Re: [PATCH 5/9] rbd: get the snapshot context for a v2 image

[Josh Durgin <josh.durgin@inktank.com>]

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>

On 09/07/2012 02:13 PM, Alex Elder wrote:
> Fetch the snapshot context for an rbd format 2 image by calling
> the "get_snapcontext" method on its header object.
>
> Signed-off-by: Alex Elder <elder@inktank.com>
> ---
>   drivers/block/rbd.c |   85
> +++++++++++++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 85 insertions(+)
>
> diff --git a/drivers/block/rbd.c b/drivers/block/rbd.c
> index d48f025..8ff84fd 100644
> --- a/drivers/block/rbd.c
> +++ b/drivers/block/rbd.c
> @@ -62,6 +62,7 @@
>   #define RBD_MINORS_PER_MAJOR	256		/* max minors per blkdev */
>
>   #define RBD_MAX_SNAP_NAME_LEN	32
> +#define RBD_MAX_SNAP_COUNT	510	/* allows max snapc to fit in 4KB */
>   #define RBD_MAX_OPT_LEN		1024
>
>   #define RBD_SNAP_HEAD_NAME	"-"
> @@ -2234,6 +2235,84 @@ static int rbd_dev_v2_features(struct rbd_device
> *rbd_dev)
>   						&rbd_dev->header.features);
>   }
>
> +static int rbd_dev_v2_snap_context(struct rbd_device *rbd_dev)
> +{
> +	size_t size;
> +	int ret;
> +	void *reply_buf;
> +	void *p;
> +	void *end;
> +	u64 seq;
> +	u32 snap_count;
> +	struct ceph_snap_context *snapc;
> +	u32 i;
> +
> +	/*
> +	 * We'll need room for the seq value (maximum snapshot id),
> +	 * snapshot count, and array of that many snapshot ids.
> +	 * For now we have a fixed upper limit on the number we're
> +	 * prepared to receive.
> +	 */
> +	size = sizeof (__le64) + sizeof (__le32) +
> +			RBD_MAX_SNAP_COUNT * sizeof (__le64);
> +	reply_buf = kzalloc(size, GFP_KERNEL);
> +	if (!reply_buf)
> +		return -ENOMEM;
> +
> +	ret = rbd_req_sync_exec(rbd_dev, rbd_dev->header_name,
> +				"rbd", "get_snapcontext",
> +				NULL, 0,
> +				reply_buf, size,
> +				CEPH_OSD_FLAG_READ, NULL);
> +	dout("%s: rbd_req_sync_exec returned %d\n", __func__, ret);
> +	if (ret < 0)
> +		goto out;
> +
> +	ret = -ERANGE;
> +	p = reply_buf;
> +	end = (char *) reply_buf + size;
> +	ceph_decode_64_safe(&p, end, seq, out);
> +	ceph_decode_32_safe(&p, end, snap_count, out);
> +
> +	/*
> +	 * Make sure the reported number of snapshot ids wouldn't go
> +	 * beyond the end of our buffer.  But before checking that,
> +	 * make sure the computed size of the snapshot context we
> +	 * allocate is representable in a size_t.
> +	 */
> +	if (snap_count > (SIZE_MAX - sizeof (struct ceph_snap_context))
> +				 / sizeof (u64)) {
> +		ret = -EINVAL;
> +		goto out;
> +	}
> +	if (!ceph_has_room(&p, end, snap_count * sizeof (__le64)))
> +		goto out;
> +
> +	size = sizeof (struct ceph_snap_context) +
> +				snap_count * sizeof (snapc->snaps[0]);
> +	snapc = kmalloc(size, GFP_KERNEL);
> +	if (!snapc) {
> +		ret = -ENOMEM;
> +		goto out;
> +	}
> +
> +	atomic_set(&snapc->nref, 1);
> +	snapc->seq = seq;
> +	snapc->num_snaps = snap_count;
> +	for (i = 0; i < snap_count; i++)
> +		snapc->snaps[i] = ceph_decode_64(&p);
> +
> +	rbd_dev->header.snapc = snapc;
> +
> +	dout("  snap context seq = %llu, snap_count = %u\n",
> +		(unsigned long long) seq, (unsigned int) snap_count);
> +
> +out:
> +	kfree(reply_buf);
> +
> +	return 0;
> +}
> +
>   /*
>    * Scan the rbd device's current snapshot list and compare it to the
>    * newly-received snapshot context.  Remove any existing snapshots
> @@ -2773,6 +2852,12 @@ static int rbd_dev_v2_probe(struct rbd_device
> *rbd_dev)
>   	ret = rbd_dev_v2_features(rbd_dev);
>   	if (ret < 0)
>   		goto out_err;
> +
> +	/* Get the snapshot context */
> +
> +	ret = rbd_dev_v2_snap_context(rbd_dev);
> +	if (ret)
> +		goto out_err;
>   	rbd_dev->image_format = 2;
>
>   	dout("discovered version 2 image, header name is %s\n",
>