Re: [PATCH v3 6/7] 9p/trans_virtio: support larger msize values

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.

Signed-off-by: Christian Schoenebeck <linux_oss@crudebyte.com>
---
 net/9p/trans_virtio.c | 61 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 61 insertions(+)

diff --git a/net/9p/trans_virtio.c b/net/9p/trans_virtio.c
index e478a34326f1..147ebf647a95 100644
--- a/net/9p/trans_virtio.c
+++ b/net/9p/trans_virtio.c

@@ -203,6 +203,31 @@ 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 +799,10 @@ 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 +825,38 @@ 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 > 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 default msize instead
+                        */
+                       if (!vq_sg_resize(&chan->vq_sg, nsgl)) {
+                               client->trans_maxsize =
+                                       PAGE_SIZE *
+                                       ((nsgl * SG_USER_PAGES_PER_LIST)
- 3);
+                       }
+               }
+#endif /* CONFIG_ARCH_NO_SG_CHAIN */
+       }
+
        client->trans = (void *)chan;
        client->status = Connected;
        chan->client = client;