Re: [RFC PATCHv5 2/7] nvme-multipath: add support for adaptive I/O policy

Hi,
   Raise some comments to see if we can keep moving this
feature forward.
   Once this feature is merged, I'll be able to build the
service-time I/O policy on top of it.


在 2025/11/5 18:33, Nilay Shroff 写道:
> This commit introduces a new I/O policy named "adaptive". Users can
> configure it by writing "adaptive" to "/sys/class/nvme-subsystem/nvme-
> subsystemX/iopolicy"
>
> The adaptive policy dynamically distributes I/O based on measured
> completion latency. The main idea is to calculate latency for each path,
> derive a weight, and then proportionally forward I/O according to those
> weights.
>
> To ensure scalability, path latency is measured per-CPU. Each CPU
> maintains its own statistics, and I/O forwarding uses these per-CPU
> values. Every ~15 seconds, a simple average latency of per-CPU batched
> samples are computed and fed into an Exponentially Weighted Moving
> Average (EWMA):
>
> avg_latency = div_u64(batch, batch_count);
> new_ewma_latency = (prev_ewma_latency * (WEIGHT-1) + avg_latency)/WEIGHT
>
> With WEIGHT = 8, this assigns 7/8 (~87.5%) weight to the previous
> latency value and 1/8 (~12.5%) to the most recent latency. This
> smoothing reduces jitter, adapts quickly to changing conditions,
> avoids storing historical samples, and works well for both low and
> high I/O rates. Path weights are then derived from the smoothed (EWMA)
> latency as follows (example with two paths A and B):
>
>      path_A_score = NSEC_PER_SEC / path_A_ewma_latency
>      path_B_score = NSEC_PER_SEC / path_B_ewma_latency
>      total_score  = path_A_score + path_B_score
>
>      path_A_weight = (path_A_score * 100) / total_score
>      path_B_weight = (path_B_score * 100) / total_score
>
> where:
>    - path_X_ewma_latency is the smoothed latency of a path in nanoseconds
>    - NSEC_PER_SEC is used as a scaling factor since valid latencies
>      are < 1 second
>    - weights are normalized to a 0–64 scale across all paths.
>
> Path credits are refilled based on this weight, with one credit
> consumed per I/O. When all credits are consumed, the credits are
> refilled again based on the current weight. This ensures that I/O is
> distributed across paths proportionally to their calculated weight.
>
> Reviewed-by: Hannes Reinecke <[email protected]>
> Signed-off-by: Nilay Shroff <[email protected]>
> ---
>   drivers/nvme/host/core.c      |  15 +-
>   drivers/nvme/host/ioctl.c     |  31 ++-
>   drivers/nvme/host/multipath.c | 425 ++++++++++++++++++++++++++++++++--
>   drivers/nvme/host/nvme.h      |  74 +++++-
>   drivers/nvme/host/pr.c        |   6 +-
>   drivers/nvme/host/sysfs.c     |   2 +-
>   6 files changed, 530 insertions(+), 23 deletions(-)
>
> diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
> index fa4181d7de73..47f375c63d2d 100644
> --- a/drivers/nvme/host/core.c
> +++ b/drivers/nvme/host/core.c
> @@ -672,6 +672,9 @@ static void nvme_free_ns_head(struct kref *ref)
>   	cleanup_srcu_struct(&head->srcu);
>   	nvme_put_subsystem(head->subsys);
>   	kfree(head->plids);
> +#ifdef CONFIG_NVME_MULTIPATH
> +	free_percpu(head->adp_path);
> +#endif
>   	kfree(head);
>   }
>   
> @@ -689,6 +692,7 @@ static void nvme_free_ns(struct kref *kref)
>   {
>   	struct nvme_ns *ns = container_of(kref, struct nvme_ns, kref);
>   
> +	nvme_free_ns_stat(ns);
>   	put_disk(ns->disk);
>   	nvme_put_ns_head(ns->head);
>   	nvme_put_ctrl(ns->ctrl);
> @@ -4137,6 +4141,9 @@ static void nvme_alloc_ns(struct nvme_ctrl *ctrl, struct nvme_ns_info *info)
>   	if (nvme_init_ns_head(ns, info))
>   		goto out_cleanup_disk;
>   
> +	if (nvme_alloc_ns_stat(ns))
