[PATCH v6 05/11] 9p/trans_virtio: support larger msize values

[Christian Schoenebeck <linux_oss@crudebyte.com>]

The virtio transport supports by default a 9p 'msize' of up to
approximately 500 kB. This patch adds support for larger 'msize'
values by resizing the amount of scatter/gather lists if required.

To be more precise, for the moment this patch increases the 'msize'
limit for the virtio transport to slightly below 4 MB, virtio
transport actually supports much more (tested successfully with an
experimental QEMU version and some dirty 9p Linux client hacks up
to msize=128MB), but client still uses linear buffers, which in
turn are limited to KMALLOC_MAX_SIZE (4M).

Signed-off-by: Christian Schoenebeck <linux_oss@crudebyte.com>
---

I am not sure if it is safe the way SG lists are resized here. I "think"
Dominique said before there should be no concurrency here, but probably
deserves a revisit.

 net/9p/trans_virtio.c | 79 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 79 insertions(+)

diff --git a/net/9p/trans_virtio.c b/net/9p/trans_virtio.c
index 5ac533f83322..921caa022570 100644
--- a/net/9p/trans_virtio.c
+++ b/net/9p/trans_virtio.c
@@ -36,6 +36,16 @@
 #include <linux/virtio_9p.h>
 #include "trans_common.h"
 
+/*
+ * Maximum amount of virtio descriptors allowed per virtio round-trip
+ * message.
+ *
+ * This effectively limits msize to (slightly below) 4M, virtio transport
+ * actually supports much more, but client still uses linear buffers, which
+ * in turn are limited to KMALLOC_MAX_SIZE (4M).
+ */
+#define VIRTIO_MAX_DESCRIPTORS 1024
+
 /**
  * struct virtqueue_sg - (chained) scatter gather lists for virtqueue data
  * transmission
@@ -203,6 +213,31 @@ static struct virtqueue_sg *vq_sg_alloc(unsigned int nsgl)
 	return vq_sg;
 }
 
+/**
+ * vq_sg_resize - resize passed virtqueue scatter/gather lists to the passed
+ * amount of lists
+ * @_vq_sg: scatter/gather lists to be resized
+ * @nsgl: new amount of scatter/gather lists
+ */
+static int vq_sg_resize(struct virtqueue_sg **_vq_sg, unsigned int nsgl)
+{
+	struct virtqueue_sg *vq_sg;
+
+	BUG_ON(!_vq_sg || !nsgl);
+	vq_sg = *_vq_sg;
+	if (vq_sg->nsgl == nsgl)
+		return 0;
+
+	/* lazy resize implementation for now */
+	vq_sg = vq_sg_alloc(nsgl);
+	if (!vq_sg)
+		return -ENOMEM;
+
+	kfree(*_vq_sg);
+	*_vq_sg = vq_sg;
+	return 0;
+}
+
 /**
  * p9_virtio_close - reclaim resources of a channel
  * @client: client instance
@@ -774,6 +809,10 @@ p9_virtio_create(struct p9_client *client, const char *devname, char *args)
 	struct virtio_chan *chan;
 	int ret = -ENOENT;
 	int found = 0;
+#if !defined(CONFIG_ARCH_NO_SG_CHAIN)
+	size_t npages;
+	size_t nsgl;
+#endif
 
 	if (devname == NULL)
 		return -EINVAL;
@@ -796,6 +835,46 @@ p9_virtio_create(struct p9_client *client, const char *devname, char *args)
 		return ret;
 	}
 
+	/*
+	 * if user supplied an 'msize' option that's larger than what this
+	 * transport supports by default, then try to allocate more sg lists
+	 */
+	if (client->msize > client->trans_maxsize) {
+#ifdef CONFIG_ARCH_NO_SG_CHAIN
+		pr_info("limiting 'msize' to %d because architecture does not "
+			"support chained scatter gather lists\n",
+			client->trans_maxsize);
+#else
+		npages = DIV_ROUND_UP(client->msize, PAGE_SIZE);
+		if (npages > VIRTIO_MAX_DESCRIPTORS)
+			npages = VIRTIO_MAX_DESCRIPTORS;
+		if (npages > chan->p9_max_pages) {
+			npages = chan->p9_max_pages;
+			pr_info("limiting 'msize' as it would exceed the max. "
+				"of %lu pages allowed on this system\n",
+				chan->p9_max_pages);
+		}
+		nsgl = DIV_ROUND_UP(npages, SG_USER_PAGES_PER_LIST);
+		if (nsgl > chan->vq_sg->nsgl) {
+			/*
+			 * if resize fails, no big deal, then just continue with
+			 * whatever we got
+			 */
+			if (!vq_sg_resize(&chan->vq_sg, nsgl)) {
+				/*
+				 * decrement 2 pages as both 9p request and 9p reply have
+				 * to fit into the virtio round-trip message
+				 */
+				client->trans_maxsize =
+					PAGE_SIZE *
+					clamp_t(int,
+						(nsgl * SG_USER_PAGES_PER_LIST) - 2,
+						0, VIRTIO_MAX_DESCRIPTORS - 2);
+			}
+		}
+#endif /* CONFIG_ARCH_NO_SG_CHAIN */
+	}
+
 	client->trans = (void *)chan;
 	client->status = Connected;
 	chan->client = client;
-- 
2.30.2