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SubjectRe: [PATCH v2 1/2] virtio-balloon: tweak config_changed implementation
On Sat, Jan 05, 2019 at 03:32:44AM +0000, Wang, Wei W wrote:
> On Friday, January 4, 2019 11:45 PM, Michael S. Tsirkin wrote:
> > > struct virtio_balloon {
> > > struct virtio_device *vdev;
> > > struct virtqueue *inflate_vq, *deflate_vq, *stats_vq, *free_page_vq;
> > > @@ -77,6 +81,8 @@ struct virtio_balloon {
> > > /* Prevent updating balloon when it is being canceled. */
> > > spinlock_t stop_update_lock;
> > > bool stop_update;
> > > + /* Bitmap to indicate if reading the related config fields are needed
> > */
> > > + unsigned long config_read_bitmap;
> > >
> > > /* The list of allocated free pages, waiting to be given back to mm */
> > > struct list_head free_page_list;
> >
> > It seems that you never initialize this bitmap. Probably harmless here but
> > generally using uninitialized memory isn't good.
>
> We've used kzalloc to allocate the vb struct, so it's already 0-initialized :)

Ah ok. We do explicitly init other fields like stop_update so
it seems cleaner to init this one too though.

> > > +
> > > static int send_free_pages(struct virtio_balloon *vb) {
> > > int err;
> > > @@ -620,6 +630,7 @@ static int send_free_pages(struct virtio_balloon *vb)
> > > * stop the reporting.
> > > */
> > > cmd_id_active = virtio32_to_cpu(vb->vdev, vb-
> > >cmd_id_active);
> > > + virtio_balloon_read_cmd_id_received(vb);
>
>
> [1]
>
>
> > > if (cmd_id_active != vb->cmd_id_received)
> > > break;
> > >
> > > @@ -637,11 +648,9 @@ static int send_free_pages(struct virtio_balloon
> > *vb)
> > > return 0;
> > > }
> > >
> > > -static void report_free_page_func(struct work_struct *work)
> > > +static void virtio_balloon_report_free_page(struct virtio_balloon
> > > +*vb)
> > > {
> > > int err;
> > > - struct virtio_balloon *vb = container_of(work, struct virtio_balloon,
> > > - report_free_page_work);
> > > struct device *dev = &vb->vdev->dev;
> > >
> > > /* Start by sending the received cmd id to host with an outbuf. */
> > > @@ -659,6 +668,22 @@ static void report_free_page_func(struct
> > work_struct *work)
> > > dev_err(dev, "Failed to send a stop id, err = %d\n", err); }
> > >
> > > +static void report_free_page_func(struct work_struct *work) {
> > > + struct virtio_balloon *vb = container_of(work, struct virtio_balloon,
> > > + report_free_page_work);
> > > +
> > > + virtio_balloon_read_cmd_id_received(vb);
> >
> > This will not achieve what you are trying to do, which is cancel reporting if it's
> > in progress.
> >
> > You need to re-read each time you compare to cmd_id_active.
>
> Yes, already did, please see [1] above

Oh right. Sorry.

>
> > An API similar to
> > u32 virtio_balloon_cmd_id_received(vb)
> > seems to be called for, and I would rename cmd_id_received to
> > cmd_id_received_cache to make sure we caught all users.
> >
>
> I'm not sure about adding "cache" here, cmd_id_received refers to the cmd id
> that the driver just received from the device. There is one called "cmd_id_active" which
> is the one that the driver is actively using.
> So cmd_id_received is already a "cache" to " cmd_id_active " in some sense.
>
> Best,
> Wei

The point is that any access to cmd_id_received should first call
virtio_balloon_read_cmd_id_received. So a better API is
to have virtio_balloon_read_cmd_id_received return the value
instead of poking at cmd_id_received afterwards.
Renaming cmd_id_received will help make sure all no
call sites were missed and help stress it's never used directly.


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