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From: Waiman Long <longman@redhat.com>
To: Alexander Viro <viro@zeniv.linux.org.uk>,
	Jonathan Corbet <corbet@lwn.net>,
	Luis Chamberlain <mcgrof@kernel.org>,
	Kees Cook <keescook@chromium.org>,
	Iurii Zaikin <yzaikin@google.com>
Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-doc@vger.kernel.org,
	Mauro Carvalho Chehab <mchehab+samsung@kernel.org>,
	Eric Biggers <ebiggers@google.com>,
	Dave Chinner <david@fromorbit.com>,
	Eric Sandeen <sandeen@redhat.com>,
	Waiman Long <longman@redhat.com>
Subject: [PATCH 09/11] fs/dcache: Don't allow small values for dentry-dir-max
Date: Wed, 26 Feb 2020 11:14:02 -0500	[thread overview]
Message-ID: <20200226161404.14136-10-longman@redhat.com> (raw)
In-Reply-To: <20200226161404.14136-1-longman@redhat.com>

A small value for "dentry-dir-max", e.g. < 10, will cause excessive
dentry count checking leading to noticeable performance degradation. In
order to make this sysctl parameter more foolproof, we are not going
to allow any positive integer value less than 256.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 Documentation/admin-guide/sysctl/fs.rst | 10 +++++-----
 fs/dcache.c                             | 24 +++++++++++++++++++-----
 2 files changed, 24 insertions(+), 10 deletions(-)

diff --git a/Documentation/admin-guide/sysctl/fs.rst b/Documentation/admin-guide/sysctl/fs.rst
index 7274a7b34ee4..e09d851f9d42 100644
--- a/Documentation/admin-guide/sysctl/fs.rst
+++ b/Documentation/admin-guide/sysctl/fs.rst
@@ -71,11 +71,11 @@ in the directory.  No restriction is placed on the number of positive
 dentries as it is naturally limited by the number of files in the
 directory.
 
-The default value is 0 which means there is no limit.  Any non-negative
-value is allowed.  However, internal tracking is done on all dentry
-types. So the value given, if non-zero, should be larger than the
-number of files in a typical large directory in order to reduce the
-tracking overhead.
+The default value is 0 which means there is no limit.  Any positive
+integer value not less than 256 is also allowed.  However, internal
+tracking is done on all dentry types. So the value given, if non-zero,
+should be larger than the number of files in a typical large directory
+in order to reduce the tracking overhead.
 
 
 dentry-state
diff --git a/fs/dcache.c b/fs/dcache.c
index 0bd5d6974f75..f470763e7fb8 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -129,10 +129,14 @@ static DEFINE_PER_CPU(long, nr_dentry_negative);
  *
  * This is sysctl parameter "dentry-dir-max" which specifies a limit on
  * the maximum number of negative dentries that are allowed under any
- * given directory.
+ * given directory. The allowable value of "dentry-dir-max" is either
+ * 0, which means no limit, or 256 and up. A low value of "dentry-dir-max"
+ * will cause excessive dentry count checking affecting system performance.
  */
-int dcache_dentry_dir_max_sysctl __read_mostly;
+int dcache_dentry_dir_max_sysctl;
 EXPORT_SYMBOL_GPL(dcache_dentry_dir_max_sysctl);
+static int negative_dentry_dir_max __read_mostly;
+#define	DENTRY_DIR_MAX_MIN	0x100
 
 static LLIST_HEAD(negative_reclaim_list);
 static DEFINE_STATIC_KEY_FALSE(negative_reclaim_enable);
@@ -206,6 +210,16 @@ int proc_dcache_dentry_dir_max(struct ctl_table *ctl, int write,
 	if (!write || ret || (dcache_dentry_dir_max_sysctl == old))
 		return ret;
 
