Date:   Fri, 6 Sep 2019 11:22:09 +0100
From:   Stefan Hajnoczi <stefanha@...hat.com>
To:     Vivek Goyal <vgoyal@...hat.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Miklos Szeredi <mszeredi@...hat.com>,
        virtualization@...ts.linux-foundation.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>
Subject: Re: [PATCH v4 15/16] virtio-fs: add virtiofs filesystem

On Thu, Sep 05, 2019 at 03:15:15PM -0400, Vivek Goyal wrote:
> On Tue, Sep 03, 2019 at 09:55:49AM -0400, Michael S. Tsirkin wrote:
> [..]
> > What's with all of the TODOs? Some of these are really scary,
> > looks like they need to be figured out before this is merged.
> 
> Hi Michael,
> 
> One of the issue I noticed is races w.r.t device removal and super
> block initialization. I am about to post a set of patches which
> take care of these races and also get rid of some of the scary
> TODOs. Other TODOs like suspend/restore, multiqueue support etc
> are improvements which we can do over a period of time.
> 
> [..]
> > > +/* Per-virtqueue state */
> > > +struct virtio_fs_vq {
> > > +	spinlock_t lock;
> > > +	struct virtqueue *vq;     /* protected by ->lock */
> > > +	struct work_struct done_work;
> > > +	struct list_head queued_reqs;
> > > +	struct delayed_work dispatch_work;
> > > +	struct fuse_dev *fud;
> > > +	bool connected;
> > > +	long in_flight;
> > > +	char name[24];
> > 
> > I'd keep names somewhere separate as they are not used on data path.
> 
> Ok, this sounds like a nice to have. Will take care of this once base
> patch gets merged.
> 
> [..]
> > > +struct virtio_fs_forget {
> > > +	struct fuse_in_header ih;
> > > +	struct fuse_forget_in arg;
> > 
> > These structures are all native endian.
> > 
> > Passing them to host will make cross-endian setups painful to support,
> > and hardware implementations impossible.
> > 
> > How about converting everything to LE?
> 
> So looks like endianness issue is now resolved (going by the other
> emails). So I will not worry about it.
> 
> [..]
> > > +/* Add a new instance to the list or return -EEXIST if tag name exists*/
> > > +static int virtio_fs_add_instance(struct virtio_fs *fs)
> > > +{
> > > +	struct virtio_fs *fs2;
> > > +	bool duplicate = false;
> > > +
> > > +	mutex_lock(&virtio_fs_mutex);
> > > +
> > > +	list_for_each_entry(fs2, &virtio_fs_instances, list) {
> > > +		if (strcmp(fs->tag, fs2->tag) == 0)
> > > +			duplicate = true;
> > > +	}
> > > +
> > > +	if (!duplicate)
> > > +		list_add_tail(&fs->list, &virtio_fs_instances);
> > 
> > 
> > This is O(N^2) as it's presumably called for each istance.
> > Doesn't scale - please switch to a tree or such.
> 
> This is O(N) and not O(N^2) right? Addition of device is O(N), search
> during mount is O(N).
> 
> This is not a frequent event at all and number of virtiofs instances
> per guest are expected to be fairly small (say less than 10). So I 
> really don't think there is any value in converting this into a tree
> (instead of a list).
> 
> [..]
> > > +static void virtio_fs_free_devs(struct virtio_fs *fs)
> > > +{
> > > +	unsigned int i;
> > > +
> > > +	/* TODO lock */
> > 
> > Doesn't inspire confidence, does it?
> 
> Agreed. Getting rid of this in set of fixes I am about to post.
> 
> > 
> > > +
> > > +	for (i = 0; i < fs->nvqs; i++) {
> > > +		struct virtio_fs_vq *fsvq = &fs->vqs[i];
> > > +
> > > +		if (!fsvq->fud)
> > > +			continue;
> > > +
> > > +		flush_work(&fsvq->done_work);
> > > +		flush_delayed_work(&fsvq->dispatch_work);
> > > +
> > > +		/* TODO need to quiesce/end_requests/decrement dev_count */
> > 
> > Indeed. Won't this crash if we don't?
> 
> Took care of this as well.
> 
> [..]
> > > +static void virtio_fs_hiprio_dispatch_work(struct work_struct *work)
> > > +{
> > > +	struct virtio_fs_forget *forget;
