scsi/scsi_device.h
@@ -589,6 +589,22 @@ static inline unsigned int sdev_id(struct scsi_device *sdev)
 #define scmd_id(scmd) sdev_id((scmd)->device)
 #define scmd_channel(scmd) sdev_channel((scmd)->device)
 
+/**
+ * scsi_device_is_pseudo_dev() - Whether a device is a pseudo SCSI device.
+ * @sdev: SCSI device to examine
+ *
+ * A pseudo SCSI device can be used to allocate SCSI commands but does not show
+ * up in sysfs. Additionally, the logical unit information in *@sdev is made up.
+ *
+ * This function tests the LUN number instead of comparing @sdev with
+ * @sdev->host->pseudo_sdev because this function may be called before
+ * @sdev->host->pseudo_sdev has been initialized.
+ */
+static inline bool scsi_device_is_pseudo_dev(struct scsi_device *sdev)
+{
+	return sdev->lun == U64_MAX;
+}
+
 /*
  * checks for positions of the SCSI state machine
  */
diff --git a/include/scsi/scsi_host.h b/include/scsi/scsi_host.h
index 91eb3f52b3d0..3bfb53cf5dfc 100644
--- a/include/scsi/scsi_host.h
+++ b/include/scsi/scsi_host.h
@@ -721,6 +721,12 @@ struct Scsi_Host {
 	/* ldm bits */
 	struct device		shost_gendev, shost_dev;
 
+	/*
+	 * A SCSI device structure used for sending internal commands to the
+	 * HBA. There is no corresponding logical unit inside the SCSI device.
+	 */
+	struct scsi_device *pseudo_sdev;
+
 	/*
 	 * Points to the transport data (if any) which is allocated
 	 * separately