[PATCH v2 0/7] net/9p: remove msize limit in virtio transport

[PATCH v2 0/7] net/9p: remove msize limit in virtio transport

[Christian Schoenebeck]

This series aims to get get rid of the current 500k 'msize' limitation in
the 9p virtio transport, which is currently a bottleneck for performance
of 9p mounts.

This is a follow-up of the following series and discussion:
https://lore.kernel.org/all/28bb651ae0349a7d57e8ddc92c1bd5e62924a912.1630770829.git.linux_oss@crudebyte.com/T/#eb647d0c013616cee3eb8ba9d87da7d8b1f476f37

Known limitation: With this series applied I can run

  QEMU host <-> 9P virtio <-> Linux guest

with up to 3 MB msize. If I try to run it with 4 MB it seems to hit some
limitation on QEMU side:

  qemu-system-x86_64: virtio: too many write descriptors in indirect table

I haven't looked into this issue yet.

Testing and feedback appreciated!

v1 -> v2:

  * Fix memory leak [patch 6].

  * Use pr_info() instead of p9_debug(P9_DEBUG_ERROR, ...) for user info
    [patch 1].

  * Show user info message if chained SG lists are not supported on
    architecture [patch 6].

  * Kernel doc fixes [patch 4], [patch 6].

Christian Schoenebeck (7):
  net/9p: show error message if user 'msize' cannot be satisfied
  9p/trans_virtio: separate allocation of scatter gather list
  9p/trans_virtio: turn amount of sg lists into runtime info
  9p/trans_virtio: introduce struct virtqueue_sg
  net/9p: add trans_maxsize to struct p9_client
  9p/trans_virtio: support larger msize values
  9p/trans_virtio: resize sg lists to whatever is possible

 include/net/9p/client.h |   2 +
 net/9p/client.c         |  17 ++-
 net/9p/trans_virtio.c   | 290 ++++++++++++++++++++++++++++++++++------
 3 files changed, 269 insertions(+), 40 deletions(-)

-- 
2.20.1

[PATCH v2 1/7] net/9p: show error message if user 'msize' cannot be satisfied

[Christian Schoenebeck]

If user supplied a large value with the 'msize' option, then
client would silently limit that 'msize' value to the maximum
value supported by transport. That's a bit confusing for users
of not having any indication why the preferred 'msize' value
could not be satisfied.

Reported-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Christian Schoenebeck <linux_oss@crudebyte.com>
---
 net/9p/client.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/net/9p/client.c b/net/9p/client.c
index 213f12ed76cd..4f4fd2098a30 100644
--- a/net/9p/client.c
+++ b/net/9p/client.c
@@ -1044,8 +1044,13 @@ struct p9_client *p9_client_create(const char *dev_name, char *options)
 	if (err)
 		goto put_trans;
 
-	if (clnt->msize > clnt->trans_mod->maxsize)
+	if (clnt->msize > clnt->trans_mod->maxsize) {
 		clnt->msize = clnt->trans_mod->maxsize;
+		pr_info("Limiting 'msize' to %d as this is the maximum "
+			"supported by transport %s\n",
+			clnt->msize, clnt->trans_mod->name
+		);
+	}
 
 	if (clnt->msize < 4096) {
 		p9_debug(P9_DEBUG_ERROR,
-- 
2.20.1

[PATCH v2 2/7] 9p/trans_virtio: separate allocation of scatter gather list

[Christian Schoenebeck]

The scatter gather list in struct virtio_chan currently
resides as compile-time constant size array directly within the
contiguous struct virtio_chan's memory space.

Separate memory space and allocation of the scatter gather list
from memory space and allocation of struct virtio_chan.

Signed-off-by: Christian Schoenebeck <linux_oss@crudebyte.com>
---
 net/9p/trans_virtio.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/net/9p/trans_virtio.c b/net/9p/trans_virtio.c
index 490a4c900339..1dbe2e921bb8 100644
--- a/net/9p/trans_virtio.c
+++ b/net/9p/trans_virtio.c
@@ -77,7 +77,7 @@ struct virtio_chan {
 	 */
 	unsigned long p9_max_pages;
 	/* Scatterlist: can be too big for stack. */
-	struct scatterlist sg[VIRTQUEUE_NUM];
+	struct scatterlist *sg;
 	/**
 	 * @tag: name to identify a mount null terminated
 	 */
@@ -574,6 +574,14 @@ static int p9_virtio_probe(struct virtio_device *vdev)
 		goto fail;
 	}
 
+	chan->sg = kmalloc_array(VIRTQUEUE_NUM,
+				 sizeof(struct scatterlist), GFP_KERNEL);
+	if (!chan->sg) {
+		pr_err("Failed to allocate virtio 9P channel\n");
+		err = -ENOMEM;
+		goto out_free_chan_shallow;
+	}
+
 	chan->vdev = vdev;
 
 	/* We expect one virtqueue, for requests. */
@@ -635,6 +643,8 @@ static int p9_virtio_probe(struct virtio_device *vdev)
 out_free_vq:
 	vdev->config->del_vqs(vdev);
 out_free_chan:
+	kfree(chan->sg);
+out_free_chan_shallow:
 	kfree(chan);
 fail:
 	return err;
@@ -728,6 +738,7 @@ static void p9_virtio_remove(struct virtio_device *vdev)
 	kobject_uevent(&(vdev->dev.kobj), KOBJ_CHANGE);
 	kfree(chan->tag);
 	kfree(chan->vc_wq);
+	kfree(chan->sg);
 	kfree(chan);
 
 }
-- 
2.20.1

[PATCH v2 3/7] 9p/trans_virtio: turn amount of sg lists into runtime info

[Christian Schoenebeck]

The size of scatter/gather lists used by the virtio transport is
currently hard coded. Refactor this to using a runtime variable.

Signed-off-by: Christian Schoenebeck <linux_oss@crudebyte.com>
---
 net/9p/trans_virtio.c | 23 +++++++++++++----------
 1 file changed, 13 insertions(+), 10 deletions(-)

diff --git a/net/9p/trans_virtio.c b/net/9p/trans_virtio.c
index 1dbe2e921bb8..3347d35a5e6e 100644
--- a/net/9p/trans_virtio.c
+++ b/net/9p/trans_virtio.c
@@ -36,7 +36,7 @@
 #include <linux/virtio_9p.h>
 #include "trans_common.h"
 
-#define VIRTQUEUE_NUM	128
+#define VIRTQUEUE_DEFAULT_NUM	128
 
 /* a single mutex to manage channel initialization and attachment */
 static DEFINE_MUTEX(virtio_9p_lock);
@@ -54,6 +54,7 @@ static atomic_t vp_pinned = ATOMIC_INIT(0);
  * @vc_wq: wait queue for waiting for thing to be added to ring buf
  * @p9_max_pages: maximum number of pinned pages
  * @sg: scatter gather list which is used to pack a request (protected?)
+ * @sg_n: amount of elements in sg array
  * @chan_list: linked list of channels
  *
  * We keep all per-channel information in a structure.
@@ -78,6 +79,7 @@ struct virtio_chan {
 	unsigned long p9_max_pages;
 	/* Scatterlist: can be too big for stack. */
 	struct scatterlist *sg;
+	size_t sg_n;
 	/**
 	 * @tag: name to identify a mount null terminated
 	 */
@@ -270,12 +272,12 @@ p9_virtio_request(struct p9_client *client, struct p9_req_t *req)
 	out_sgs = in_sgs = 0;
 	/* Handle out VirtIO ring buffers */
 	out = pack_sg_list(chan->sg, 0,
-			   VIRTQUEUE_NUM, req->tc.sdata, req->tc.size);
+			   chan->sg_n, req->tc.sdata, req->tc.size);
 	if (out)
 		sgs[out_sgs++] = chan->sg;
 
 	in = pack_sg_list(chan->sg, out,
-			  VIRTQUEUE_NUM, req->rc.sdata, req->rc.capacity);
+			  chan->sg_n, req->rc.sdata, req->rc.capacity);
 	if (in)
 		sgs[out_sgs + in_sgs++] = chan->sg + out;
 
