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SubjectRe: [PATCH v2] virtiofs: propagate sync() to file server
On Mon, Apr 26, 2021 at 05:10:11PM +0200, Greg Kurz wrote:
> Even if POSIX doesn't mandate it, linux users legitimately expect
> sync() to flush all data and metadata to physical storage when it
> is located on the same system. This isn't happening with virtiofs
> though : sync() inside the guest returns right away even though
> data still needs to be flushed from the host page cache.
>
> This is easily demonstrated by doing the following in the guest:
>
> $ dd if=/dev/zero of=/mnt/foo bs=1M count=5K ; strace -T -e sync sync
> 5120+0 records in
> 5120+0 records out
> 5368709120 bytes (5.4 GB, 5.0 GiB) copied, 5.22224 s, 1.0 GB/s
> sync() = 0 <0.024068>
> +++ exited with 0 +++
>
> and start the following in the host when the 'dd' command completes
> in the guest:
>
> $ strace -T -e fsync /usr/bin/sync virtiofs/foo
> fsync(3) = 0 <10.371640>
> +++ exited with 0 +++
>
> There are no good reasons not to honor the expected behavior of
> sync() actually : it gives an unrealistic impression that virtiofs
> is super fast and that data has safely landed on HW, which isn't
> the case obviously.
>
> Implement a ->sync_fs() superblock operation that sends a new
> FUSE_SYNC request type for this purpose. Provision a 64-bit
> flags field for possible future extensions. Since the file
> server cannot handle the wait == 0 case, we skip it to avoid a
> gratuitous roundtrip.
>
> Like with FUSE_FSYNC and FUSE_FSYNCDIR, lack of support for
> FUSE_SYNC in the file server is treated as permanent success.
> This ensures compatibility with older file servers : the client
> will get the current behavior of sync() not being propagated to
> the file server.
>
> Note that such an operation allows the file server to DoS sync().
> Since a typical FUSE file server is an untrusted piece of software
> running in userspace, this is disabled by default. Only enable it
> with virtiofs for now since virtiofsd is supposedly trusted by the
> guest kernel.
>
> Reported-by: Robert Krawitz <rlk@redhat.com>
> Signed-off-by: Greg Kurz <groug@kaod.org>
> ---
>
> v2: - clarify compatibility with older servers in changelog (Vivek)
> - ignore the wait == 0 case (Miklos)
> - 64-bit aligned argument structure (Vivek, Miklos)
>
> fs/fuse/fuse_i.h | 3 +++
> fs/fuse/inode.c | 35 +++++++++++++++++++++++++++++++++++
> fs/fuse/virtio_fs.c | 1 +
> include/uapi/linux/fuse.h | 10 +++++++++-
> 4 files changed, 48 insertions(+), 1 deletion(-)
>
> diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
> index 63d97a15ffde..68e9ae96cbd4 100644
> --- a/fs/fuse/fuse_i.h
> +++ b/fs/fuse/fuse_i.h
> @@ -755,6 +755,9 @@ struct fuse_conn {
> /* Auto-mount submounts announced by the server */
> unsigned int auto_submounts:1;
>
> + /* Propagate syncfs() to server */
> + unsigned int sync_fs:1;
> +
> /** The number of requests waiting for completion */
> atomic_t num_waiting;
>
> diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
> index b0e18b470e91..ac184069b40f 100644
> --- a/fs/fuse/inode.c
> +++ b/fs/fuse/inode.c
