[PATCH v9 27/37] virtio_balloon: submit reported pages as individual buffers

["Michael S. Tsirkin" <mst@redhat.com>]

Submit each reported page as a separate virtqueue buffer instead
of one buffer with an sg list of all pages. This avoids indirect
descriptor allocation (kmalloc in the reporting path) and gives
per-page used length feedback from the device.

On error, the already-queued pages are kicked and drained
before the error is returned. The caller (page_reporting_drain)
then marks the batch as unreported, which is conservative
but correct.

Note: if the virtqueue is broken, wait_event on
virtqueue_get_buf hangs.  This is a pre-existing issue:
the old single-buffer code had the same hang.  EVENT_IDX
is not a concern: callbacks were never disabled, so the
virtqueue manages used_event automatically.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Assisted-by: Claude:claude-opus-4-6
---
 drivers/virtio/virtio_balloon.c | 40 +++++++++++++++++++++------------
 1 file changed, 26 insertions(+), 14 deletions(-)

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index 6a1a610c2cb1..53b4a3984e7d 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -187,7 +187,9 @@ static void tell_host(struct virtio_balloon *vb, struct virtqueue *vq)
 
 	sg_init_one(&sg, vb->pfns, sizeof(vb->pfns[0]) * vb->num_pfns);
 
-	/* We should always be able to add one buffer to an empty queue. */
+	/* We made sure the vq is large enough so we should always
+	 * be able to add one buffer to an empty queue.
+	 */
 	virtqueue_add_outbuf(vq, &sg, 1, vb, GFP_KERNEL);
 	virtqueue_kick(vq);
 
@@ -202,25 +204,35 @@ static int virtballoon_free_page_report(struct page_reporting_dev_info *pr_dev_i
 	struct virtio_balloon *vb =
 		container_of(pr_dev_info, struct virtio_balloon, pr_dev_info);
 	struct virtqueue *vq = vb->reporting_vq;
-	unsigned int unused, err;
+	unsigned int i, err = 0;
 
 	/* We should always be able to add these buffers to an empty queue. */
-	err = virtqueue_add_inbuf(vq, sg, nents, vb, GFP_NOWAIT);
+	for (i = 0; i < nents; i++) {
+		struct scatterlist one;
 
-	/*
-	 * In the extremely unlikely case that something has occurred and we
-	 * are able to trigger an error we will simply display a warning
-	 * and exit without actually processing the pages.
-	 */
-	if (WARN_ON_ONCE(err))
-		return err;
+		sg_init_table(&one, 1);
+		sg_set_page(&one, sg_page(&sg[i]), sg[i].length,
+			    sg[i].offset);
+		err = virtqueue_add_inbuf(vq, &one, 1, &sg[i], GFP_NOWAIT);
+		if (WARN_ON_ONCE(err)) {
+			nents = i;
+			break;
+		}
+	}
 
-	virtqueue_kick(vq);
+	if (nents) {
+		virtqueue_kick(vq);
 
-	/* When host has read buffer, this completes via balloon_ack */
-	wait_event(vb->acked, virtqueue_get_buf(vq, &unused));
+		/* When host has read buffer, this completes via balloon_ack */
+		for (i = 0; i < nents; i++) {
+			unsigned int unused;
 
-	return 0;
+			wait_event(vb->acked,
+				   virtqueue_get_buf(vq, &unused));
+		}
+	}
+
+	return err;
 }
 
 static void set_page_pfns(struct virtio_balloon *vb,
-- 
MST