@@ -447,14 +449,14 @@ p9_virtio_zc_request(struct p9_client *client, struct p9_req_t *req,
 
 	/* out data */
 	out = pack_sg_list(chan->sg, 0,
-			   VIRTQUEUE_NUM, req->tc.sdata, req->tc.size);
+			   chan->sg_n, req->tc.sdata, req->tc.size);
 
 	if (out)
 		sgs[out_sgs++] = chan->sg;
 
 	if (out_pages) {
 		sgs[out_sgs++] = chan->sg + out;
-		out += pack_sg_list_p(chan->sg, out, VIRTQUEUE_NUM,
+		out += pack_sg_list_p(chan->sg, out, chan->sg_n,
 				      out_pages, out_nr_pages, offs, outlen);
 	}
 
@@ -466,13 +468,13 @@ p9_virtio_zc_request(struct p9_client *client, struct p9_req_t *req,
 	 * allocated memory and payload onto the user buffer.
 	 */
 	in = pack_sg_list(chan->sg, out,
-			  VIRTQUEUE_NUM, req->rc.sdata, in_hdr_len);
+			  chan->sg_n, req->rc.sdata, in_hdr_len);
 	if (in)
 		sgs[out_sgs + in_sgs++] = chan->sg + out;
 
 	if (in_pages) {
 		sgs[out_sgs + in_sgs++] = chan->sg + out + in;
-		in += pack_sg_list_p(chan->sg, out + in, VIRTQUEUE_NUM,
+		in += pack_sg_list_p(chan->sg, out + in, chan->sg_n,
 				     in_pages, in_nr_pages, offs, inlen);
 	}
 
@@ -574,13 +576,14 @@ static int p9_virtio_probe(struct virtio_device *vdev)
 		goto fail;
 	}
 
-	chan->sg = kmalloc_array(VIRTQUEUE_NUM,
+	chan->sg = kmalloc_array(VIRTQUEUE_DEFAULT_NUM,
 				 sizeof(struct scatterlist), GFP_KERNEL);
 	if (!chan->sg) {
 		pr_err("Failed to allocate virtio 9P channel\n");
 		err = -ENOMEM;
 		goto out_free_chan_shallow;
 	}
+	chan->sg_n = VIRTQUEUE_DEFAULT_NUM;
 
 	chan->vdev = vdev;
 
@@ -593,7 +596,7 @@ static int p9_virtio_probe(struct virtio_device *vdev)
 	chan->vq->vdev->priv = chan;
 	spin_lock_init(&chan->lock);
 
-	sg_init_table(chan->sg, VIRTQUEUE_NUM);
+	sg_init_table(chan->sg, chan->sg_n);
 
 	chan->inuse = false;
 	if (virtio_has_feature(vdev, VIRTIO_9P_MOUNT_TAG)) {
@@ -777,7 +780,7 @@ static struct p9_trans_module p9_virtio_trans = {
 	 * that are not at page boundary, that can result in an extra
 	 * page in zero copy.
 	 */
-	.maxsize = PAGE_SIZE * (VIRTQUEUE_NUM - 3),
+	.maxsize = PAGE_SIZE * (VIRTQUEUE_DEFAULT_NUM - 3),
 	.def = 1,
 	.owner = THIS_MODULE,
 };
-- 
2.20.1

[PATCH v2 4/7] 9p/trans_virtio: introduce struct virtqueue_sg

[Christian Schoenebeck]

The amount of elements in a scatter/gather list is limited to
approximately 128 elements. To allow going beyond that limit
with subsequent patches, pave the way by turning the one-
dimensional sg list array into a two-dimensional array, i.e:

  sg[128]

becomes

  sgl[nsgl][SG_MAX_SINGLE_ALLOC]

As the value of 'nsgl' is exactly (still) 1 in this commit
and the compile-time (compiler and architecture dependent)
value of 'SG_MAX_SINGLE_ALLOC' equals approximately the
previous hard coded 128 elements, this commit is therefore
more of a preparatory refactoring then actual behaviour
change.

A custom struct virtqueue_sg is defined instead of using
shared API struct sg_table, because the latter would not
allow to resize the table after allocation. sg_append_table
API OTOH would not fit either, because it requires a list
of pages beforehand upon allocation. And both APIs only
support all-or-nothing allocation.

Signed-off-by: Christian Schoenebeck <linux_oss@crudebyte.com>
---
 net/9p/trans_virtio.c | 190 ++++++++++++++++++++++++++++++++----------
 1 file changed, 144 insertions(+), 46 deletions(-)

diff --git a/net/9p/trans_virtio.c b/net/9p/trans_virtio.c
index 3347d35a5e6e..0db8de84bd51 100644
--- a/net/9p/trans_virtio.c
+++ b/net/9p/trans_virtio.c
@@ -36,7 +36,31 @@
 #include <linux/virtio_9p.h>
 #include "trans_common.h"
 
-#define VIRTQUEUE_DEFAULT_NUM	128
+/**
+ * struct virtqueue_sg - (chained) scatter gather lists for virtqueue data
+ * transmission
+ * @nsgl: amount of elements (in first dimension) of array field @sgl
+ * @sgl: two-dimensional array, i.e. sgl[nsgl][SG_MAX_SINGLE_ALLOC]
+ */
+struct virtqueue_sg {
+	unsigned int nsgl;
+	struct scatterlist *sgl[];
+};
+
+/*
+ * Default value for field nsgl in struct virtqueue_sg, which defines the
+ * initial virtio data transmission capacity when this virtio transport is
+ * probed.
+ */
+#define VIRTQUEUE_SG_NSGL_DEFAULT 1
+
+/* maximum value for field nsgl in struct virtqueue_sg */
+#define VIRTQUEUE_SG_NSGL_MAX						\
+	((PAGE_SIZE - sizeof(struct virtqueue_sg)) /			\
+	sizeof(struct scatterlist *))					\
+
+/* last entry per sg list is used for chaining (pointer to next list) */
+#define SG_USER_PAGES_PER_LIST	(SG_MAX_SINGLE_ALLOC - 1)
 
 /* a single mutex to manage channel initialization and attachment */
 static DEFINE_MUTEX(virtio_9p_lock);
@@ -53,8 +77,7 @@ static atomic_t vp_pinned = ATOMIC_INIT(0);
  * @ring_bufs_avail: flag to indicate there is some available in the ring buf
  * @vc_wq: wait queue for waiting for thing to be added to ring buf
  * @p9_max_pages: maximum number of pinned pages
- * @sg: scatter gather list which is used to pack a request (protected?)
- * @sg_n: amount of elements in sg array
+ * @vq_sg: table of scatter gather lists, which are used to pack a request
  * @chan_list: linked list of channels
  *
  * We keep all per-channel information in a structure.
@@ -77,9 +100,7 @@ struct virtio_chan {
 	 * will be placing it in each channel.
 	 */
 	unsigned long p9_max_pages;
-	/* Scatterlist: can be too big for stack. */
-	struct scatterlist *sg;
-	size_t sg_n;
+	struct virtqueue_sg *vq_sg;
 	/**
 	 * @tag: name to identify a mount null terminated
 	 */
@@ -96,6 +117,89 @@ static unsigned int rest_of_page(void *data)
 	return PAGE_SIZE - offset_in_page(data);
 }
 