+	/*
+	 * A non-zero value must be >= DENTRY_DIR_MAX_MIN.
+	 */
+	if (dcache_dentry_dir_max_sysctl &&
+	   (dcache_dentry_dir_max_sysctl < DENTRY_DIR_MAX_MIN)) {
+		dcache_dentry_dir_max_sysctl = old;
+		return -EINVAL;
+	}
+
+	negative_dentry_dir_max = dcache_dentry_dir_max_sysctl;
 	if (!old && dcache_dentry_dir_max_sysctl)
 		static_branch_enable(&negative_reclaim_enable);
 	else if (old && !dcache_dentry_dir_max_sysctl)
@@ -1396,7 +1410,7 @@ static void reclaim_negative_dentry(struct dentry *parent, int *quota,
 				    struct list_head *dispose)
 {
 	struct dentry *child;
-	int limit = READ_ONCE(dcache_dentry_dir_max_sysctl);
+	int limit = READ_ONCE(negative_dentry_dir_max);
 	int cnt, npositive;
 
 	lockdep_assert_held(&parent->d_lock);
@@ -1405,7 +1419,7 @@ static void reclaim_negative_dentry(struct dentry *parent, int *quota,
 
 	/*
 	 * Compute # of negative dentries to be reclaimed
-	 * An extra 1/8 of dcache_dentry_dir_max_sysctl is added.
+	 * An extra 1/8 of negative_dentry_dir_max is added.
 	 */
 	if (cnt <= limit)
 		return;
@@ -1537,7 +1551,7 @@ static void negative_reclaim_workfn(struct work_struct *work)
 static void negative_reclaim_check(struct dentry *parent)
 	__releases(rcu)
 {
-	int limit = dcache_dentry_dir_max_sysctl;
+	int limit = negative_dentry_dir_max;
 	struct reclaim_dentry *dentry_node;
 
 	if (!limit)
-- 
2.18.1


  parent reply	other threads:[~2020-02-26 16:15 UTC|newest]

Thread overview: 34+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-26 16:13 [PATCH 00/11] fs/dcache: Limit # of negative dentries Waiman Long
2020-02-26 16:13 ` [PATCH 01/11] fs/dcache: Fix incorrect accounting " Waiman Long
2020-02-26 16:13 ` [PATCH 02/11] fs/dcache: Simplify __dentry_kill() Waiman Long
2020-02-26 16:13 ` [PATCH 03/11] fs/dcache: Add a counter to track number of children Waiman Long
2020-02-26 16:13 ` [PATCH 04/11] fs/dcache: Add sysctl parameter dentry-dir-max Waiman Long
2020-02-26 16:13 ` [PATCH 05/11] fs/dcache: Reclaim excessive negative dentries in directories Waiman Long
2020-02-26 16:13 ` [PATCH 06/11] fs/dcache: directory opportunistically stores # of positive dentries Waiman Long
2020-02-26 16:14 ` [PATCH 07/11] fs/dcache: Add static key negative_reclaim_enable Waiman Long
2020-02-26 16:14 ` [PATCH 08/11] fs/dcache: Limit dentry reclaim count in negative_reclaim_workfn() Waiman Long
2020-02-26 16:14 ` Waiman Long [this message]
2020-02-26 16:14 ` [PATCH 10/11] fs/dcache: Kill off dentry as last resort Waiman Long
2020-02-26 16:14 ` [PATCH 11/11] fs/dcache: Track # of negative dentries reclaimed & killed Waiman Long
2020-02-26 16:29 ` [PATCH 00/11] fs/dcache: Limit # of negative dentries Matthew Wilcox
2020-02-26 19:19   ` Waiman Long
2020-02-26 21:28     ` Matthew Wilcox
2020-02-26 21:28   ` Andreas Dilger
2020-02-26 21:45     ` Matthew Wilcox
2020-02-27  8:07       ` Dave Chinner
2020-02-27  9:55     ` Ian Kent
2020-02-28  3:34       ` Matthew Wilcox
2020-02-28  4:16         ` Ian Kent
2020-02-28  4:36           ` Ian Kent
2020-02-28  4:52             ` Al Viro
2020-02-28  4:22         ` Al Viro
2020-02-28  4:52           ` Ian Kent
2020-02-28 15:32         ` Waiman Long
2020-02-28 15:39           ` Matthew Wilcox
2020-02-28 19:32         ` Theodore Y. Ts'o
2020-02-27 19:04   ` Eric Sandeen
2020-02-27 22:39     ` Dave Chinner
2020-02-27  8:30 ` Dave Chinner
2020-02-28 15:47   ` Waiman Long
2020-03-15  3:46 ` Matthew Wilcox
2020-03-21 10:17   ` Konstantin Khlebnikov

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