> +		goto out_unlink_ns;
> +
>   	/*
>   	 * If multipathing is enabled, the device name for all disks and not
>   	 * just those that represent shared namespaces needs to be based on the
> @@ -4161,7 +4168,7 @@ static void nvme_alloc_ns(struct nvme_ctrl *ctrl, struct nvme_ns_info *info)
>   	}
>   
>   	if (nvme_update_ns_info(ns, info))
> -		goto out_unlink_ns;
> +		goto out_free_ns_stat;
>   
>   	mutex_lock(&ctrl->namespaces_lock);
>   	/*
> @@ -4170,7 +4177,7 @@ static void nvme_alloc_ns(struct nvme_ctrl *ctrl, struct nvme_ns_info *info)
>   	 */
>   	if (test_bit(NVME_CTRL_FROZEN, &ctrl->flags)) {
>   		mutex_unlock(&ctrl->namespaces_lock);
> -		goto out_unlink_ns;
> +		goto out_free_ns_stat;
>   	}
>   	nvme_ns_add_to_ctrl_list(ns);
>   	mutex_unlock(&ctrl->namespaces_lock);
> @@ -4201,6 +4208,8 @@ static void nvme_alloc_ns(struct nvme_ctrl *ctrl, struct nvme_ns_info *info)
>   	list_del_rcu(&ns->list);
>   	mutex_unlock(&ctrl->namespaces_lock);
>   	synchronize_srcu(&ctrl->srcu);
> +out_free_ns_stat:
> +	nvme_free_ns_stat(ns);
>    out_unlink_ns:
>   	mutex_lock(&ctrl->subsys->lock);
>   	list_del_rcu(&ns->siblings);
> @@ -4244,6 +4253,8 @@ static void nvme_ns_remove(struct nvme_ns *ns)
>   	 */
>   	synchronize_srcu(&ns->head->srcu);
>   
> +	nvme_mpath_cancel_adaptive_path_weight_work(ns);
> +

Here cancel weight_work first and only then clear the NVME_NS_PATH_STAT 
flag.

During this window, nvme_mpath_end_request() may see that the 
NVME_NS_PATH_STAT

flag is still set and re-queue weight_work.

Therefore, you should clear the NVME_NS_PATH_STAT flag first and then 
cancel weight_work.

>   	/* wait for concurrent submissions */
>   	if (nvme_mpath_clear_current_path(ns))
>   		synchronize_srcu(&ns->head->srcu);
> diff --git a/drivers/nvme/host/ioctl.c b/drivers/nvme/host/ioctl.c
> index c212fa952c0f..759d147d9930 100644
> --- a/drivers/nvme/host/ioctl.c
> +++ b/drivers/nvme/host/ioctl.c
> @@ -700,18 +700,29 @@ static int nvme_ns_head_ctrl_ioctl(struct nvme_ns *ns, unsigned int cmd,
>   int nvme_ns_head_ioctl(struct block_device *bdev, blk_mode_t mode,
>   		unsigned int cmd, unsigned long arg)
>   {
> +	u8 opcode;
>   	struct nvme_ns_head *head = bdev->bd_disk->private_data;
>   	bool open_for_write = mode & BLK_OPEN_WRITE;
>   	void __user *argp = (void __user *)arg;
>   	struct nvme_ns *ns;
>   	int srcu_idx, ret = -EWOULDBLOCK;
>   	unsigned int flags = 0;
> +	unsigned int op_type = NVME_STAT_OTHER;
>   
>   	if (bdev_is_partition(bdev))
>   		flags |= NVME_IOCTL_PARTITION;
>   
> +	if (cmd == NVME_IOCTL_SUBMIT_IO) {
> +		if (get_user(opcode, (u8 *)argp))
> +			return -EFAULT;
> +		if (opcode == nvme_cmd_write)
> +			op_type = NVME_STAT_WRITE;
> +		else if (opcode == nvme_cmd_read)
> +			op_type = NVME_STAT_READ;
> +	}
> +
>   	srcu_idx = srcu_read_lock(&head->srcu);
> -	ns = nvme_find_path(head);
> +	ns = nvme_find_path(head, op_type);
>   	if (!ns)
>   		goto out_unlock;
>   
> @@ -733,6 +744,7 @@ int nvme_ns_head_ioctl(struct block_device *bdev, blk_mode_t mode,
>   long nvme_ns_head_chr_ioctl(struct file *file, unsigned int cmd,
>   		unsigned long arg)
>   {
> +	u8 opcode;
>   	bool open_for_write = file->f_mode & FMODE_WRITE;
>   	struct cdev *cdev = file_inode(file)->i_cdev;
>   	struct nvme_ns_head *head =
> @@ -740,9 +752,19 @@ long nvme_ns_head_chr_ioctl(struct file *file, unsigned int cmd,
>   	void __user *argp = (void __user *)arg;
>   	struct nvme_ns *ns;
>   	int srcu_idx, ret = -EWOULDBLOCK;
> +	unsigned int op_type = NVME_STAT_OTHER;
> +
> +	if (cmd == NVME_IOCTL_SUBMIT_IO) {
> +		if (get_user(opcode, (u8 *)argp))
> +			return -EFAULT;
> +		if (opcode == nvme_cmd_write)