+/**
+ * vq_sg_page - returns user page for given page index
+ * @vq_sg: scatter gather lists used by this transport
+ * @page: user page index across all scatter gather lists
+ */
+static struct scatterlist *vq_sg_page(struct virtqueue_sg *vq_sg, size_t page)
+{
+	unsigned int node = page / SG_USER_PAGES_PER_LIST;
+	unsigned int leaf = page % SG_USER_PAGES_PER_LIST;
+	BUG_ON(node >= VIRTQUEUE_SG_NSGL_MAX);
+	return &vq_sg->sgl[node][leaf];
+}
+
+/**
+ * vq_sg_npages - returns total number of individual user pages in passed
+ * scatter gather lists
+ * @vq_sg: scatter gather lists to be counted
+ */
+static size_t vq_sg_npages(struct virtqueue_sg *vq_sg)
+{
+	return vq_sg->nsgl * SG_USER_PAGES_PER_LIST;
+}
+
+/**
+ * vq_sg_free - free all memory previously allocated for @vq_sg
+ * @vq_sg: scatter gather lists to be freed
+ */
+static void vq_sg_free(struct virtqueue_sg *vq_sg)
+{
+	unsigned int i;
+
+	for (i = 0; i < vq_sg->nsgl; ++i) {
+		kfree(vq_sg->sgl[i]);
+	}
+	kfree(vq_sg);
+}
+
+/**
+ * vq_sg_alloc - allocates and returns @nsgl scatter gather lists
+ * @nsgl: amount of scatter gather lists to be allocated
+ * If @nsgl is larger than one then chained lists are used if supported by
+ * architecture.
+ */
+static struct virtqueue_sg *vq_sg_alloc(unsigned int nsgl)
+{
+	struct virtqueue_sg *vq_sg;
+	unsigned int i;
+
+	BUG_ON(!nsgl || nsgl > VIRTQUEUE_SG_NSGL_MAX);
+#ifdef CONFIG_ARCH_NO_SG_CHAIN
+	if (WARN_ON_ONCE(nsgl > 1))
+		return NULL;
+#endif
+
+	vq_sg = kzalloc(sizeof(struct virtqueue_sg) +
+			nsgl * sizeof(struct scatterlist *),
+			GFP_KERNEL);
+
+	if (!vq_sg)
+		return NULL;
+
+	vq_sg->nsgl = nsgl;
+
+	for (i = 0; i < nsgl; ++i) {
+		vq_sg->sgl[i] = kmalloc_array(
+			SG_MAX_SINGLE_ALLOC, sizeof(struct scatterlist),
+			GFP_KERNEL
+		);
+		if (!vq_sg->sgl[i]) {
+			vq_sg_free(vq_sg);
+			return NULL;
+		}
+		sg_init_table(vq_sg->sgl[i], SG_MAX_SINGLE_ALLOC);
+		if (i) {
+			/* chain the lists */
+			sg_chain(vq_sg->sgl[i - 1], SG_MAX_SINGLE_ALLOC,
+				 vq_sg->sgl[i]);
+		}
+	}
+	sg_mark_end(&vq_sg->sgl[nsgl - 1][SG_MAX_SINGLE_ALLOC - 1]);
+	return vq_sg;
+}
+
 /**
  * p9_virtio_close - reclaim resources of a channel
  * @client: client instance
@@ -158,9 +262,8 @@ static void req_done(struct virtqueue *vq)
 
 /**
  * pack_sg_list - pack a scatter gather list from a linear buffer
- * @sg: scatter/gather list to pack into
+ * @vq_sg: scatter/gather lists to pack into
  * @start: which segment of the sg_list to start at
- * @limit: maximum segment to pack data to
  * @data: data to pack into scatter/gather list
  * @count: amount of data to pack into the scatter/gather list
  *
@@ -170,11 +273,12 @@ static void req_done(struct virtqueue *vq)
  *
  */
 
-static int pack_sg_list(struct scatterlist *sg, int start,
-			int limit, char *data, int count)
+static int pack_sg_list(struct virtqueue_sg *vq_sg, int start,
+			char *data, int count)
 {
 	int s;
 	int index = start;
+	size_t limit = vq_sg_npages(vq_sg);
 
 	while (count) {
 		s = rest_of_page(data);
@@ -182,13 +286,13 @@ static int pack_sg_list(struct scatterlist *sg, int start,
 			s = count;
 		BUG_ON(index >= limit);
 		/* Make sure we don't terminate early. */
-		sg_unmark_end(&sg[index]);
-		sg_set_buf(&sg[index++], data, s);
+		sg_unmark_end(vq_sg_page(vq_sg, index));
+		sg_set_buf(vq_sg_page(vq_sg, index++), data, s);
 		count -= s;
 		data += s;
 	}
 	if (index-start)
-		sg_mark_end(&sg[index - 1]);
+		sg_mark_end(vq_sg_page(vq_sg, index - 1));
 	return index-start;
 }
 
@@ -208,21 +312,21 @@ static int p9_virtio_cancelled(struct p9_client *client, struct p9_req_t *req)
 /**
  * pack_sg_list_p - Just like pack_sg_list. Instead of taking a buffer,
  * this takes a list of pages.
- * @sg: scatter/gather list to pack into
+ * @vq_sg: scatter/gather lists to pack into
  * @start: which segment of the sg_list to start at
- * @limit: maximum number of pages in sg list.
  * @pdata: a list of pages to add into sg.
  * @nr_pages: number of pages to pack into the scatter/gather list
  * @offs: amount of data in the beginning of first page _not_ to pack
  * @count: amount of data to pack into the scatter/gather list
  */
 static int
-pack_sg_list_p(struct scatterlist *sg, int start, int limit,
+pack_sg_list_p(struct virtqueue_sg *vq_sg, int start,
 	       struct page **pdata, int nr_pages, size_t offs, int count)
 {
 	int i = 0, s;
 	int data_off = offs;
 	int index = start;
+	size_t limit = vq_sg_npages(vq_sg);
 
 	BUG_ON(nr_pages > (limit - start));
 	/*
@@ -235,15 +339,16 @@ pack_sg_list_p(struct scatterlist *sg, int start, int limit,
 			s = count;
 		BUG_ON(index >= limit);
 		/* Make sure we don't terminate early. */
-		sg_unmark_end(&sg[index]);
-		sg_set_page(&sg[index++], pdata[i++], s, data_off);
+		sg_unmark_end(vq_sg_page(vq_sg, index));
+		sg_set_page(vq_sg_page(vq_sg, index++), pdata[i++], s,
+			    data_off);
 		data_off = 0;
 		count -= s;
 		nr_pages--;
 	}
 
 	if (index-start)
-		sg_mark_end(&sg[index - 1]);
+		sg_mark_end(vq_sg_page(vq_sg, index - 1));
 	return index - start;
 }
 
@@ -271,15 +376,13 @@ p9_virtio_request(struct p9_client *client, struct p9_req_t *req)
 
 	out_sgs = in_sgs = 0;
 	/* Handle out VirtIO ring buffers */
-	out = pack_sg_list(chan->sg, 0,
-			   chan->sg_n, req->tc.sdata, req->tc.size);
+	out = pack_sg_list(chan->vq_sg, 0, req->tc.sdata, req->tc.size);
 	if (out)
-		sgs[out_sgs++] = chan->sg;
+		sgs[out_sgs++] = vq_sg_page(chan->vq_sg, 0);
 
