[Virtio-fs] [RFC 5/5] fuse: Implement crossmounts
Max Reitz
mreitz at redhat.com
Wed May 6 15:36:37 UTC 2020
With this commit, FUSE servers can indicate crossmount points by putting
the mounted filesystem's st_dev into st_rdev (which is otherwise unused
for directories). The inode will then be marked as S_AUTOMOUNT, and the
.d_automount implementation creates a new submount at that location, so
that the submount gets a distinct st_dev value.
Note that all submounts get a distinct superblock and a distinct st_dev
value, so for virtio-fs, even if the same filesystem is mounted more
than once on the host, none of its mount points will have the same
st_dev. We need distinct superblocks because the superblock points to
the root node, but the different host mounts may show different trees
(e.g. due to submounts in some of them, but not in others).
We could keep a mapping of "server-reported st_rdev" to "anonymous
st_dev" (as returned by get_anon_bdev()), and thus use the same st_dev
for multiple superblocks, but that probably has pitfalls that are not
worth risking for something that most likely nobody needs.
Right now, this behavior is only enabled when fuse_conn.auto_submounts
is set, which is the case only for virtio-fs.
Signed-off-by: Max Reitz <mreitz at redhat.com>
---
fs/fuse/fuse_i.h | 3 ++
fs/fuse/dir.c | 85 +++++++++++++++++++++++++++++++++++++++++++++
fs/fuse/inode.c | 6 ++--
fs/fuse/virtio_fs.c | 1 +
4 files changed, 93 insertions(+), 2 deletions(-)
diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index d8528aa97386..af73d60253d8 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -723,6 +723,9 @@ struct fuse_conn {
/* Do not show mount options */
unsigned int no_mount_options:1;
+ /* Auto-mount submounts announced by the server */
+ unsigned int auto_submounts:1;
+
/** The number of requests waiting for completion */
atomic_t num_waiting;
diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
index fb0e9204060c..882fc76834b2 100644
--- a/fs/fuse/dir.c
+++ b/fs/fuse/dir.c
@@ -10,6 +10,7 @@
#include <linux/pagemap.h>
#include <linux/file.h>
+#include <linux/fs_context.h>
#include <linux/sched.h>
#include <linux/namei.h>
#include <linux/slab.h>
@@ -299,6 +300,89 @@ static int fuse_dentry_delete(const struct dentry *dentry)
return time_before64(fuse_dentry_time(dentry), get_jiffies_64());
}
+static int fuse_submount_set_super(struct super_block *sb,
+ struct fs_context *fsc)
+{
+ int err;
+
+ err = get_anon_bdev(&sb->s_dev);
+ if (!err)
+ fuse_mount_get(fsc->s_fs_info);
+
+ return err;
+}
+
+/*
+ * Create a fuse_mount object with a new superblock (with path->dentry
+ * as the root), and return that mount so it can be auto-mounted on
+ * @path.
+ */
+static struct vfsmount *fuse_dentry_automount(struct path *path)
+{
+ struct fs_context *fsc;
+ struct fuse_mount *parent_fm = get_fuse_mount_super(path->mnt->mnt_sb);
+ struct fuse_conn *fc = parent_fm->fc;
+ struct fuse_mount *fm;
+ struct vfsmount *mnt;
+ struct fuse_inode *mp_finode = get_fuse_inode(d_inode(path->dentry));
+ struct super_block *sb;
+ int err;
+
+ fsc = fs_context_for_submount(path->mnt->mnt_sb->s_type, path->dentry);
+ if (IS_ERR(fsc)) {
+ err = PTR_ERR(fsc);
+ goto out;
+ }
+
+ err = -ENOMEM;
+ fm = kzalloc(sizeof(struct fuse_mount), GFP_KERNEL);
+ if (!fm)
+ goto out_put_fsc;
+
+ /* Create new fuse_mount for the existing fuse_conn */
+ fuse_mount_init(fm, fc);
+
+ /* Get (new) superblock for this new fuse_mount */
+ fsc->s_fs_info = fm;
+ sb = sget_fc(fsc, NULL, fuse_submount_set_super);
+ fuse_mount_put(fm);
+ if (IS_ERR(sb)) {
+ err = PTR_ERR(sb);
+ goto out_put_fsc;
+ }
+
+ /* Initialize superblock, making @mp_finode its root */
+ err = fuse_fill_super_common(sb, NULL, mp_finode);
+ if (err)
+ goto out_put_sb;
+
+ sb->s_flags |= SB_ACTIVE;
+ fsc->root = dget(sb->s_root);
+ /* We are done configuring the superblock, so unlock it */
+ up_write(&sb->s_umount);
+
+ /* Create the submount */
+ mnt = vfs_create_mount(fsc);
+ if (IS_ERR(mnt)) {
+ err = PTR_ERR(mnt);
+ goto out_put_fsc;
+ }
+
+ mntget(mnt);
+
+ put_fs_context(fsc);
+ return mnt;
+
+out_put_sb:
+ /* Only jump here when fsc->root is NULL and sb is still locked
+ * (otherwise put_fs_context() will put the superblock) */
+ deactivate_locked_super(sb);
+out_put_fsc:
+ put_fs_context(fsc);
+out:
+ return ERR_PTR(err);
+}
+
const struct dentry_operations fuse_dentry_operations = {
.d_revalidate = fuse_dentry_revalidate,
.d_delete = fuse_dentry_delete,
@@ -306,6 +390,7 @@ const struct dentry_operations fuse_dentry_operations = {
.d_init = fuse_dentry_init,
.d_release = fuse_dentry_release,
#endif
+ .d_automount = fuse_dentry_automount,
};
const struct dentry_operations fuse_root_dentry_operations = {
diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
index ff30d30b3faf..77a386cad12f 100644
--- a/fs/fuse/inode.c
+++ b/fs/fuse/inode.c
@@ -253,9 +253,11 @@ static void fuse_init_inode(struct inode *inode, struct fuse_attr *attr)
if (S_ISREG(inode->i_mode)) {
fuse_init_common(inode);
fuse_init_file_inode(inode);
- } else if (S_ISDIR(inode->i_mode))
+ } else if (S_ISDIR(inode->i_mode)) {
fuse_init_dir(inode);
- else if (S_ISLNK(inode->i_mode))
+ if (attr->rdev && get_fuse_conn(inode)->auto_submounts)
+ inode->i_flags |= S_AUTOMOUNT;
+ } else if (S_ISLNK(inode->i_mode))
fuse_init_symlink(inode);
else if (S_ISCHR(inode->i_mode) || S_ISBLK(inode->i_mode) ||
S_ISFIFO(inode->i_mode) || S_ISSOCK(inode->i_mode)) {
diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
index 38100f096421..53dae0ee5ab1 100644
--- a/fs/fuse/virtio_fs.c
+++ b/fs/fuse/virtio_fs.c
@@ -1193,6 +1193,7 @@ static int virtio_fs_get_tree(struct fs_context *fsc)
fs);
fc->release = fuse_free_conn;
fc->delete_stale = true;
+ fc->auto_submounts = true;
fuse_mount_init(fm, fc);
fuse_conn_put(fc);
--
2.26.2
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