[PATCH v2 1/3] vfs: introduce the concept of inode number domains

Amir Goldstein
2018-11-08T11:49:40Z

Added super_block fields s_max_ino_bits and s_ino_domain that may be
used by stacking filesystems that want to harness unique inode numbers
of underlying filesystem.

s_ino_domain is a key that described a unique inode numbers domain.
For non stacked filesystem, s_ino_domain defaults to s_dev.

Stacking filesystem may "inherit" s_ino_domain from underlying
filesystem when inode numbers of stacked inode are the same as the
underlying inode numbers.

A filesystem may advertise the maximum used inode number bits by
setting s_max_ino_bits to non zero value.
Stacking filesystems may use this information to multiplex several
inode number domains into a new domain of unique inode numbers.

Change the type of s_stack_depth from int to unsigned short to pack
the "stacked" context variables nicely. FILESYSTEM_MAX_STACK_DEPTH
is not likely to grow far beyond the current value of 2.

Suggested-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---
fs/overlayfs/super.c | 3 ++-
fs/super.c | 2 ++
include/linux/fs.h | 16 +++++++++++++++-
3 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index 0116735cc321..1afe67c4599a 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -775,7 +775,8 @@ static int ovl_check_namelen(struct path *path, struct ovl_fs *ofs,
}

static int ovl_lower_dir(const char *name, struct path *path,
- struct ovl_fs *ofs, int *stack_depth, bool *remote)
+ struct ovl_fs *ofs, unsigned short *stack_depth,
+ bool *remote)
{
int fh_type;
int err;
diff --git a/fs/super.c b/fs/super.c
index ca53a08497ed..860ca3311f05 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -529,6 +529,8 @@ struct super_block *sget_userns(struct file_system_type *type,
return ERR_PTR(err);
}
s->s_type = type;
+ /* May be overridden by stacked filesystem */
+ s->s_ino_domain = s->s_dev;
strlcpy(s->s_id, type->name, sizeof(s->s_id));
list_add_tail(&s->s_list, &super_blocks);
hlist_add_head(&s->s_instances, &type->fs_supers);
diff --git a/include/linux/fs.h b/include/linux/fs.h
index c95c0807471f..7d45b5158cd0 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1487,7 +1487,21 @@ struct super_block {
/*
* Indicates how deep in a filesystem stack this SB is
*/
- int s_stack_depth;
+ unsigned short s_stack_depth;
+ /*
+ * s_max_ino_bits and s_ino_domain may be used by stacking filesystems
+ * that want to harness unique inode numbers of underlying filesystem.
+ * For non stacked filesystem, s_ino_domain defaults to s_dev.
+ * Stacking filesystem may "inherit" s_ino_domain from underlying
+ * filesystem when inode numbers of stacked inode are the same as the
+ * underlying inode numbers.
+ * A filesystem may advertise the maximum used inode number bits by
+ * setting s_max_ino_bits to non zero value.
+ * Stacking filesystems may use this information to multiplex several
+ * inode number domains into a new domain of unique inode numbers.
+ */
+ unsigned short s_max_ino_bits;
+ dev_t s_ino_domain;

/* s_inode_list_lock protects s_inodes */
spinlock_t s_inode_list_lock ____cacheline_aligned_in_smp;
--
2.17.1