ovl: persistent overlay inode nlink for indexed inodes

With inodes index enabled, an overlay inode nlink counts the union of upper
and non-covered lower hardlinks. During the lifetime of a non-pure upper
inode, the following nlink modifying operations can happen:

1. Lower hardlink copy up
2. Upper hardlink created, unlinked or renamed over
3. Lower hardlink whiteout or renamed over

For the first, copy up case, the union nlink does not change, whether the
operation succeeds or fails, but the upper inode nlink may change.
Therefore, before copy up, we store the union nlink value relative to the
lower inode nlink in the index inode xattr trusted.overlay.nlink.

For the second, upper hardlink case, the union nlink should be incremented
or decremented IFF the operation succeeds, aligned with nlink change of the
upper inode. Therefore, before link/unlink/rename, we store the union nlink
value relative to the upper inode nlink in the index inode.

For the last, lower cover up case, we simplify things by preceding the
whiteout or cover up with copy up. This makes sure that there is an index
upper inode where the nlink xattr can be stored before the copied up upper
entry is unlink.

Return the overlay inode nlinks for indexed upper inodes on stat(2).

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

1 files changed, 5 insertions, 0 deletions

diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
index 751b36a5c22f..c1321ab38224 100644
--- a/fs/overlayfs/overlayfs.h
+++ b/fs/overlayfs/overlayfs.h
@@ -25,6 +25,7 @@ enum ovl_path_type {
 #define OVL_XATTR_REDIRECT OVL_XATTR_PREFIX "redirect"
 #define OVL_XATTR_ORIGIN OVL_XATTR_PREFIX "origin"
 #define OVL_XATTR_IMPURE OVL_XATTR_PREFIX "impure"
+#define OVL_XATTR_NLINK OVL_XATTR_PREFIX "nlink"
 
 enum ovl_flag {
 	OVL_IMPURE,
@@ -229,6 +230,8 @@ void ovl_set_flag(unsigned long flag, struct inode *inode);
 bool ovl_test_flag(unsigned long flag, struct inode *inode);
 bool ovl_inuse_trylock(struct dentry *dentry);
 void ovl_inuse_unlock(struct dentry *dentry);
+int ovl_nlink_start(struct dentry *dentry, bool *locked);
+void ovl_nlink_end(struct dentry *dentry, bool locked);
 
 static inline bool ovl_is_impuredir(struct dentry *dentry)
 {
@@ -258,6 +261,8 @@ int ovl_indexdir_cleanup(struct dentry *dentry, struct vfsmount *mnt,
 			 struct path *lowerstack, unsigned int numlower);
 
 /* inode.c */
+int ovl_set_nlink_upper(struct dentry *dentry);
+int ovl_set_nlink_lower(struct dentry *dentry);
 int ovl_setattr(struct dentry *dentry, struct iattr *attr);
 int ovl_getattr(const struct path *path, struct kstat *stat,
 		u32 request_mask, unsigned int flags);