> +			op_type = NVME_STAT_WRITE;
> +		else if (opcode == nvme_cmd_read)
> +			op_type = NVME_STAT_READ;
> +	}
>   
>   	srcu_idx = srcu_read_lock(&head->srcu);
> -	ns = nvme_find_path(head);
> +	ns = nvme_find_path(head, op_type);
>   	if (!ns)
>   		goto out_unlock;
>   
> @@ -762,7 +784,10 @@ int nvme_ns_head_chr_uring_cmd(struct io_uring_cmd *ioucmd,
>   	struct cdev *cdev = file_inode(ioucmd->file)->i_cdev;
>   	struct nvme_ns_head *head = container_of(cdev, struct nvme_ns_head, cdev);
>   	int srcu_idx = srcu_read_lock(&head->srcu);
> -	struct nvme_ns *ns = nvme_find_path(head);
> +	const struct nvme_uring_cmd *cmd = io_uring_sqe_cmd(ioucmd->sqe);
> +	struct nvme_ns *ns = nvme_find_path(head,
> +			READ_ONCE(cmd->opcode) & 1 ?
> +			NVME_STAT_WRITE : NVME_STAT_READ);
Here using nvme_cmd's opcode to find the path, but on the completion 
side using
req_op(rq) to count.

For passthrough IOs, the req_op(rq) is REQ_OP_DRV_IN or REQ_OP_DRV_OUT, 
this cause
nvme_data_dir() return OTHER when the IO is nvme read, and also broken 
to flush and write_zeros.
>   	int ret = -EINVAL;
>   
>   	if (ns)
> diff --git a/drivers/nvme/host/multipath.c b/drivers/nvme/host/multipath.c
> index 543e17aead12..55dc28375662 100644
> --- a/drivers/nvme/host/multipath.c
> +++ b/drivers/nvme/host/multipath.c
> @@ -6,6 +6,9 @@
>   #include <linux/backing-dev.h>
>   #include <linux/moduleparam.h>
>   #include <linux/vmalloc.h>
> +#include <linux/blk-mq.h>
> +#include <linux/math64.h>
> +#include <linux/rculist.h>
>   #include <trace/events/block.h>
>   #include "nvme.h"
>   
> @@ -66,9 +69,10 @@ MODULE_PARM_DESC(multipath_always_on,
>   	"create multipath node always except for private namespace with non-unique nsid; note that this also implicitly enables native multipath support");
>   
>   static const char *nvme_iopolicy_names[] = {
> -	[NVME_IOPOLICY_NUMA]	= "numa",
> -	[NVME_IOPOLICY_RR]	= "round-robin",
> -	[NVME_IOPOLICY_QD]      = "queue-depth",
> +	[NVME_IOPOLICY_NUMA]	 = "numa",
> +	[NVME_IOPOLICY_RR]	 = "round-robin",
> +	[NVME_IOPOLICY_QD]       = "queue-depth",
> +	[NVME_IOPOLICY_ADAPTIVE] = "adaptive",
>   };
>   
>   static int iopolicy = NVME_IOPOLICY_NUMA;
> @@ -83,6 +87,8 @@ static int nvme_set_iopolicy(const char *val, const struct kernel_param *kp)
>   		iopolicy = NVME_IOPOLICY_RR;
>   	else if (!strncmp(val, "queue-depth", 11))
>   		iopolicy = NVME_IOPOLICY_QD;
> +	else if (!strncmp(val, "adaptive", 8))
> +		iopolicy = NVME_IOPOLICY_ADAPTIVE;
>   	else
>   		return -EINVAL;
>   
> @@ -198,6 +204,204 @@ void nvme_mpath_start_request(struct request *rq)
>   }
>   EXPORT_SYMBOL_GPL(nvme_mpath_start_request);
>   
> +static void nvme_mpath_weight_work(struct work_struct *weight_work)
> +{
> +	int cpu, srcu_idx;
> +	u32 weight;
> +	struct nvme_ns *ns;
> +	struct nvme_path_stat *stat;
> +	struct nvme_path_work *work = container_of(weight_work,
> +			struct nvme_path_work, weight_work);
> +	struct nvme_ns_head *head = work->ns->head;
> +	int op_type = work->op_type;
> +	u64 total_score = 0;
> +
> +	cpu = get_cpu();
> +
> +	srcu_idx = srcu_read_lock(&head->srcu);
> +	list_for_each_entry_srcu(ns, &head->list, siblings,
> +			srcu_read_lock_held(&head->srcu)) {
> +
> +		stat = &this_cpu_ptr(ns->info)[op_type].stat;
> +		if (!READ_ONCE(stat->slat_ns)) {
> +			stat->score = 0;
> +			continue;
> +		}
> +		/*
> +		 * Compute the path score as the inverse of smoothed
> +		 * latency, scaled by NSEC_PER_SEC. Floating point
> +		 * math is unavailable in the kernel, so fixed-point
> +		 * scaling is used instead. NSEC_PER_SEC is chosen
> +		 * because valid latencies are always < 1 second; longer
> +		 * latencies are ignored.
