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Date: Fri, 1 Jun 2018 00:11:15 -0400
From: Mike Snitzer <snitzer@...hat.com>
To: Christoph Hellwig <hch@....de>
Cc: Sagi Grimberg <sagi@...mberg.me>,
Johannes Thumshirn <jthumshirn@...e.de>,
Keith Busch <keith.busch@...el.com>,
Hannes Reinecke <hare@...e.de>,
Laurence Oberman <loberman@...hat.com>,
Ewan Milne <emilne@...hat.com>,
James Smart <james.smart@...adcom.com>,
Linux Kernel Mailinglist <linux-kernel@...r.kernel.org>,
Linux NVMe Mailinglist <linux-nvme@...ts.infradead.org>,
"Martin K . Petersen" <martin.petersen@...cle.com>,
Martin George <marting@...app.com>,
John Meneghini <John.Meneghini@...app.com>
Subject: Re: [PATCH 0/3] Provide more fine grained control over multipathing
On Thu, May 31 2018 at 12:34pm -0400,
Christoph Hellwig <hch@....de> wrote:
> On Thu, May 31, 2018 at 08:37:39AM -0400, Mike Snitzer wrote:
> > I saw your reply to the 1/3 patch.. I do agree it is broken for not
> > checking if any handles are active. But that is easily fixed no?
>
> Doing a switch at runtime simply is a really bad idea. If for some
> reason we end up with a good per-controller switch it would have
> to be something set at probe time, and to get it on a controller
> you'd need to reset it first.
Yes, I see that now. And the implementation would need to be something
yourself or other more seasoned NVMe developers pursued. NVMe code is
pretty unforgiving.
I took a crack at aspects of this, my head hurts. While testing I hit
some "interesting" lack of self-awareness about NVMe resources that are
in use. So lots of associations are able to be torn down rather than
graceful failure. Could be nvme_fcloop specific, but it is pretty easy
to do the following using mptest's lib/unittests/nvme_4port_create.sh
followed by: modprobe -r nvme_fcloop
Results in an infinite spew of:
[14245.345759] nvme_fcloop: fcloop_exit: Failed deleting remote port
[14245.351851] nvme_fcloop: fcloop_exit: Failed deleting target port
[14245.357944] nvme_fcloop: fcloop_exit: Failed deleting remote port
[14245.364038] nvme_fcloop: fcloop_exit: Failed deleting target port
Another fun one is to lib/unittests/nvme_4port_delete.sh while the
native NVMe multipath device (created from nvme_4port_create.sh) was
still in use by an xfs mount, so:
./nvme_4port_create.sh
mount /dev/nvme1n1 /mnt
./nvme_4port_delete.sh
umount /mnt
Those were clear screwups on my part but I wouldn't have expected them
to cause nvme to blow through so many stop signs.
Anyway, I put enough time to trying to make the previously thought
"simple" mpath_personality switch safe -- in the face of active handles
(issue Sagi pointed out) -- that it is clear NVMe just doesn't have
enough state to do it in a clean way. Would require a deeper
understanding of the code that I don't have. Most every NVMe function
returns void so there is basically no potential for error handling (in
the face of a resource being in use).
The following is my WIP patch (built ontop of the 3 patches from
this thread's series) that has cured me of wanting to continue pursuit
of a robust implementation of the runtime 'mpath_personality' switch:
diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
index 1e018d0..80103b3 100644
--- a/drivers/nvme/host/core.c
+++ b/drivers/nvme/host/core.c
@@ -2146,10 +2146,8 @@ static ssize_t __nvme_subsys_store_mpath_personality(struct nvme_subsystem *subs
goto out;
}
- if (subsys->native_mpath != native_mpath) {
- subsys->native_mpath = native_mpath;
- ret = nvme_mpath_change_personality(subsys);
- }
+ if (subsys->native_mpath != native_mpath)
+ ret = nvme_mpath_change_personality(subsys, native_mpath);
out:
return ret ? ret : count;
}
diff --git a/drivers/nvme/host/multipath.c b/drivers/nvme/host/multipath.c
index 53d2610..017c924 100644
--- a/drivers/nvme/host/multipath.c
+++ b/drivers/nvme/host/multipath.c
@@ -247,26 +247,57 @@ void nvme_mpath_remove_disk(struct nvme_ns_head *head)
put_disk(head->disk);
}
-int nvme_mpath_change_personality(struct nvme_subsystem *subsys)
+static bool __nvme_subsys_in_use(struct nvme_subsystem *subsys)
{
struct nvme_ctrl *ctrl;
- int ret = 0;
+ struct nvme_ns *ns, *next;
-restart:
- mutex_lock(&subsys->lock);
list_for_each_entry(ctrl, &subsys->ctrls, subsys_entry) {
- if (!list_empty(&ctrl->namespaces)) {
- mutex_unlock(&subsys->lock);
- nvme_remove_namespaces(ctrl);
- goto restart;
+ down_write(&ctrl->namespaces_rwsem);
+ list_for_each_entry_safe(ns, next, &ctrl->namespaces, list) {
+ if ((kref_read(&ns->kref) > 1) ||
+ // FIXME: need to compare with N paths
+ (ns->head && (kref_read(&ns->head->ref) > 1))) {
+ printk("ns->kref = %d", kref_read(&ns->kref));
+ printk("ns->head->ref = %d", kref_read(&ns->head->ref));
+ up_write(&ctrl->namespaces_rwsem);
+ mutex_unlock(&subsys->lock);
+ return true;
+ }
}
+ up_write(&ctrl->namespaces_rwsem);
}
- mutex_unlock(&subsys->lock);
+
+ return false;
+}
+
+int nvme_mpath_change_personality(struct nvme_subsystem *subsys, bool native)
+{
+ struct nvme_ctrl *ctrl;
mutex_lock(&subsys->lock);
- list_for_each_entry(ctrl, &subsys->ctrls, subsys_entry)
- nvme_queue_scan(ctrl);
+
+ if (__nvme_subsys_in_use(subsys)) {
+ mutex_unlock(&subsys->lock);
+ return -EBUSY;
+ }
+
+ // FIXME: racey, subsys could now be in use here.
+ // Interlock against use needs work from an NVMe developer (hch?) :)
+
+ list_for_each_entry(ctrl, &subsys->ctrls, subsys_entry) {
+ cancel_work_sync(&ctrl->reset_work);
+ flush_work(&ctrl->reset_work);
+ nvme_stop_ctrl(ctrl);
+ }
+
+ subsys->native_mpath = native;
mutex_unlock(&subsys->lock);
- return ret;
+ list_for_each_entry(ctrl, &subsys->ctrls, subsys_entry) {
+ nvme_remove_namespaces(ctrl);
+ nvme_start_ctrl(ctrl);
+ }
+
+ return 0;
}
diff --git a/drivers/nvme/host/nvme.h b/drivers/nvme/host/nvme.h
index 81e4e71..97a6b08 100644
--- a/drivers/nvme/host/nvme.h
+++ b/drivers/nvme/host/nvme.h
@@ -452,7 +452,7 @@ void nvme_set_disk_name(char *disk_name, struct nvme_ns *ns,
int nvme_mpath_alloc_disk(struct nvme_ctrl *ctrl,struct nvme_ns_head *head);
void nvme_mpath_add_disk(struct nvme_ns_head *head);
void nvme_mpath_remove_disk(struct nvme_ns_head *head);
-int nvme_mpath_change_personality(struct nvme_subsystem *subsys);
+int nvme_mpath_change_personality(struct nvme_subsystem *subsys, bool native);
static inline void nvme_mpath_clear_current_path(struct nvme_ns *ns)
{
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