ovl: don't cache acl on overlay layer

Some operations (setxattr/chmod) can make the cached acl stale.  We either
need to clear overlay's acl cache for the affected inode or prevent acl
caching on the overlay altogether.  Preventing caching has the following
advantages:

 - no double caching, less memory used

 - overlay cache doesn't go stale when fs clears it's own cache

Possible disadvantage is performance loss.  If that becomes a problem
get_acl() can be optimized for overlayfs.

This patch disables caching by pre setting i_*acl to a value that

  - has bit 0 set, so is_uncached_acl() will return true

  - is not equal to ACL_NOT_CACHED, so get_acl() will not overwrite it

The constant -3 was chosen for this purpose.

Fixes: 39a25b2b37 ("ovl: define ->get_acl() for overlay inodes")
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/overlayfs/inode.c

@@ -416,6 +416,9 @@ static void ovl_fill_inode(struct inode *inode, umode_t mode)
 	inode->i_ino = get_next_ino();
 	inode->i_mode = mode;
 	inode->i_flags |= S_NOCMTIME;
+#ifdef CONFIG_FS_POSIX_ACL
+	inode->i_acl = inode->i_default_acl = ACL_DONT_CACHE;
+#endif
 
 	mode &= S_IFMT;
 	switch (mode) {

include/linux/fs.h

@@ -574,6 +574,7 @@ static inline void mapping_allow_writable(struct address_space *mapping)
 
 struct posix_acl;
 #define ACL_NOT_CACHED ((void *)(-1))
+#define ACL_DONT_CACHE ((void *)(-3))
 
 static inline struct posix_acl *
 uncached_acl_sentinel(struct task_struct *task)