> +		 */
> +		stat->score = div_u64(NSEC_PER_SEC, READ_ONCE(stat->slat_ns));
> +
> +		/* Compute total score. */
> +		total_score += stat->score;
> +	}
> +
> +	if (!total_score)
> +		goto out;
> +
> +	/*
> +	 * After computing the total slatency, we derive per-path weight
> +	 * (normalized to the range 0–64). The weight represents the
> +	 * relative share of I/O the path should receive.
> +	 *
> +	 *   - lower smoothed latency -> higher weight
> +	 *   - higher smoothed slatency -> lower weight
> +	 *
> +	 * Next, while forwarding I/O, we assign "credits" to each path
> +	 * based on its weight (please also refer nvme_adaptive_path()):
> +	 *   - Initially, credits = weight.
> +	 *   - Each time an I/O is dispatched on a path, its credits are
> +	 *     decremented proportionally.
> +	 *   - When a path runs out of credits, it becomes temporarily
> +	 *     ineligible until credit is refilled.
> +	 *
> +	 * I/O distribution is therefore governed by available credits,
> +	 * ensuring that over time the proportion of I/O sent to each
> +	 * path matches its weight (and thus its performance).
> +	 */
> +	list_for_each_entry_srcu(ns, &head->list, siblings,
> +			srcu_read_lock_held(&head->srcu)) {
> +
> +		stat = &this_cpu_ptr(ns->info)[op_type].stat;
> +		weight = div_u64(stat->score * 64, total_score);
> +
> +		/*
> +		 * Ensure the path weight never drops below 1. A weight
> +		 * of 0 is used only for newly added paths. During
> +		 * bootstrap, a few I/Os are sent to such paths to
> +		 * establish an initial weight. Enforcing a minimum
> +		 * weight of 1 guarantees that no path is forgotten and
> +		 * that each path is probed at least occasionally.
> +		 */
> +		if (!weight)
> +			weight = 1;
> +
> +		WRITE_ONCE(stat->weight, weight);
> +	}
> +out:
> +	srcu_read_unlock(&head->srcu, srcu_idx);
> +	put_cpu();
> +}
> +
> +/*
> + * Formula to calculate the EWMA (Exponentially Weighted Moving Average):
> + * ewma = (old_ewma * (EWMA_SHIFT - 1) + (EWMA_SHIFT)) / EWMA_SHIFT
> + * For instance, with EWMA_SHIFT = 3, this assigns 7/8 (~87.5 %) weight to
> + * the existing/old ewma and 1/8 (~12.5%) weight to the new sample.
