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Date: Mon, 22 Mar 2021 15:01:45 -0400
From: Vivek Goyal <vgoyal@...hat.com>
To: Miklos Szeredi <miklos@...redi.hu>
Cc: Connor Kuehl <ckuehl@...hat.com>, virtio-fs@...hat.com,
linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
virtualization@...ts.linux-foundation.org, stefanha@...hat.com,
jasowang@...hat.com, mst@...hat.com
Subject: Re: [PATCH 2/3] virtiofs: split requests that exceed virtqueue size
On Thu, Mar 18, 2021 at 04:17:51PM +0100, Miklos Szeredi wrote:
> 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@...hat.com>
> 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@...hat.com>
> Signed-off-by: Miklos Szeredi <mszeredi@...hat.com>
> ---
Looks good to me.
Reviewed-by: Vivek Goyal <vgoyal@...hat.com>
Vivek
> 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) {
>
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