-	in = pack_sg_list(chan->sg, out,
-			  chan->sg_n, req->rc.sdata, req->rc.capacity);
+	in = pack_sg_list(chan->vq_sg, out, req->rc.sdata, req->rc.capacity);
 	if (in)
-		sgs[out_sgs + in_sgs++] = chan->sg + out;
+		sgs[out_sgs + in_sgs++] = vq_sg_page(chan->vq_sg, out);
 
 	err = virtqueue_add_sgs(chan->vq, sgs, out_sgs, in_sgs, req,
 				GFP_ATOMIC);
@@ -448,16 +551,15 @@ p9_virtio_zc_request(struct p9_client *client, struct p9_req_t *req,
 	out_sgs = in_sgs = 0;
 
 	/* out data */
-	out = pack_sg_list(chan->sg, 0,
-			   chan->sg_n, req->tc.sdata, req->tc.size);
+	out = pack_sg_list(chan->vq_sg, 0, req->tc.sdata, req->tc.size);
 
 	if (out)
-		sgs[out_sgs++] = chan->sg;
+		sgs[out_sgs++] = vq_sg_page(chan->vq_sg, 0);
 
 	if (out_pages) {
-		sgs[out_sgs++] = chan->sg + out;
-		out += pack_sg_list_p(chan->sg, out, chan->sg_n,
-				      out_pages, out_nr_pages, offs, outlen);
+		sgs[out_sgs++] = vq_sg_page(chan->vq_sg, out);
+		out += pack_sg_list_p(chan->vq_sg, out, out_pages,
+				      out_nr_pages, offs, outlen);
 	}
 
 	/*
@@ -467,15 +569,14 @@ p9_virtio_zc_request(struct p9_client *client, struct p9_req_t *req,
 	 * Arrange in such a way that server places header in the
 	 * allocated memory and payload onto the user buffer.
 	 */
-	in = pack_sg_list(chan->sg, out,
-			  chan->sg_n, req->rc.sdata, in_hdr_len);
+	in = pack_sg_list(chan->vq_sg, out, req->rc.sdata, in_hdr_len);
 	if (in)
-		sgs[out_sgs + in_sgs++] = chan->sg + out;
+		sgs[out_sgs + in_sgs++] = vq_sg_page(chan->vq_sg, out);
 
 	if (in_pages) {
-		sgs[out_sgs + in_sgs++] = chan->sg + out + in;
-		in += pack_sg_list_p(chan->sg, out + in, chan->sg_n,
-				     in_pages, in_nr_pages, offs, inlen);
+		sgs[out_sgs + in_sgs++] = vq_sg_page(chan->vq_sg, out + in);
+		in += pack_sg_list_p(chan->vq_sg, out + in, in_pages,
+				     in_nr_pages, offs, inlen);
 	}
 
 	BUG_ON(out_sgs + in_sgs > ARRAY_SIZE(sgs));
@@ -576,14 +677,12 @@ static int p9_virtio_probe(struct virtio_device *vdev)
 		goto fail;
 	}
 
-	chan->sg = kmalloc_array(VIRTQUEUE_DEFAULT_NUM,
-				 sizeof(struct scatterlist), GFP_KERNEL);
-	if (!chan->sg) {
+	chan->vq_sg = vq_sg_alloc(VIRTQUEUE_SG_NSGL_DEFAULT);
+	if (!chan->vq_sg) {
 		pr_err("Failed to allocate virtio 9P channel\n");
 		err = -ENOMEM;
 		goto out_free_chan_shallow;
 	}
-	chan->sg_n = VIRTQUEUE_DEFAULT_NUM;
 
 	chan->vdev = vdev;
 
@@ -596,8 +695,6 @@ static int p9_virtio_probe(struct virtio_device *vdev)
 	chan->vq->vdev->priv = chan;
 	spin_lock_init(&chan->lock);
 
-	sg_init_table(chan->sg, chan->sg_n);
-
 	chan->inuse = false;
 	if (virtio_has_feature(vdev, VIRTIO_9P_MOUNT_TAG)) {
 		virtio_cread(vdev, struct virtio_9p_config, tag_len, &tag_len);
@@ -646,7 +743,7 @@ static int p9_virtio_probe(struct virtio_device *vdev)
 out_free_vq:
 	vdev->config->del_vqs(vdev);
 out_free_chan:
-	kfree(chan->sg);
+	vq_sg_free(chan->vq_sg);
 out_free_chan_shallow:
 	kfree(chan);
 fail:
@@ -741,7 +838,7 @@ static void p9_virtio_remove(struct virtio_device *vdev)
 	kobject_uevent(&(vdev->dev.kobj), KOBJ_CHANGE);
 	kfree(chan->tag);
 	kfree(chan->vc_wq);
-	kfree(chan->sg);
+	vq_sg_free(chan->vq_sg);
 	kfree(chan);
 
 }
@@ -780,7 +877,8 @@ static struct p9_trans_module p9_virtio_trans = {
 	 * that are not at page boundary, that can result in an extra
 	 * page in zero copy.
 	 */
-	.maxsize = PAGE_SIZE * (VIRTQUEUE_DEFAULT_NUM - 3),
+	.maxsize = PAGE_SIZE *
+		((VIRTQUEUE_SG_NSGL_DEFAULT * SG_USER_PAGES_PER_LIST) - 3),
 	.def = 1,
 	.owner = THIS_MODULE,
 };
-- 
2.20.1

[PATCH v2 5/7] net/9p: add trans_maxsize to struct p9_client

[Christian Schoenebeck]

This new field 'trans_maxsize' optionally allows transport to
update it to reflect the actual maximum msize supported by
allocated transport channel.

Signed-off-by: Christian Schoenebeck <linux_oss@crudebyte.com>
---
 include/net/9p/client.h |  2 ++
 net/9p/client.c         | 12 ++++++++++--
 2 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/include/net/9p/client.h b/include/net/9p/client.h
index e1c308d8d288..e48c4cdf9be0 100644
--- a/include/net/9p/client.h
+++ b/include/net/9p/client.h
@@ -89,6 +89,7 @@ struct p9_req_t {
  * struct p9_client - per client instance state
  * @lock: protect @fids and @reqs
  * @msize: maximum data size negotiated by protocol
+ * @trans_maxsize: actual maximum msize supported by transport channel
  * @proto_version: 9P protocol version to use
  * @trans_mod: module API instantiated with this client
  * @status: connection state
@@ -103,6 +104,7 @@ struct p9_req_t {
 struct p9_client {
 	spinlock_t lock;
 	unsigned int msize;
+	unsigned int trans_maxsize;
 	unsigned char proto_version;
 	struct p9_trans_module *trans_mod;
 	enum p9_trans_status status;
diff --git a/net/9p/client.c b/net/9p/client.c
index 4f4fd2098a30..a75034fa249b 100644
--- a/net/9p/client.c
+++ b/net/9p/client.c
@@ -1037,6 +1037,14 @@ struct p9_client *p9_client_create(const char *dev_name, char *options)
 		goto free_client;
 	}
 
+	/*
+	 * transport will get a chance to increase trans_maxsize (if
+	 * necessary) and it may update trans_maxsize in create() function
+	 * below accordingly to reflect the actual maximum size supported by
+	 * the allocated transport channel
+	 */
+	clnt->trans_maxsize = clnt->trans_mod->maxsize;
+
 	p9_debug(P9_DEBUG_MUX, "clnt %p trans %p msize %d protocol %d\n",
 		 clnt, clnt->trans_mod, clnt->msize, clnt->proto_version);
 
@@ -1044,8 +1052,8 @@ struct p9_client *p9_client_create(const char *dev_name, char *options)
 	if (err)
 		goto put_trans;
 