> > > +	struct virtio_fs_vq *fsvq = container_of(work, struct virtio_fs_vq,
> > > +						 dispatch_work.work);
> > > +	struct virtqueue *vq = fsvq->vq;
> > > +	struct scatterlist sg;
> > > +	struct scatterlist *sgs[] = {&sg};
> > > +	bool notify;
> > > +	int ret;
> > > +
> > > +	pr_debug("virtio-fs: worker %s called.\n", __func__);
> > > +	while (1) {
> > > +		spin_lock(&fsvq->lock);
> > > +		forget = list_first_entry_or_null(&fsvq->queued_reqs,
> > > +					struct virtio_fs_forget, list);
> > > +		if (!forget) {
> > > +			spin_unlock(&fsvq->lock);
> > > +			return;
> > > +		}
> > > +
> > > +		list_del(&forget->list);
> > > +		if (!fsvq->connected) {
> > > +			spin_unlock(&fsvq->lock);
> > > +			kfree(forget);
> > > +			continue;
> > > +		}
> > > +
> > > +		sg_init_one(&sg, forget, sizeof(*forget));
> > 
> > This passes to host a structure including "struct list_head list";
> > 
> > Not a good idea.
> 
> Ok, host does not have to see "struct list_head list". Its needed for
> guest. Will look into getting rid of this.
> 
> > 
> > 
> > > +
> > > +		/* Enqueue the request */
> > > +		dev_dbg(&vq->vdev->dev, "%s\n", __func__);
> > > +		ret = virtqueue_add_sgs(vq, sgs, 1, 0, forget, GFP_ATOMIC);
> > 
> > 
> > This is easier as add_outbuf.
> 
> Will look into it.
> 
> > 
> > Also - why GFP_ATOMIC?
> 
> Hmm..., may be it can be GFP_KERNEL. I don't see atomic context here. Will
> look into it.
> 
> > 
> > > +		if (ret < 0) {
> > > +			if (ret == -ENOMEM || ret == -ENOSPC) {
> > > +				pr_debug("virtio-fs: Could not queue FORGET: err=%d. Will try later\n",
> > > +					 ret);
> > > +				list_add_tail(&forget->list,
> > > +						&fsvq->queued_reqs);
> > > +				schedule_delayed_work(&fsvq->dispatch_work,
> > > +						msecs_to_jiffies(1));
> > 
> > Can't we we requeue after some buffers get consumed?
> 
> That's what dispatch work is doing. It tries to requeue the request after
> a while.
> 
> [..]
> > > +static int virtio_fs_probe(struct virtio_device *vdev)
> > > +{
> > > +	struct virtio_fs *fs;
> > > +	int ret;
> > > +
> > > +	fs = devm_kzalloc(&vdev->dev, sizeof(*fs), GFP_KERNEL);
> > > +	if (!fs)
> > > +		return -ENOMEM;
> > > +	vdev->priv = fs;
> > > +
> > > +	ret = virtio_fs_read_tag(vdev, fs);
> > > +	if (ret < 0)
> > > +		goto out;
> > > +
> > > +	ret = virtio_fs_setup_vqs(vdev, fs);
> > > +	if (ret < 0)
> > > +		goto out;
> > > +
> > > +	/* TODO vq affinity */
> > > +	/* TODO populate notifications vq */
> > 
> > what's notifications vq?
> 
> It has not been implemented yet. At some point of time we want to have
> a notion of notification queue so that host can send notifications to
> guest. Will get rid of this comment for now.
> 
> [..]
> > > +#ifdef CONFIG_PM_SLEEP
> > > +static int virtio_fs_freeze(struct virtio_device *vdev)
> > > +{
> > > +	return 0; /* TODO */
> > > +}
> > > +
> > > +static int virtio_fs_restore(struct virtio_device *vdev)
> > > +{
> > > +	return 0; /* TODO */
> > > +}
> > 
> > Is this really a good idea? I'd rather it was implemented,
> > but if not possible at all disabling PM seems better than just
> > keep going.
> 
> I agree. Will look into disabling it.
> 
> > 
> > > +#endif /* CONFIG_PM_SLEEP */
> > > +
> > > +const static struct virtio_device_id id_table[] = {
> > > +	{ VIRTIO_ID_FS, VIRTIO_DEV_ANY_ID },
> > > +	{},
> > > +};
> > > +
> > > +const static unsigned int feature_table[] = {};
> > > +
> > > +static struct virtio_driver virtio_fs_driver = {
> > > +	.driver.name		= KBUILD_MODNAME,
> > > +	.driver.owner		= THIS_MODULE,
> > > +	.id_table		= id_table,
> > > +	.feature_table		= feature_table,
> > > +	.feature_table_size	= ARRAY_SIZE(feature_table),
> > > +	/* TODO validate config_get != NULL */
> > 
> > Why?
> 
> Don't know. Stefan, do you remember why did you put this comment? If not,
> I will get rid of it.

