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Subject[PATCH 09/34] union-mount: Add two superblock fields for union mounts
Add two fields to struct super_block to support union mounts.
s_union_lower_mnts is a pointer to a cloned vfsmount tree of all the
lower (read-only) mounts unioned with the topmost (read-write)
vfsmount. These mounts may have submounts which will also be unioned;
hence we copy the entire vfsmount tree, not just the root vfsmounts.
s_union_count is the number of lower mounts unioned at the root of the
file system. This count is the maximum number of directories that
will ever be unioned with a single directory. We use it to allocate a
union stack of the correct size for each directory.

Signed-off-by: Valerie Aurora <>
include/linux/fs.h | 12 ++++++++++++
1 files changed, 12 insertions(+), 0 deletions(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 2e3d745..5e35b03 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1397,6 +1397,18 @@ struct super_block {
* Decremented by free_vfsmnt() if MNT_HARD_READONLY is set.
int s_hard_readonly_users;
+ /*
+ * Root of the private cloned vfsmount tree of the read-only
+ * mounts in this union (set in topmost vfsmount only)
+ */
+ struct vfsmount *s_union_lower_mnts;
+ /*
+ * Number of layers in this union, not counting the topmost or
+ * submounts.
+ */
+ unsigned int s_union_count;

extern struct timespec current_fs_time(struct super_block *sb);

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