-	if (clnt->msize > clnt->trans_mod->maxsize) {
-		clnt->msize = clnt->trans_mod->maxsize;
+	if (clnt->msize > clnt->trans_maxsize) {
+		clnt->msize = clnt->trans_maxsize;
 		pr_info("Limiting 'msize' to %d as this is the maximum "
 			"supported by transport %s\n",
 			clnt->msize, clnt->trans_mod->name
-- 
2.20.1

[PATCH v2 6/7] 9p/trans_virtio: support larger msize values

[Christian Schoenebeck]

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 | 57 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 57 insertions(+)

diff --git a/net/9p/trans_virtio.c b/net/9p/trans_virtio.c
index 0db8de84bd51..4cb75f45aa15 100644
--- a/net/9p/trans_virtio.c
+++ b/net/9p/trans_virtio.c
@@ -200,6 +200,31 @@ 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
@@ -771,6 +796,10 @@ 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;
@@ -793,6 +822,34 @@ 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;
+		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;
-- 
2.20.1

[PATCH v2 7/7] 9p/trans_virtio: resize sg lists to whatever is possible

[Christian Schoenebeck]

Right now vq_sg_resize() used a lazy implementation following
the all-or-nothing princible. So it either resized exactly to
the requested new amount of sg lists, or it did not resize at
all.

The problem with this is if a user supplies a very large msize
value, resize would simply fail and the user would stick to
the default maximum msize supported by the virtio transport.

To resolve this potential issue, change vq_sg_resize() to resize
the passed sg list to whatever is possible on the machine.

Signed-off-by: Christian Schoenebeck <linux_oss@crudebyte.com>
---
 net/9p/trans_virtio.c | 65 ++++++++++++++++++++++++++++++++++++-------
 1 file changed, 55 insertions(+), 10 deletions(-)

diff --git a/net/9p/trans_virtio.c b/net/9p/trans_virtio.c
index 4cb75f45aa15..b9bab7ed2768 100644
--- a/net/9p/trans_virtio.c
+++ b/net/9p/trans_virtio.c
@@ -205,24 +205,66 @@ static struct virtqueue_sg *vq_sg_alloc(unsigned int nsgl)
  * amount of lists
  * @_vq_sg: scatter/gather lists to be resized
  * @nsgl: new amount of scatter/gather lists
+ *
+ * Old scatter/gather lists are retained. Only growing the size is supported.
+ * If the requested amount cannot be satisfied, then lists are increased to
+ * whatever is possible.
  */
 static int vq_sg_resize(struct virtqueue_sg **_vq_sg, unsigned int nsgl)
 {
 	struct virtqueue_sg *vq_sg;
+	unsigned int i;
+	size_t sz;
+	int ret = 0;
 
 	BUG_ON(!_vq_sg || !nsgl);
 	vq_sg = *_vq_sg;
+	if (nsgl > VIRTQUEUE_SG_NSGL_MAX)
+		nsgl = VIRTQUEUE_SG_NSGL_MAX;
 	if (vq_sg->nsgl == nsgl)
 		return 0;
+	if (vq_sg->nsgl > nsgl)
+		return -ENOTSUPP;
+
+	vq_sg = kzalloc(sizeof(struct virtqueue_sg) +
+			nsgl * sizeof(struct scatterlist *),
+			GFP_KERNEL);
 
-	/* lazy resize implementation for now */
-	vq_sg = vq_sg_alloc(nsgl);
 	if (!vq_sg)
 		return -ENOMEM;
 
-	kfree(*_vq_sg);
+	/* copy over old scatter gather lists */
+	sz = sizeof(struct virtqueue_sg) +
+		(*_vq_sg)->nsgl * sizeof(struct scatterlist *);
+	memcpy(vq_sg, *_vq_sg, sz);
+
+	vq_sg->nsgl = nsgl;
+
+	for (i = (*_vq_sg)->nsgl; i < nsgl; ++i) {
+		vq_sg->sgl[i] = kmalloc_array(
+			SG_MAX_SINGLE_ALLOC, sizeof(struct scatterlist),
+			GFP_KERNEL
+		);
+		/*
+		 * handle failed allocation as soft error, we take whatever
+		 * we get
+		 */
+		if (!vq_sg->sgl[i]) {
+			ret = -ENOMEM;
+			vq_sg->nsgl = nsgl = i;
+			break;
+		}
+		sg_init_table(vq_sg->sgl[i], SG_MAX_SINGLE_ALLOC);
+		if (i) {
+			/* chain the lists */
+			sg_chain(vq_sg->sgl[i - 1], SG_MAX_SINGLE_ALLOC,
+				 vq_sg->sgl[i]);
+		}
+	}
+	sg_mark_end(&vq_sg->sgl[nsgl - 1][SG_MAX_SINGLE_ALLOC - 1]);
+
 	*_vq_sg = vq_sg;
-	return 0;
+	return ret;
 }
 
 /**
@@ -839,13 +881,16 @@ p9_virtio_create(struct p9_client *client, const char *devname, char *args)
 		if (nsgl > chan->vq_sg->nsgl) {
 			/*
 			 * if resize fails, no big deal, then just
-			 * continue with default msize instead
+			 * continue with whatever we got
 			 */
-			if (!vq_sg_resize(&chan->vq_sg, nsgl)) {
-				client->trans_maxsize =
-					PAGE_SIZE *
-					((nsgl * SG_USER_PAGES_PER_LIST) - 3);
-			}
+			vq_sg_resize(&chan->vq_sg, nsgl);
+			/*
+			 * actual allocation size might be less than
+			 * requested, so use vq_sg->nsgl instead of nsgl
+			 */
+			client->trans_maxsize =
+				PAGE_SIZE * ((chan->vq_sg->nsgl *
+				SG_USER_PAGES_PER_LIST) - 3);
 		}
 #endif /* CONFIG_ARCH_NO_SG_CHAIN */
 	}
-- 
2.20.1

Re: [PATCH v2 4/7] 9p/trans_virtio: introduce struct virtqueue_sg

[Christian Schoenebeck]

