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From: Hao Li <>
To: <>, <>
Cc: <>, <>,
	<>, <>,
	<>, <>
Subject: [PATCH v2] fs: Kill DCACHE_DONTCACHE dentry even if DCACHE_REFERENCED is set
Date: Thu, 24 Sep 2020 13:59:58 +0800	[thread overview]
Message-ID: <> (raw)

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 <>
v1 is split into two standalone patch as discussed in [1], and the first
patch has been reviewed in [2]. This is the second patch.


 fs/dcache.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index ea0485861d93..97e81a844a96 100644
--- a/fs/dcache.c
+++ b/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.
 	d_flags = READ_ONCE(dentry->d_flags);
 	/* Nothing to do? Dropping the reference was all we needed? */
 	if (d_flags == (DCACHE_REFERENCED | DCACHE_LRU_LIST) && !d_unhashed(dentry))

             reply	other threads:[~2020-09-24  6:00 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-24  5:59 Hao Li [this message]
2020-09-24 14:58 ` Jan Kara
2020-10-21 13:51   ` Jan Kara
2020-11-05 10:54     ` Li, Hao
2020-09-24 15:27 ` Ira Weiny

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