> + */
> +static inline u64 ewma_update(u64 old, u64 new)
> +{
> +	return (old * ((1 << NVME_DEFAULT_ADP_EWMA_SHIFT) - 1)
> +			+ new) >> NVME_DEFAULT_ADP_EWMA_SHIFT;
> +}
> +
> +static void nvme_mpath_add_sample(struct request *rq, struct nvme_ns *ns)
> +{
> +	int cpu;
> +	unsigned int op_type;
> +	struct nvme_path_info *info;
> +	struct nvme_path_stat *stat;
> +	u64 now, latency, slat_ns, avg_lat_ns;
> +	struct nvme_ns_head *head = ns->head;
> +
> +	if (list_is_singular(&head->list))
> +		return;
> +
> +	now = ktime_get_ns();
> +	latency = now >= rq->io_start_time_ns ? now - rq->io_start_time_ns : 0;
> +	if (!latency)
> +		return;
> +
> +	/*
> +	 * As completion code path is serialized(i.e. no same completion queue
> +	 * update code could run simultaneously on multiple cpu) we can safely
> +	 * access per cpu nvme path stat here from another cpu (in case the
> +	 * completion cpu is different from submission cpu).
> +	 * The only field which could be accessed simultaneously here is the
> +	 * path ->weight which may be accessed by this function as well as I/O
> +	 * submission path during path selection logic and we protect ->weight
> +	 * using READ_ONCE/WRITE_ONCE. Yes this may not be 100% accurate but
> +	 * we also don't need to be so accurate here as the path credit would
> +	 * be anyways refilled, based on path weight, once path consumes all
> +	 * its credits. And we limit path weight/credit max up to 100. Please
> +	 * also refer nvme_adaptive_path().
> +	 */
> +	cpu = blk_mq_rq_cpu(rq);
> +	op_type = nvme_data_dir(req_op(rq));
> +	info = &per_cpu_ptr(ns->info, cpu)[op_type];
> +	stat = &info->stat;
> +
> +	/*
> +	 * If latency > ~1s then ignore this sample to prevent EWMA from being
> +	 * skewed by pathological outliers (multi-second waits, controller
> +	 * timeouts etc.). This keeps path scores representative of normal
> +	 * performance and avoids instability from rare spikes. If such high
> +	 * latency is real, ANA state reporting or keep-alive error counters
> +	 * will mark the path unhealthy and remove it from the head node list,
> +	 * so we safely skip such sample here.
> +	 */
> +	if (unlikely(latency > NSEC_PER_SEC)) {
> +		stat->nr_ignored++;
> +		dev_warn_ratelimited(ns->ctrl->device,
> +			"ignoring sample with >1s latency (possible controller stall or timeout)\n");
> +		return;
> +	}
> +
> +	/*
> +	 * Accumulate latency samples and increment the batch count for each
> +	 * ~15 second interval. When the interval expires, compute the simple
> +	 * average latency over that window, then update the smoothed (EWMA)
> +	 * latency. The path weight is recalculated based on this smoothed
> +	 * latency.
> +	 */
> +	stat->batch += latency;
> +	stat->batch_count++;
> +	stat->nr_samples++;
> +
> +	if (now > stat->last_weight_ts &&
> +	    (now - stat->last_weight_ts) >= NVME_DEFAULT_ADP_WEIGHT_TIMEOUT) {
> +
> +		stat->last_weight_ts = now;
> +
> +		/*
> +		 * Find simple average latency for the last epoch (~15 sec
> +		 * interval).
> +		 */
> +		avg_lat_ns = div_u64(stat->batch, stat->batch_count);
> +
> +		/*
> +		 * Calculate smooth/EWMA (Exponentially Weighted Moving Average)
> +		 * latency. EWMA is preferred over simple average latency
> +		 * because it smooths naturally, reduces jitter from sudden
> +		 * spikes, and adapts faster to changing conditions. It also
> +		 * avoids storing historical samples, and works well for both
> +		 * slow and fast I/O rates.
> +		 * Formula:
> +		 * slat_ns = (prev_slat_ns * (WEIGHT - 1) + (latency)) / WEIGHT
> +		 * With WEIGHT = 8, this assigns 7/8 (~87.5 %) weight to the
> +		 * existing latency and 1/8 (~12.5%) weight to the new latency.
> +		 */
> +		if (unlikely(!stat->slat_ns))
> +			WRITE_ONCE(stat->slat_ns, avg_lat_ns);
> +		else {
> +			slat_ns = ewma_update(stat->slat_ns, avg_lat_ns);
> +			WRITE_ONCE(stat->slat_ns, slat_ns);
> +		}
> +
> +		stat->batch = stat->batch_count = 0;
> +
> +		/*
> +		 * Defer calculation of the path weight in per-cpu workqueue.