On Montag, 20. September 2021 18:44:00 CEST Christian Schoenebeck wrote:
> The amount of elements in a scatter/gather list is limited to
> approximately 128 elements. To allow going beyond that limit
> with subsequent patches, pave the way by turning the one-
> dimensional sg list array into a two-dimensional array, i.e:
> 
>   sg[128]
> 
> becomes
> 
>   sgl[nsgl][SG_MAX_SINGLE_ALLOC]
> 
> As the value of 'nsgl' is exactly (still) 1 in this commit
> and the compile-time (compiler and architecture dependent)
> value of 'SG_MAX_SINGLE_ALLOC' equals approximately the
> previous hard coded 128 elements, this commit is therefore
> more of a preparatory refactoring then actual behaviour
> change.
> 
> A custom struct virtqueue_sg is defined instead of using
> shared API struct sg_table, because the latter would not
> allow to resize the table after allocation. sg_append_table
> API OTOH would not fit either, because it requires a list
> of pages beforehand upon allocation. And both APIs only
> support all-or-nothing allocation.
> 
> Signed-off-by: Christian Schoenebeck <linux_oss@crudebyte.com>
> ---
>  net/9p/trans_virtio.c | 190 ++++++++++++++++++++++++++++++++----------
>  1 file changed, 144 insertions(+), 46 deletions(-)
> 
> diff --git a/net/9p/trans_virtio.c b/net/9p/trans_virtio.c
> index 3347d35a5e6e..0db8de84bd51 100644
> --- a/net/9p/trans_virtio.c
> +++ b/net/9p/trans_virtio.c
> @@ -36,7 +36,31 @@
>  #include <linux/virtio_9p.h>
>  #include "trans_common.h"
> 
> -#define VIRTQUEUE_DEFAULT_NUM	128
> +/**
> + * struct virtqueue_sg - (chained) scatter gather lists for virtqueue data
> + * transmission
> + * @nsgl: amount of elements (in first dimension) of array field @sgl
> + * @sgl: two-dimensional array, i.e. sgl[nsgl][SG_MAX_SINGLE_ALLOC]
> + */
> +struct virtqueue_sg {
> +	unsigned int nsgl;
> +	struct scatterlist *sgl[];
> +};
> +
> +/*
> + * Default value for field nsgl in struct virtqueue_sg, which defines the
> + * initial virtio data transmission capacity when this virtio transport is
> + * probed.
> + */
> +#define VIRTQUEUE_SG_NSGL_DEFAULT 1
> +
> +/* maximum value for field nsgl in struct virtqueue_sg */
> +#define VIRTQUEUE_SG_NSGL_MAX						\
> +	((PAGE_SIZE - sizeof(struct virtqueue_sg)) /			\
> +	sizeof(struct scatterlist *))					\
> +
> +/* last entry per sg list is used for chaining (pointer to next list) */
> +#define SG_USER_PAGES_PER_LIST	(SG_MAX_SINGLE_ALLOC - 1)
> 
>  /* a single mutex to manage channel initialization and attachment */
>  static DEFINE_MUTEX(virtio_9p_lock);
> @@ -53,8 +77,7 @@ static atomic_t vp_pinned = ATOMIC_INIT(0);
>   * @ring_bufs_avail: flag to indicate there is some available in the ring
> buf * @vc_wq: wait queue for waiting for thing to be added to ring buf *
> @p9_max_pages: maximum number of pinned pages
> - * @sg: scatter gather list which is used to pack a request (protected?)
> - * @sg_n: amount of elements in sg array
> + * @vq_sg: table of scatter gather lists, which are used to pack a request
>   * @chan_list: linked list of channels
>   *
>   * We keep all per-channel information in a structure.
> @@ -77,9 +100,7 @@ struct virtio_chan {
>  	 * will be placing it in each channel.
>  	 */
>  	unsigned long p9_max_pages;
> -	/* Scatterlist: can be too big for stack. */
> -	struct scatterlist *sg;
> -	size_t sg_n;
> +	struct virtqueue_sg *vq_sg;
>  	/**
>  	 * @tag: name to identify a mount null terminated
>  	 */
> @@ -96,6 +117,89 @@ static unsigned int rest_of_page(void *data)
>  	return PAGE_SIZE - offset_in_page(data);
>  }
> 
> +/**
> + * vq_sg_page - returns user page for given page index
> + * @vq_sg: scatter gather lists used by this transport
> + * @page: user page index across all scatter gather lists
> + */
> +static struct scatterlist *vq_sg_page(struct virtqueue_sg *vq_sg, size_t
> page) +{
> +	unsigned int node = page / SG_USER_PAGES_PER_LIST;
> +	unsigned int leaf = page % SG_USER_PAGES_PER_LIST;
> +	BUG_ON(node >= VIRTQUEUE_SG_NSGL_MAX);
> +	return &vq_sg->sgl[node][leaf];
> +}
> +
> +/**
> + * vq_sg_npages - returns total number of individual user pages in passed
> + * scatter gather lists
> + * @vq_sg: scatter gather lists to be counted
> + */
> +static size_t vq_sg_npages(struct virtqueue_sg *vq_sg)
> +{
> +	return vq_sg->nsgl * SG_USER_PAGES_PER_LIST;
> +}
> +
> +/**
> + * vq_sg_free - free all memory previously allocated for @vq_sg
> + * @vq_sg: scatter gather lists to be freed
> + */
> +static void vq_sg_free(struct virtqueue_sg *vq_sg)
> +{
> +	unsigned int i;
> +

Here should be a

	if (!vq_sg) return;

Not a real issue right now as vq_sg_free() is never called with NULL anywhere, 
but it is common expactation that free calls should safely handle NULL 
arguments.