This comment can be removed.

> > > +static void virtio_fs_wake_pending_and_unlock(struct fuse_iqueue *fiq)
> > > +__releases(fiq->waitq.lock)
> > > +{
> > > +	unsigned int queue_id = VQ_REQUEST; /* TODO multiqueue */
> > > +	struct virtio_fs *fs;
> > > +	struct fuse_conn *fc;
> > > +	struct fuse_req *req;
> > > +	struct fuse_pqueue *fpq;
> > > +	int ret;
> > > +
> > > +	WARN_ON(list_empty(&fiq->pending));
> > > +	req = list_last_entry(&fiq->pending, struct fuse_req, list);
> > > +	clear_bit(FR_PENDING, &req->flags);
> > > +	list_del_init(&req->list);
> > > +	WARN_ON(!list_empty(&fiq->pending));
> > > +	spin_unlock(&fiq->waitq.lock);
> > > +
> > > +	fs = fiq->priv;
> > > +	fc = fs->vqs[queue_id].fud->fc;
> > > +
> > > +	dev_dbg(&fs->vqs[queue_id].vq->vdev->dev,
> > > +		"%s: opcode %u unique %#llx nodeid %#llx in.len %u out.len %u\n",
> > > +		__func__, req->in.h.opcode, req->in.h.unique, req->in.h.nodeid,
> > > +		req->in.h.len, fuse_len_args(req->out.numargs, req->out.args));
> > > +
> > > +	fpq = &fs->vqs[queue_id].fud->pq;
> > > +	spin_lock(&fpq->lock);
> > > +	if (!fpq->connected) {
> > > +		spin_unlock(&fpq->lock);
> > > +		req->out.h.error = -ENODEV;
> > > +		pr_err("virtio-fs: %s disconnected\n", __func__);
> > > +		fuse_request_end(fc, req);
> > > +		return;
> > > +	}
> > > +	list_add_tail(&req->list, fpq->processing);
> > > +	spin_unlock(&fpq->lock);
> > > +	set_bit(FR_SENT, &req->flags);
> > > +	/* matches barrier in request_wait_answer() */
> > > +	smp_mb__after_atomic();
> > > +	/* TODO check for FR_INTERRUPTED? */
> > 
> > 
> > ?
> 
> hmm... we don't support FR_INTERRUPTED. Stefan, do you remember why
> this TODO is here. If not, I will get rid of it.

We don't support FUSE_INTERRUPT yet.  The purpose of this comment is
that when we do support FUSE_INTERRUPT we'll need to follow
fuse_dev_do_read() in queuing a FUSE_INTERRUPT here.

Stefan

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