fs: Kill DCACHE_DONTCACHE dentry even if DCACHE_REFERENCED is set

If DCACHE_REFERENCED is set, fast_dput() will return true, and then
retain_dentry() have no chance to check DCACHE_DONTCACHE. As a result,
the dentry won't be killed and the corresponding inode can't be evicted.
In the following example, the DAX policy can't take effects unless we
do a drop_caches manually.

  # DCACHE_LRU_LIST will be set
  echo abcdefg > test.txt

  # DCACHE_REFERENCED will be set and DCACHE_DONTCACHE can't do anything
  xfs_io -c 'chattr +x' test.txt

  # Drop caches to make DAX changing take effects
  echo 2 > /proc/sys/vm/drop_caches

What this patch does is preventing fast_dput() from returning true if
DCACHE_DONTCACHE is set. Then retain_dentry() will detect the
DCACHE_DONTCACHE and will return false. As a result, the dentry will be
killed and the inode will be evicted. In this way, if we change per-file
DAX policy, it will take effects automatically after this file is closed
by all processes.

I also add some comments to make the code more clear.

Signed-off-by: Hao Li <lihao2018.fnst@cn.fujitsu.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/dcache.c

@@ -793,10 +793,17 @@ static inline bool fast_dput(struct dentry *dentry)
 	 * a reference to the dentry and change that, but
 	 * our work is done - we can leave the dentry
 	 * around with a zero refcount.
+	 *
+	 * Nevertheless, there are two cases that we should kill
+	 * the dentry anyway.
+	 * 1. free disconnected dentries as soon as their refcount
+	 *    reached zero.
+	 * 2. free dentries if they should not be cached.
 	 */
 	smp_rmb();
 	d_flags = READ_ONCE(dentry->d_flags);
-	d_flags &= DCACHE_REFERENCED | DCACHE_LRU_LIST | DCACHE_DISCONNECTED;
+	d_flags &= DCACHE_REFERENCED | DCACHE_LRU_LIST |
+			DCACHE_DISCONNECTED | DCACHE_DONTCACHE;
 
 	/* Nothing to do? Dropping the reference was all we needed? */
 	if (d_flags == (DCACHE_REFERENCED | DCACHE_LRU_LIST) && !d_unhashed(dentry))