> +	for (i = 0; i < vq_sg->nsgl; ++i) {
> +		kfree(vq_sg->sgl[i]);
> +	}
> +	kfree(vq_sg);
> +}
> +
> +/**
> + * vq_sg_alloc - allocates and returns @nsgl scatter gather lists
> + * @nsgl: amount of scatter gather lists to be allocated
> + * If @nsgl is larger than one then chained lists are used if supported by
> + * architecture.
> + */
> +static struct virtqueue_sg *vq_sg_alloc(unsigned int nsgl)
> +{
> +	struct virtqueue_sg *vq_sg;
> +	unsigned int i;
> +
> +	BUG_ON(!nsgl || nsgl > VIRTQUEUE_SG_NSGL_MAX);
> +#ifdef CONFIG_ARCH_NO_SG_CHAIN
> +	if (WARN_ON_ONCE(nsgl > 1))
> +		return NULL;
> +#endif
> +
> +	vq_sg = kzalloc(sizeof(struct virtqueue_sg) +
> +			nsgl * sizeof(struct scatterlist *),
> +			GFP_KERNEL);
> +
> +	if (!vq_sg)
> +		return NULL;
> +
> +	vq_sg->nsgl = nsgl;
> +
> +	for (i = 0; i < nsgl; ++i) {
> +		vq_sg->sgl[i] = kmalloc_array(
> +			SG_MAX_SINGLE_ALLOC, sizeof(struct scatterlist),
> +			GFP_KERNEL
> +		);
> +		if (!vq_sg->sgl[i]) {
> +			vq_sg_free(vq_sg);
> +			return NULL;
> +		}
> +		sg_init_table(vq_sg->sgl[i], SG_MAX_SINGLE_ALLOC);
> +		if (i) {
> +			/* chain the lists */
> +			sg_chain(vq_sg->sgl[i - 1], SG_MAX_SINGLE_ALLOC,
> +				 vq_sg->sgl[i]);
> +		}
> +	}
> +	sg_mark_end(&vq_sg->sgl[nsgl - 1][SG_MAX_SINGLE_ALLOC - 1]);
> +	return vq_sg;
> +}
> +
>  /**
>   * p9_virtio_close - reclaim resources of a channel
>   * @client: client instance
> @@ -158,9 +262,8 @@ static void req_done(struct virtqueue *vq)
> 
>  /**
>   * pack_sg_list - pack a scatter gather list from a linear buffer
> - * @sg: scatter/gather list to pack into
> + * @vq_sg: scatter/gather lists to pack into
>   * @start: which segment of the sg_list to start at
> - * @limit: maximum segment to pack data to
>   * @data: data to pack into scatter/gather list
>   * @count: amount of data to pack into the scatter/gather list
>   *
> @@ -170,11 +273,12 @@ static void req_done(struct virtqueue *vq)
>   *
>   */
> 
> -static int pack_sg_list(struct scatterlist *sg, int start,
> -			int limit, char *data, int count)
> +static int pack_sg_list(struct virtqueue_sg *vq_sg, int start,
> +			char *data, int count)
>  {
>  	int s;
>  	int index = start;
> +	size_t limit = vq_sg_npages(vq_sg);
> 
>  	while (count) {
>  		s = rest_of_page(data);
> @@ -182,13 +286,13 @@ static int pack_sg_list(struct scatterlist *sg, int
> start, s = count;
>  		BUG_ON(index >= limit);
>  		/* Make sure we don't terminate early. */
> -		sg_unmark_end(&sg[index]);
> -		sg_set_buf(&sg[index++], data, s);
> +		sg_unmark_end(vq_sg_page(vq_sg, index));
> +		sg_set_buf(vq_sg_page(vq_sg, index++), data, s);
>  		count -= s;
>  		data += s;
>  	}
>  	if (index-start)
> -		sg_mark_end(&sg[index - 1]);
> +		sg_mark_end(vq_sg_page(vq_sg, index - 1));
>  	return index-start;
>  }
> 
> @@ -208,21 +312,21 @@ static int p9_virtio_cancelled(struct p9_client
> *client, struct p9_req_t *req) /**
>   * pack_sg_list_p - Just like pack_sg_list. Instead of taking a buffer,
>   * this takes a list of pages.
> - * @sg: scatter/gather list to pack into
> + * @vq_sg: scatter/gather lists to pack into
>   * @start: which segment of the sg_list to start at
> - * @limit: maximum number of pages in sg list.
>   * @pdata: a list of pages to add into sg.
>   * @nr_pages: number of pages to pack into the scatter/gather list
>   * @offs: amount of data in the beginning of first page _not_ to pack
>   * @count: amount of data to pack into the scatter/gather list
>   */
>  static int
> -pack_sg_list_p(struct scatterlist *sg, int start, int limit,
> +pack_sg_list_p(struct virtqueue_sg *vq_sg, int start,
>  	       struct page **pdata, int nr_pages, size_t offs, int count)
>  {
>  	int i = 0, s;
>  	int data_off = offs;
>  	int index = start;
> +	size_t limit = vq_sg_npages(vq_sg);
> 
>  	BUG_ON(nr_pages > (limit - start));
>  	/*
> @@ -235,15 +339,16 @@ pack_sg_list_p(struct scatterlist *sg, int start, int
> limit, s = count;
>  		BUG_ON(index >= limit);
>  		/* Make sure we don't terminate early. */
> -		sg_unmark_end(&sg[index]);
> -		sg_set_page(&sg[index++], pdata[i++], s, data_off);
> +		sg_unmark_end(vq_sg_page(vq_sg, index));
> +		sg_set_page(vq_sg_page(vq_sg, index++), pdata[i++], s,
> +			    data_off);
>  		data_off = 0;
>  		count -= s;
>  		nr_pages--;
>  	}
> 
>  	if (index-start)
> -		sg_mark_end(&sg[index - 1]);
> +		sg_mark_end(vq_sg_page(vq_sg, index - 1));
>  	return index - start;
>  }
> 
> @@ -271,15 +376,13 @@ p9_virtio_request(struct p9_client *client, struct
> p9_req_t *req)
> 
>  	out_sgs = in_sgs = 0;
>  	/* Handle out VirtIO ring buffers */
> -	out = pack_sg_list(chan->sg, 0,
> -			   chan->sg_n, req->tc.sdata, req->tc.size);
> +	out = pack_sg_list(chan->vq_sg, 0, req->tc.sdata, req->tc.size);
>  	if (out)
> -		sgs[out_sgs++] = chan->sg;
> +		sgs[out_sgs++] = vq_sg_page(chan->vq_sg, 0);
> 
> -	in = pack_sg_list(chan->sg, out,
> -			  chan->sg_n, req->rc.sdata, req->rc.capacity);
> +	in = pack_sg_list(chan->vq_sg, out, req->rc.sdata, req->rc.capacity);
>  	if (in)
> -		sgs[out_sgs + in_sgs++] = chan->sg + out;
> +		sgs[out_sgs + in_sgs++] = vq_sg_page(chan->vq_sg, out);
> 
>  	err = virtqueue_add_sgs(chan->vq, sgs, out_sgs, in_sgs, req,
>  				GFP_ATOMIC);
> @@ -448,16 +551,15 @@ p9_virtio_zc_request(struct p9_client *client, struct
> p9_req_t *req, out_sgs = in_sgs = 0;
> 
>  	/* out data */
> -	out = pack_sg_list(chan->sg, 0,
> -			   chan->sg_n, req->tc.sdata, req->tc.size);
> +	out = pack_sg_list(chan->vq_sg, 0, req->tc.sdata, req->tc.size);
> 
>  	if (out)
> -		sgs[out_sgs++] = chan->sg;
> +		sgs[out_sgs++] = vq_sg_page(chan->vq_sg, 0);
> 
>  	if (out_pages) {
> -		sgs[out_sgs++] = chan->sg + out;
> -		out += pack_sg_list_p(chan->sg, out, chan->sg_n,
> -				      out_pages, out_nr_pages, offs, outlen);
> +		sgs[out_sgs++] = vq_sg_page(chan->vq_sg, out);
> +		out += pack_sg_list_p(chan->vq_sg, out, out_pages,
> +				      out_nr_pages, offs, outlen);
>  	}
> 
>  	/*
> @@ -467,15 +569,14 @@ p9_virtio_zc_request(struct p9_client *client, struct
> p9_req_t *req, * Arrange in such a way that server places header in the
>  	 * allocated memory and payload onto the user buffer.
>  	 */
> -	in = pack_sg_list(chan->sg, out,
> -			  chan->sg_n, req->rc.sdata, in_hdr_len);
> +	in = pack_sg_list(chan->vq_sg, out, req->rc.sdata, in_hdr_len);
>  	if (in)
> -		sgs[out_sgs + in_sgs++] = chan->sg + out;
> +		sgs[out_sgs + in_sgs++] = vq_sg_page(chan->vq_sg, out);
> 
>  	if (in_pages) {
> -		sgs[out_sgs + in_sgs++] = chan->sg + out + in;
> -		in += pack_sg_list_p(chan->sg, out + in, chan->sg_n,
> -				     in_pages, in_nr_pages, offs, inlen);
> +		sgs[out_sgs + in_sgs++] = vq_sg_page(chan->vq_sg, out + in);
> +		in += pack_sg_list_p(chan->vq_sg, out + in, in_pages,
> +				     in_nr_pages, offs, inlen);
>  	}
> 
>  	BUG_ON(out_sgs + in_sgs > ARRAY_SIZE(sgs));
> @@ -576,14 +677,12 @@ static int p9_virtio_probe(struct virtio_device *vdev)
> goto fail;
>  	}
> 
> -	chan->sg = kmalloc_array(VIRTQUEUE_DEFAULT_NUM,
> -				 sizeof(struct scatterlist), GFP_KERNEL);
> -	if (!chan->sg) {
> +	chan->vq_sg = vq_sg_alloc(VIRTQUEUE_SG_NSGL_DEFAULT);
> +	if (!chan->vq_sg) {
>  		pr_err("Failed to allocate virtio 9P channel\n");
>  		err = -ENOMEM;
>  		goto out_free_chan_shallow;
>  	}
> -	chan->sg_n = VIRTQUEUE_DEFAULT_NUM;
> 
>  	chan->vdev = vdev;
> 
> @@ -596,8 +695,6 @@ static int p9_virtio_probe(struct virtio_device *vdev)
>  	chan->vq->vdev->priv = chan;
>  	spin_lock_init(&chan->lock);
> 
> -	sg_init_table(chan->sg, chan->sg_n);
> -
>  	chan->inuse = false;
>  	if (virtio_has_feature(vdev, VIRTIO_9P_MOUNT_TAG)) {
>  		virtio_cread(vdev, struct virtio_9p_config, tag_len, &tag_len);
> @@ -646,7 +743,7 @@ static int p9_virtio_probe(struct virtio_device *vdev)
>  out_free_vq:
>  	vdev->config->del_vqs(vdev);
>  out_free_chan:
> -	kfree(chan->sg);
> +	vq_sg_free(chan->vq_sg);
>  out_free_chan_shallow:
>  	kfree(chan);
>  fail:
> @@ -741,7 +838,7 @@ static void p9_virtio_remove(struct virtio_device *vdev)
> kobject_uevent(&(vdev->dev.kobj), KOBJ_CHANGE);
>  	kfree(chan->tag);
>  	kfree(chan->vc_wq);
> -	kfree(chan->sg);
> +	vq_sg_free(chan->vq_sg);
>  	kfree(chan);
> 
>  }
> @@ -780,7 +877,8 @@ static struct p9_trans_module p9_virtio_trans = {
>  	 * that are not at page boundary, that can result in an extra
>  	 * page in zero copy.
>  	 */
> -	.maxsize = PAGE_SIZE * (VIRTQUEUE_DEFAULT_NUM - 3),
> +	.maxsize = PAGE_SIZE *
> +		((VIRTQUEUE_SG_NSGL_DEFAULT * SG_USER_PAGES_PER_LIST) - 3),
>  	.def = 1,
>  	.owner = THIS_MODULE,
>  };

