[PATCH v2 0/2] nvme-multipath: requeue work fixes

[PATCH v2 0/2] nvme-multipath: requeue work fixes

[Qiliang Yuan]

Patch 1 fails I/O queued without a path as soon as delayed_removal_secs
is cleared, instead of when the delayed removal runs.

Patch 2 stops nvme_mpath_set_live() from waiting for an SRCU grace
period per namespace, which makes an ANA change with many namespaces
take tens to hundreds of milliseconds.

Signed-off-by: Qiliang Yuan <qiliang.yuan@linux.dev>
---
V1 -> V2:
- Add patch 1
- Take the SRCU cookie under head->lock in patch 2 (Sashiko)
- Rerun the tests

v1: https://lore.kernel.org/r/20261110-bug-nvme-mpath-set-live-srcu-v1-1-1f58b405b0f8@linux.dev

---
Qiliang Yuan (2):
      nvme-multipath: fail queued I/O when delayed_removal_secs is cleared
      nvme-multipath: don't wait for a grace period in nvme_mpath_set_live()

 drivers/nvme/host/multipath.c | 19 ++++++++++++++++++-
 drivers/nvme/host/nvme.h      |  1 +
 2 files changed, 19 insertions(+), 1 deletion(-)
---
base-commit: b3dfbde62cf1981c4638939c159dfd087b7a72e9
change-id: 20261110-bug-nvme-mpath-set-live-srcu-cbd8965f1457

Best regards,
-- 
Qiliang Yuan <qiliang.yuan@linux.dev>

[PATCH v2 1/2] nvme-multipath: fail queued I/O when delayed_removal_secs is cleared

[Qiliang Yuan]

With delayed_removal_secs set, bios submitted after the last path goes
away are queued on head->requeue_list, in case a path comes back before
the delayed removal of the head.

Writing 0 to delayed_removal_secs clears NVME_NSHEAD_QUEUE_IF_NO_PATH
but doesn't kick the requeue work, so the bios already queued stay
there until the removal work runs, up to the old delay later, instead
of failing right away.

Kick the requeue work when the attribute is cleared, as
fail_on_ctrl_loss_store() does when it is set, so that the queued bios
are resubmitted and fail.

With delayed_removal_secs at 30 and no path left, a direct read is
queued and delayed_removal_secs cleared two seconds later. The read
then fails after (mean of two runs, min-max in parentheses):

  before (ms)            after (ms)   change
  26234 (26209-26258)    3.5 (3-4)    -99.99%

Fixes: 62188639ec160 ("nvme-multipath: introduce delayed removal of the multipath head node")
Signed-off-by: Qiliang Yuan <qiliang.yuan@linux.dev>
---
 drivers/nvme/host/multipath.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/nvme/host/multipath.c b/drivers/nvme/host/multipath.c
index ae8059366b57d..a92a05b1f925a 100644
--- a/drivers/nvme/host/multipath.c
+++ b/drivers/nvme/host/multipath.c
@@ -1249,6 +1249,10 @@ static ssize_t delayed_removal_secs_store(struct device *dev,
 	 */
 	synchronize_srcu(&head->srcu);
 
+	/* Fail the I/O queued while waiting for a path to come back */
+	if (!sec)
+		kblockd_schedule_work(&head->requeue_work);
+
 	return count;
 }
 

-- 
2.43.0

[PATCH v2 2/2] nvme-multipath: don't wait for a grace period in nvme_mpath_set_live()

[Qiliang Yuan]

When a path goes live, nvme_mpath_set_live() waits for an SRCU grace
period of the namespace head before kicking the requeue work, so that
bios which found no usable path are on the requeue list by the time it
runs.

The ANA work calls nvme_mpath_set_live() for every namespace that
becomes optimized or non-optimized, one after another, so an ANA change
waits for one grace period per namespace.

Start the grace period with start_poll_synchronize_srcu() instead and
have nvme_requeue_work() wait for it only if it has not completed yet.
Take the cookie under head->lock, so that concurrent callers store
their cookies in order and the requeue work never sees an older one.
The grace periods of all namespaces then run in parallel, and requeue
work kicked for other reasons finds the cookie completed and does not
wait.

Flipping an ANA group from inaccessible to optimized ten times over
nvme-tcp loopback, nvme_read_ana_log() takes (median, min-max in
parentheses):

  namespaces  before (ms)          after (ms)        change
         256  51.9 (42.5-85.4)     8.9 (2.6-12.0)      -83%
        1024  157.5 (133.5-179.6)  49.5 (32.8-67.1)    -69%

Signed-off-by: Qiliang Yuan <qiliang.yuan@linux.dev>
---
 drivers/nvme/host/multipath.c | 15 ++++++++++++++-
 drivers/nvme/host/nvme.h      |  1 +
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/drivers/nvme/host/multipath.c b/drivers/nvme/host/multipath.c
index a92a05b1f925a..9c1023102902c 100644
--- a/drivers/nvme/host/multipath.c
+++ b/drivers/nvme/host/multipath.c
@@ -712,6 +712,11 @@ static void nvme_requeue_work(struct work_struct *work)
 		container_of(work, struct nvme_ns_head, requeue_work);
 	struct bio *bio, *next;
 
+	/* Let bios that found no usable path reach the requeue list first. */
+	if (!poll_state_synchronize_srcu(&head->srcu,
+					 READ_ONCE(head->requeue_srcu_cookie)))
+		synchronize_srcu(&head->srcu);
+
 	spin_lock_irq(&head->requeue_lock);
 	next = bio_list_get(&head->requeue_list);
 	spin_unlock_irq(&head->requeue_lock);
@@ -767,6 +772,7 @@ int nvme_mpath_alloc_disk(struct nvme_ctrl *ctrl, struct nvme_ns_head *head)
 	mutex_init(&head->lock);
 	spin_lock_init(&head->requeue_lock);
 	INIT_WORK(&head->requeue_work, nvme_requeue_work);
+	head->requeue_srcu_cookie = get_completed_synchronize_srcu();
 	INIT_WORK(&head->partition_scan_work, nvme_partition_scan_work);
 	INIT_DELAYED_WORK(&head->remove_work, nvme_remove_head_work);
 
@@ -849,9 +855,16 @@ static void nvme_mpath_set_live(struct nvme_ns *ns)
 			__nvme_find_path(head, node);
 		srcu_read_unlock(&head->srcu, srcu_idx);
 	}
+	/*
+	 * Have nvme_requeue_work() wait for the grace period instead of
+	 * blocking here, so that namespaces going live together, e.g. on
+	 * an ANA change, do not wait for one grace period each. Start it
+	 * under head->lock so that the newest cookie is stored last.
+	 */
+	WRITE_ONCE(head->requeue_srcu_cookie,
+		   start_poll_synchronize_srcu(&head->srcu));
 	mutex_unlock(&head->lock);
 
-	synchronize_srcu(&head->srcu);
 	nvme_mpath_revalidate_zones(head);
 	kblockd_schedule_work(&head->requeue_work);
 }
diff --git a/drivers/nvme/host/nvme.h b/drivers/nvme/host/nvme.h
index 8833a463f1c84..3fff9b394b1cd 100644
--- a/drivers/nvme/host/nvme.h
+++ b/drivers/nvme/host/nvme.h
@@ -587,6 +587,7 @@ struct nvme_ns_head {
 		__guarded_by(&requeue_lock);
 	spinlock_t		requeue_lock;
 	struct work_struct	requeue_work;
+	unsigned long		requeue_srcu_cookie;
 	struct work_struct	partition_scan_work;
 	struct mutex		lock;
 	unsigned long		flags;

-- 
2.43.0