> +		 */
> +		schedule_work_on(cpu, &info->work.weight_work);
> +	}
> +}
> +
>   void nvme_mpath_end_request(struct request *rq)
>   {
>   	struct nvme_ns *ns = rq->q->queuedata;
> @@ -205,6 +409,9 @@ void nvme_mpath_end_request(struct request *rq)
>   	if (nvme_req(rq)->flags & NVME_MPATH_CNT_ACTIVE)
>   		atomic_dec_if_positive(&ns->ctrl->nr_active);
>   
> +	if (test_bit(NVME_NS_PATH_STAT, &ns->flags))
> +		nvme_mpath_add_sample(rq, ns);
> +
>   	if (!(nvme_req(rq)->flags & NVME_MPATH_IO_STATS))
>   		return;
>   	bdev_end_io_acct(ns->head->disk->part0, req_op(rq),
> @@ -238,6 +445,62 @@ static const char *nvme_ana_state_names[] = {
>   	[NVME_ANA_CHANGE]		= "change",
>   };
>   
> +static void nvme_mpath_reset_adaptive_path_stat(struct nvme_ns *ns)
> +{
> +	int i, cpu;
> +	struct nvme_path_stat *stat;
> +
> +	for_each_possible_cpu(cpu) {
> +		for (i = 0; i < NVME_NUM_STAT_GROUPS; i++) {
> +			stat = &per_cpu_ptr(ns->info, cpu)[i].stat;
> +			memset(stat, 0, sizeof(struct nvme_path_stat));
> +		}
> +	}
> +}
> +
> +void nvme_mpath_cancel_adaptive_path_weight_work(struct nvme_ns *ns)
> +{
> +	int i, cpu;
> +	struct nvme_path_info *info;
> +
> +	if (!test_bit(NVME_NS_PATH_STAT, &ns->flags))
> +		return;
> +
> +	for_each_online_cpu(cpu) {
> +		for (i = 0; i < NVME_NUM_STAT_GROUPS; i++) {
> +			info = &per_cpu_ptr(ns->info, cpu)[i];
> +			cancel_work_sync(&info->work.weight_work);
> +		}
> +	}
> +}
> +
> +static bool nvme_mpath_enable_adaptive_path_policy(struct nvme_ns *ns)
> +{
> +	struct nvme_ns_head *head = ns->head;
> +
> +	if (!head->disk || head->subsys->iopolicy != NVME_IOPOLICY_ADAPTIVE)
> +		return false;
> +
> +	if (test_and_set_bit(NVME_NS_PATH_STAT, &ns->flags))
> +		return false;
> +
> +	blk_queue_flag_set(QUEUE_FLAG_SAME_FORCE, ns->queue);
> +	blk_stat_enable_accounting(ns->queue);
> +	return true;
> +}
> +
> +static bool nvme_mpath_disable_adaptive_path_policy(struct nvme_ns *ns)
> +{
> +
> +	if (!test_and_clear_bit(NVME_NS_PATH_STAT, &ns->flags))
> +		return false;
> +
> +	blk_stat_disable_accounting(ns->queue);
> +	blk_queue_flag_clear(QUEUE_FLAG_SAME_FORCE, ns->queue);
> +	nvme_mpath_reset_adaptive_path_stat(ns);
The adp_path still hold the ns's pointer, should clear too,
otherwise, we will access a freed ns.
> +	return true;
> +}
> +
>   bool nvme_mpath_clear_current_path(struct nvme_ns *ns)
>   {
>   	struct nvme_ns_head *head = ns->head;
> @@ -253,6 +516,8 @@ bool nvme_mpath_clear_current_path(struct nvme_ns *ns)
>   			changed = true;
>   		}
>   	}
> +	if (nvme_mpath_disable_adaptive_path_policy(ns))
> +		changed = true;
>   out:
>   	return changed;
>   }
> @@ -271,6 +536,45 @@ void nvme_mpath_clear_ctrl_paths(struct nvme_ctrl *ctrl)
>   	srcu_read_unlock(&ctrl->srcu, srcu_idx);
>   }
>   
> +int nvme_alloc_ns_stat(struct nvme_ns *ns)
> +{
> +	int i, cpu;
> +	struct nvme_path_work *work;
> +	gfp_t gfp = GFP_KERNEL | __GFP_ZERO;
> +
> +	if (!ns->head->disk)
> +		return 0;
> +
> +	ns->info = __alloc_percpu_gfp(NVME_NUM_STAT_GROUPS *
> +			sizeof(struct nvme_path_info),
> +			__alignof__(struct nvme_path_info), gfp);
Should alloc this only when the user use the adaptive io policy?

Best Regards,
Guixin Liu
>
>