Re: [PATCH v2 6/7] 9p/trans_virtio: support larger msize values

[Christian Schoenebeck]

On Montag, 20. September 2021 18:44:13 CEST Christian Schoenebeck wrote:
> 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 | 57 +++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 57 insertions(+)
> 
> diff --git a/net/9p/trans_virtio.c b/net/9p/trans_virtio.c
> index 0db8de84bd51..4cb75f45aa15 100644
> --- a/net/9p/trans_virtio.c
> +++ b/net/9p/trans_virtio.c
> @@ -200,6 +200,31 @@ 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
> @@ -771,6 +796,10 @@ 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;
> @@ -793,6 +822,34 @@ 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;
> +		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);
> +			}
> +		}

Maybe an ...

			} else {
				pr_info(...);
			}

would be useful here to explain the user why transport refrained from 
increasing its max size even though user's msize option demanded it.

> +#endif /* CONFIG_ARCH_NO_SG_CHAIN */
> +	}
> +
>  	client->trans = (void *)chan;
>  	client->status = Connected;
>  	chan->client = client;

Re: [PATCH v2 7/7] 9p/trans_virtio: resize sg lists to whatever is possible

[Christian Schoenebeck]

On Montag, 20. September 2021 18:44:20 CEST Christian Schoenebeck wrote:
> Right now vq_sg_resize() used a lazy implementation following
> the all-or-nothing princible. So it either resized exactly to
> the requested new amount of sg lists, or it did not resize at
> all.
> 
> The problem with this is if a user supplies a very large msize
> value, resize would simply fail and the user would stick to
> the default maximum msize supported by the virtio transport.
> 
> To resolve this potential issue, change vq_sg_resize() to resize
> the passed sg list to whatever is possible on the machine.
> 
> Signed-off-by: Christian Schoenebeck <linux_oss@crudebyte.com>
> ---
>  net/9p/trans_virtio.c | 65 ++++++++++++++++++++++++++++++++++++-------
>  1 file changed, 55 insertions(+), 10 deletions(-)
> 
> diff --git a/net/9p/trans_virtio.c b/net/9p/trans_virtio.c
> index 4cb75f45aa15..b9bab7ed2768 100644
> --- a/net/9p/trans_virtio.c
> +++ b/net/9p/trans_virtio.c
> @@ -205,24 +205,66 @@ static struct virtqueue_sg *vq_sg_alloc(unsigned int
> nsgl) * amount of lists
>   * @_vq_sg: scatter/gather lists to be resized
>   * @nsgl: new amount of scatter/gather lists
> + *
> + * Old scatter/gather lists are retained. Only growing the size is
> supported. + * If the requested amount cannot be satisfied, then lists are
> increased to + * whatever is possible.
>   */
>  static int vq_sg_resize(struct virtqueue_sg **_vq_sg, unsigned int nsgl)
>  {
>  	struct virtqueue_sg *vq_sg;
> +	unsigned int i;
> +	size_t sz;
> +	int ret = 0;
> 
>  	BUG_ON(!_vq_sg || !nsgl);
>  	vq_sg = *_vq_sg;
> +	if (nsgl > VIRTQUEUE_SG_NSGL_MAX)
> +		nsgl = VIRTQUEUE_SG_NSGL_MAX;
>  	if (vq_sg->nsgl == nsgl)
>  		return 0;
> +	if (vq_sg->nsgl > nsgl)
> +		return -ENOTSUPP;
> +
> +	vq_sg = kzalloc(sizeof(struct virtqueue_sg) +
> +			nsgl * sizeof(struct scatterlist *),
> +			GFP_KERNEL);
> 
> -	/* lazy resize implementation for now */
> -	vq_sg = vq_sg_alloc(nsgl);
>  	if (!vq_sg)
>  		return -ENOMEM;
> 
> -	kfree(*_vq_sg);
> +	/* copy over old scatter gather lists */
> +	sz = sizeof(struct virtqueue_sg) +
> +		(*_vq_sg)->nsgl * sizeof(struct scatterlist *);
> +	memcpy(vq_sg, *_vq_sg, sz);

Missing

	kfree(*_vq_sg);

here.

> +
> +	vq_sg->nsgl = nsgl;
> +
> +	for (i = (*_vq_sg)->nsgl; i < nsgl; ++i) {
> +		vq_sg->sgl[i] = kmalloc_array(
> +			SG_MAX_SINGLE_ALLOC, sizeof(struct scatterlist),
> +			GFP_KERNEL
> +		);
> +		/*
> +		 * handle failed allocation as soft error, we take whatever
> +		 * we get
> +		 */
> +		if (!vq_sg->sgl[i]) {
> +			ret = -ENOMEM;
> +			vq_sg->nsgl = nsgl = i;
> +			break;
> +		}
> +		sg_init_table(vq_sg->sgl[i], SG_MAX_SINGLE_ALLOC);
> +		if (i) {
> +			/* chain the lists */
> +			sg_chain(vq_sg->sgl[i - 1], SG_MAX_SINGLE_ALLOC,
> +				 vq_sg->sgl[i]);
> +		}
> +	}
> +	sg_mark_end(&vq_sg->sgl[nsgl - 1][SG_MAX_SINGLE_ALLOC - 1]);
> +
>  	*_vq_sg = vq_sg;
> -	return 0;
> +	return ret;
>  }
> 
>  /**
> @@ -839,13 +881,16 @@ p9_virtio_create(struct p9_client *client, const char
> *devname, char *args) if (nsgl > chan->vq_sg->nsgl) {
>  			/*
>  			 * if resize fails, no big deal, then just
> -			 * continue with default msize instead
> +			 * continue with whatever we got
>  			 */
> -			if (!vq_sg_resize(&chan->vq_sg, nsgl)) {
> -				client->trans_maxsize =
> -					PAGE_SIZE *
> -					((nsgl * SG_USER_PAGES_PER_LIST) - 3);
> -			}
> +			vq_sg_resize(&chan->vq_sg, nsgl);
> +			/*
> +			 * actual allocation size might be less than
> +			 * requested, so use vq_sg->nsgl instead of nsgl
> +			 */
> +			client->trans_maxsize =
> +				PAGE_SIZE * ((chan->vq_sg->nsgl *
> +				SG_USER_PAGES_PER_LIST) - 3);

As with patch 6, here should probably be an additional

		if (chan->vq_sg->nsgl < nsgl) {
			pr_inf(...);
		}

to explain the user that not all required sg lists could be allocated suiting 
user's requested msize option.

>  		}
>  #endif /* CONFIG_ARCH_NO_SG_CHAIN */
>  	}