lkml.org 
[lkml]   [2020]   [Sep]   [24]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
From
Subject[PATCH v2] fs: Kill DCACHE_DONTCACHE dentry even if DCACHE_REFERENCED is set
Date
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>
---
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.

[1]: https://lore.kernel.org/linux-fsdevel/20200831003407.GE12096@dread.disaster.area/
[2]: https://lore.kernel.org/linux-fsdevel/20200906214002.GI12131@dread.disaster.area/

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.
*/
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))
--
2.28.0


\
 
 \ /
  Last update: 2020-09-24 08:00    [W:0.071 / U:0.928 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site