Re: [PATCH 2/3] virtiofs: split requests that exceed virtqueue size

[Miklos Szeredi]

On Thu, Mar 18, 2021 at 08:52:22AM -0500, Connor Kuehl wrote:
> If an incoming FUSE request can't fit on the virtqueue, the request is
> placed onto a workqueue so a worker can try to resubmit it later where
> there will (hopefully) be space for it next time.
> 
> This is fine for requests that aren't larger than a virtqueue's maximum
> capacity. However, if a request's size exceeds the maximum capacity of
> the virtqueue (even if the virtqueue is empty), it will be doomed to a
> life of being placed on the workqueue, removed, discovered it won't fit,
> and placed on the workqueue yet again.
> 
> Furthermore, from section 2.6.5.3.1 (Driver Requirements: Indirect
> Descriptors) of the virtio spec:
> 
>   "A driver MUST NOT create a descriptor chain longer than the Queue
>   Size of the device."
> 
> To fix this, limit the number of pages FUSE will use for an overall
> request. This way, each request can realistically fit on the virtqueue
> when it is decomposed into a scattergather list and avoid violating
> section 2.6.5.3.1 of the virtio spec.

I removed the conditional compilation and renamed the limit.  Also made
virtio_fs_get_tree() bail out if it hit the WARN_ON().  Updated patch below.

The virtio_ring patch in this series should probably go through the respective
subsystem tree.


Thanks,
Miklos

---
From: Connor Kuehl <ckuehl@xxxxxxxxxx>
Subject: virtiofs: split requests that exceed virtqueue size
Date: Thu, 18 Mar 2021 08:52:22 -0500

If an incoming FUSE request can't fit on the virtqueue, the request is
placed onto a workqueue so a worker can try to resubmit it later where
there will (hopefully) be space for it next time.

This is fine for requests that aren't larger than a virtqueue's maximum
capacity.  However, if a request's size exceeds the maximum capacity of the
virtqueue (even if the virtqueue is empty), it will be doomed to a life of
being placed on the workqueue, removed, discovered it won't fit, and placed
on the workqueue yet again.

Furthermore, from section 2.6.5.3.1 (Driver Requirements: Indirect
Descriptors) of the virtio spec:

  "A driver MUST NOT create a descriptor chain longer than the Queue
  Size of the device."

To fix this, limit the number of pages FUSE will use for an overall
request.  This way, each request can realistically fit on the virtqueue
when it is decomposed into a scattergather list and avoid violating section
2.6.5.3.1 of the virtio spec.

Signed-off-by: Connor Kuehl <ckuehl@xxxxxxxxxx>
Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxxxxx>
---
 fs/fuse/fuse_i.h    |    3 +++
 fs/fuse/inode.c     |    3 ++-
 fs/fuse/virtio_fs.c |   19 +++++++++++++++++--
 3 files changed, 22 insertions(+), 3 deletions(-)

--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -555,6 +555,9 @@ struct fuse_conn {
 	/** Maxmum number of pages that can be used in a single request */
 	unsigned int max_pages;
 
+	/** Constrain ->max_pages to this value during feature negotiation */
+	unsigned int max_pages_limit;
+
 	/** Input queue */
 	struct fuse_iqueue iq;
 
--- a/fs/fuse/inode.c
+++ b/fs/fuse/inode.c
@@ -712,6 +712,7 @@ void fuse_conn_init(struct fuse_conn *fc
 	fc->pid_ns = get_pid_ns(task_active_pid_ns(current));
 	fc->user_ns = get_user_ns(user_ns);
 	fc->max_pages = FUSE_DEFAULT_MAX_PAGES_PER_REQ;
+	fc->max_pages_limit = FUSE_MAX_MAX_PAGES;
 
 	INIT_LIST_HEAD(&fc->mounts);
 	list_add(&fm->fc_entry, &fc->mounts);
@@ -1040,7 +1041,7 @@ static void process_init_reply(struct fu
 				fc->abort_err = 1;
 			if (arg->flags & FUSE_MAX_PAGES) {
 				fc->max_pages =
-					min_t(unsigned int, FUSE_MAX_MAX_PAGES,
+					min_t(unsigned int, fc->max_pages_limit,
 					max_t(unsigned int, arg->max_pages, 1));
 			}
 			if (IS_ENABLED(CONFIG_FUSE_DAX) &&
--- a/fs/fuse/virtio_fs.c
+++ b/fs/fuse/virtio_fs.c
@@ -18,6 +18,12 @@
 #include <linux/uio.h>
 #include "fuse_i.h"
 
+/* Used to help calculate the FUSE connection's max_pages limit for a request's
+ * size. Parts of the struct fuse_req are sliced into scattergather lists in
+ * addition to the pages used, so this can help account for that overhead.
+ */
+#define FUSE_HEADER_OVERHEAD    4
+
 /* List of virtio-fs device instances and a lock for the list. Also provides
  * mutual exclusion in device removal and mounting path
  */
@@ -1413,9 +1419,10 @@ static int virtio_fs_get_tree(struct fs_
 {
 	struct virtio_fs *fs;
 	struct super_block *sb;
-	struct fuse_conn *fc;
+	struct fuse_conn *fc = NULL;
 	struct fuse_mount *fm;
-	int err;
+	unsigned int virtqueue_size;
+	int err = -EIO;
 
 	/* This gets a reference on virtio_fs object. This ptr gets installed
 	 * in fc->iq->priv. Once fuse_conn is going away, it calls ->put()
@@ -1427,6 +1434,10 @@ static int virtio_fs_get_tree(struct fs_
 		return -EINVAL;
 	}
 
+	virtqueue_size = virtqueue_get_vring_size(fs->vqs[VQ_REQUEST].vq);
+	if (WARN_ON(virtqueue_size <= FUSE_HEADER_OVERHEAD))
+		goto out_err;
+
 	err = -ENOMEM;
 	fc = kzalloc(sizeof(struct fuse_conn), GFP_KERNEL);
 	if (!fc)
@@ -1442,6 +1453,10 @@ static int virtio_fs_get_tree(struct fs_
 	fc->delete_stale = true;
 	fc->auto_submounts = true;
 
+	/* Tell FUSE to split requests that exceed the virtqueue's size */
+	fc->max_pages_limit = min_t(unsigned int, fc->max_pages_limit,
+				    virtqueue_size - FUSE_HEADER_OVERHEAD);
+
 	fsc->s_fs_info = fm;
 	sb = sget_fc(fsc, virtio_fs_test_super, set_anon_super_fc);
 	if (fsc->s_fs_info) {