> @@ -506,6 +506,40 @@ static int fuse_statfs(struct dentry *dentry, struct kstatfs *buf)
> return err;
> }
>
> +static int fuse_sync_fs(struct super_block *sb, int wait)
> +{
> + struct fuse_mount *fm = get_fuse_mount_super(sb);
> + struct fuse_conn *fc = fm->fc;
> + struct fuse_syncfs_in inarg;
> + FUSE_ARGS(args);
> + int err;
> +
> + /*
> + * Userspace cannot handle the wait == 0 case. Avoid a
> + * gratuitous roundtrip.
> + */
> + if (!wait)
> + return 0;
> +
> + if (!fc->sync_fs)
> + return 0;
> +
> + memset(&inarg, 0, sizeof(inarg));
> + args.in_numargs = 1;
> + args.in_args[0].size = sizeof(inarg);
> + args.in_args[0].value = &inarg;
> + args.opcode = FUSE_SYNCFS;
> + args.out_numargs = 0;
> +
> + err = fuse_simple_request(fm, &args);
> + if (err == -ENOSYS) {
> + fc->sync_fs = 0;
> + err = 0;
> + }
> +
> + return err;
> +}
> +
> enum {
> OPT_SOURCE,
> OPT_SUBTYPE,
> @@ -909,6 +943,7 @@ static const struct super_operations fuse_super_operations = {
> .put_super = fuse_put_super,
> .umount_begin = fuse_umount_begin,
> .statfs = fuse_statfs,
> + .sync_fs = fuse_sync_fs,
> .show_options = fuse_show_options,
> };
>
> diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
> index 4ee6f734ba83..a3c025308743 100644
> --- a/fs/fuse/virtio_fs.c
> +++ b/fs/fuse/virtio_fs.c
> @@ -1441,6 +1441,7 @@ static int virtio_fs_get_tree(struct fs_context *fsc)
> fc->release = fuse_free_conn;
> fc->delete_stale = true;
> fc->auto_submounts = true;
> + fc->sync_fs = true;
>
> fsc->s_fs_info = fm;
> sb = sget_fc(fsc, virtio_fs_test_super, set_anon_super_fc);
> diff --git a/include/uapi/linux/fuse.h b/include/uapi/linux/fuse.h
> index 54442612c48b..1265ca17620c 100644
> --- a/include/uapi/linux/fuse.h
> +++ b/include/uapi/linux/fuse.h
> @@ -179,6 +179,9 @@
> * 7.33
> * - add FUSE_HANDLE_KILLPRIV_V2, FUSE_WRITE_KILL_SUIDGID, FATTR_KILL_SUIDGID
> * - add FUSE_OPEN_KILL_SUIDGID
> + *
> + * 7.34
> + * - add FUSE_SYNCFS
> */
>
> #ifndef _LINUX_FUSE_H
> @@ -214,7 +217,7 @@
> #define FUSE_KERNEL_VERSION 7
>
> /** Minor version number of this interface */
> -#define FUSE_KERNEL_MINOR_VERSION 33
> +#define FUSE_KERNEL_MINOR_VERSION 34
>
> /** The node ID of the root inode */
> #define FUSE_ROOT_ID 1
> @@ -499,6 +502,7 @@ enum fuse_opcode {
> FUSE_COPY_FILE_RANGE = 47,
> FUSE_SETUPMAPPING = 48,
> FUSE_REMOVEMAPPING = 49,
> + FUSE_SYNCFS = 50,
>
> /* CUSE specific operations */
> CUSE_INIT = 4096,
> @@ -957,4 +961,8 @@ struct fuse_removemapping_one {
> #define FUSE_REMOVEMAPPING_MAX_ENTRY \
> (PAGE_SIZE / sizeof(struct fuse_removemapping_one))
>
> +struct fuse_syncfs_in {
> + uint64_t flags;
> +};
> +

Hi Greg,

Will it be better if 32bits are for flags and reset 32 are
padding and can be used in whatever manner.

struct fuse_syncfs_in {
uint32_t flags;
uint32_t padding;
};

This will increase the flexibility if we were to send more information
in future.

I already see bunch of structures where flags are 32 bit and reset
are padding bits. fuse_read_in, fuse_write_in, fuse_rename2_in etc.

Thanks
Vivek

> #endif /* _LINUX_FUSE_H */
> --
> 2.26.3
>

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