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Date:   Tue, 7 May 2019 13:25:54 -0700
From:   Jakub Staroń <jstaron@...gle.com>
To:     Pankaj Gupta <pagupta@...hat.com>, linux-nvdimm@...ts.01.org,
        linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-acpi@...r.kernel.org,
        qemu-devel@...gnu.org, linux-ext4@...r.kernel.org,
        linux-xfs@...r.kernel.org
Cc:     jack@...e.cz, mst@...hat.com, jasowang@...hat.com,
        david@...morbit.com, lcapitulino@...hat.com,
        adilger.kernel@...ger.ca, zwisler@...nel.org, aarcange@...hat.com,
        dave.jiang@...el.com, darrick.wong@...cle.com,
        vishal.l.verma@...el.com, david@...hat.com, willy@...radead.org,
        hch@...radead.org, jmoyer@...hat.com, nilal@...hat.com,
        lenb@...nel.org, kilobyte@...band.pl, riel@...riel.com,
        yuval.shaia@...cle.com, stefanha@...hat.com, pbonzini@...hat.com,
        dan.j.williams@...el.com, kwolf@...hat.com, tytso@....edu,
        xiaoguangrong.eric@...il.com, cohuck@...hat.com, rjw@...ysocki.net,
        imammedo@...hat.com, smbarber@...gle.com
Subject: Re: [Qemu-devel] [PATCH v7 2/6] virtio-pmem: Add virtio pmem driver

On 4/25/19 10:00 PM, Pankaj Gupta wrote:

> +void host_ack(struct virtqueue *vq)
> +{
> +	unsigned int len;
> +	unsigned long flags;
> +	struct virtio_pmem_request *req, *req_buf;
> +	struct virtio_pmem *vpmem = vq->vdev->priv;
> +
> +	spin_lock_irqsave(&vpmem->pmem_lock, flags);
> +	while ((req = virtqueue_get_buf(vq, &len)) != NULL) {
> +		req->done = true;
> +		wake_up(&req->host_acked);
> +
> +		if (!list_empty(&vpmem->req_list)) {
> +			req_buf = list_first_entry(&vpmem->req_list,
> +					struct virtio_pmem_request, list);
> +			list_del(&vpmem->req_list);

Shouldn't it be rather `list_del(vpmem->req_list.next)`? We are trying to unlink
first element of the list and `vpmem->req_list` is just the list head.

> +int virtio_pmem_flush(struct nd_region *nd_region)
> +{
> +	int err;
> +	unsigned long flags;
> +	struct scatterlist *sgs[2], sg, ret;
> +	struct virtio_device *vdev = nd_region->provider_data;
> +	struct virtio_pmem *vpmem = vdev->priv;
> +	struct virtio_pmem_request *req;
> +
> +	might_sleep();
> +	req = kmalloc(sizeof(*req), GFP_KERNEL);
> +	if (!req)
> +		return -ENOMEM;
> +
> +	req->done = req->wq_buf_avail = false;
> +	strcpy(req->name, "FLUSH");
> +	init_waitqueue_head(&req->host_acked);
> +	init_waitqueue_head(&req->wq_buf);
> +	sg_init_one(&sg, req->name, strlen(req->name));
> +	sgs[0] = &sg;
> +	sg_init_one(&ret, &req->ret, sizeof(req->ret));
> +	sgs[1] = &ret;
> +
> +	spin_lock_irqsave(&vpmem->pmem_lock, flags);
> +	err = virtqueue_add_sgs(vpmem->req_vq, sgs, 1, 1, req, GFP_ATOMIC);
> +	if (err) {
> +		dev_err(&vdev->dev, "failed to send command to virtio pmem device\n");
> +
> +		list_add_tail(&vpmem->req_list, &req->list);
> +		spin_unlock_irqrestore(&vpmem->pmem_lock, flags);
> +
> +		/* When host has read buffer, this completes via host_ack */
> +		wait_event(req->wq_buf, req->wq_buf_avail);
> +		spin_lock_irqsave(&vpmem->pmem_lock, flags);
> +	}

Aren't the arguments in `list_add_tail` swapped? The element we are adding should
be first, the list should be second. Also, shouldn't we resubmit the request after
waking up from `wait_event(req->wq_buf, req->wq_buf_avail)`?

I propose rewriting it like that:

diff --git a/drivers/nvdimm/virtio_pmem.c b/drivers/nvdimm/virtio_pmem.c
index 66b582f751a3..ff0556b04e86 100644
--- a/drivers/nvdimm/virtio_pmem.c
+++ b/drivers/nvdimm/virtio_pmem.c
@@ -25,7 +25,7 @@ void host_ack(struct virtqueue *vq)
 		if (!list_empty(&vpmem->req_list)) {
 			req_buf = list_first_entry(&vpmem->req_list,
 					struct virtio_pmem_request, list);
-			list_del(&vpmem->req_list);
+			list_del(vpmem->req_list.next);
 			req_buf->wq_buf_avail = true;
 			wake_up(&req_buf->wq_buf);
 		}
@@ -59,17 +59,33 @@ int virtio_pmem_flush(struct nd_region *nd_region)
 	sgs[1] = &ret;
 
 	spin_lock_irqsave(&vpmem->pmem_lock, flags);
-	err = virtqueue_add_sgs(vpmem->req_vq, sgs, 1, 1, req, GFP_ATOMIC);
-	if (err) {
-		dev_err(&vdev->dev, "failed to send command to virtio pmem device\n");
+	/*
+	 * If virtqueue_add_sgs returns -ENOSPC then req_vq virtual queue does not
+	 * have free descriptor slots. We add the request to req_list and wait
+	 * for host_ack to wake us up when free slots are available.
+	 */
+	while ((err = virtqueue_add_sgs(vpmem->req_vq, sgs, 1, 1, req, GFP_ATOMIC)) == -ENOSPC) {
+		dev_err(&vdev->dev, "failed to send command to virtio pmem device, no free slots in the virtqueue, postponing request\n");
+		req->wq_buf_avail = false;
 
-		list_add_tail(&vpmem->req_list, &req->list);
+		list_add_tail(&req->list, &vpmem->req_list);
 		spin_unlock_irqrestore(&vpmem->pmem_lock, flags);
 
 		/* When host has read buffer, this completes via host_ack */
 		wait_event(req->wq_buf, req->wq_buf_avail);
 		spin_lock_irqsave(&vpmem->pmem_lock, flags);
 	}
+
+	/*
+	 * virtqueue_add_sgs failed with error different than -ENOSPC, we can't
+	 * do anything about that.
+	 */
+	if (err) {
+		dev_info(&vdev->dev, "failed to send command to virtio pmem device, error code %d\n", err);
+		spin_unlock_irqrestore(&vpmem->pmem_lock, flags);
+		err = -EIO;
+		goto ret;
+	}
 	err = virtqueue_kick(vpmem->req_vq);
 	spin_unlock_irqrestore(&vpmem->pmem_lock, flags);


Let me know if it looks reasonable to you.

Thank you,
Jakub Staron

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