linux-kernel - [PATCH 03/11] fs/dcache: Add a counter to track number of children

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Date:   Wed, 26 Feb 2020 11:13:56 -0500
From:   Waiman Long <longman@...hat.com>
To:     Alexander Viro <viro@...iv.linux.org.uk>,
        Jonathan Corbet <corbet@....net>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Iurii Zaikin <yzaikin@...gle.com>
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-doc@...r.kernel.org,
        Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
        Eric Biggers <ebiggers@...gle.com>,
        Dave Chinner <david@...morbit.com>,
        Eric Sandeen <sandeen@...hat.com>,
        Waiman Long <longman@...hat.com>
Subject: [PATCH 03/11] fs/dcache: Add a counter to track number of children

Add a new field d_nchildren to struct dentry to track the number of
children in a directory.

Theoretically, we could use reference count (d_lockref.count) as a
proxy for the number of children. Normally the reference count should
be quite close to the number of children. However, when the directory
dentry is heavily contended, the reference count can differ from the
number of children by quite a bit.

The d_nchildren field is updated only when d_lock has already been held,
so the performance cost of this tracking should be negligible.

Signed-off-by: Waiman Long <longman@...hat.com>
---
 fs/dcache.c            | 16 ++++++++++++----
 include/linux/dcache.h |  7 ++++---
 2 files changed, 16 insertions(+), 7 deletions(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index a977f9e05840..0ee5aa2c31cf 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -526,6 +526,8 @@ static inline void dentry_unlist(struct dentry *dentry, struct dentry *parent)
 	if (unlikely(list_empty(&dentry->d_child)))
 		return;
 	__list_del_entry(&dentry->d_child);
+	parent->d_nchildren--;
+
 	/*
 	 * Cursors can move around the list of children.  While we'd been
 	 * a normal list member, it didn't matter - ->d_child.next would've
@@ -1738,6 +1740,7 @@ static struct dentry *__d_alloc(struct super_block *sb, const struct qstr *name)
 	dentry->d_sb = sb;
 	dentry->d_op = NULL;
 	dentry->d_fsdata = NULL;
+	dentry->d_nchildren = 0;
 	INIT_HLIST_BL_NODE(&dentry->d_hash);
 	INIT_LIST_HEAD(&dentry->d_lru);
 	INIT_LIST_HEAD(&dentry->d_subdirs);
@@ -1782,6 +1785,7 @@ struct dentry *d_alloc(struct dentry * parent, const struct qstr *name)
 	__dget_dlock(parent);
 	dentry->d_parent = parent;
 	list_add(&dentry->d_child, &parent->d_subdirs);
+	parent->d_nchildren++;
 	spin_unlock(&parent->d_lock);
 
 	return dentry;
@@ -2762,10 +2766,10 @@ static void swap_names(struct dentry *dentry, struct dentry *target)
 			 * Both are internal.
 			 */
 			unsigned int i;
-			BUILD_BUG_ON(!IS_ALIGNED(DNAME_INLINE_LEN, sizeof(long)));
-			for (i = 0; i < DNAME_INLINE_LEN / sizeof(long); i++) {
-				swap(((long *) &dentry->d_iname)[i],
-				     ((long *) &target->d_iname)[i]);
+			BUILD_BUG_ON(!IS_ALIGNED(DNAME_INLINE_LEN, sizeof(int)));
+			for (i = 0; i < DNAME_INLINE_LEN / sizeof(int); i++) {
+				swap(((int *) &dentry->d_iname)[i],
+				     ((int *) &target->d_iname)[i]);
 			}
 		}
 	}
@@ -2855,6 +2859,10 @@ static void __d_move(struct dentry *dentry, struct dentry *target,
 		dentry->d_parent->d_lockref.count++;
 		if (dentry != old_parent) /* wasn't IS_ROOT */
 			WARN_ON(!--old_parent->d_lockref.count);
+
+		/* Adjust d_nchildren */
+		dentry->d_parent->d_nchildren++;
+		old_parent->d_nchildren--;
 	} else {
 		target->d_parent = old_parent;
 		swap_names(dentry, target);
diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index 2762ca2508f9..e9e66eb50d1a 100644
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -75,12 +75,12 @@ extern struct dentry_stat_t dentry_stat;
  * large memory footprint increase).
  */
 #ifdef CONFIG_64BIT
-# define DNAME_INLINE_LEN 32 /* 192 bytes */
+# define DNAME_INLINE_LEN 28 /* 192 bytes */
 #else
 # ifdef CONFIG_SMP
-#  define DNAME_INLINE_LEN 36 /* 128 bytes */
+#  define DNAME_INLINE_LEN 32 /* 128 bytes */
 # else
-#  define DNAME_INLINE_LEN 40 /* 128 bytes */
+#  define DNAME_INLINE_LEN 36 /* 128 bytes */
 # endif
 #endif
 
@@ -96,6 +96,7 @@ struct dentry {
 	struct inode *d_inode;		/* Where the name belongs to - NULL is
 					 * negative */
 	unsigned char d_iname[DNAME_INLINE_LEN];	/* small names */
+	unsigned int d_nchildren;	/* # of children (directory only) */
 
 	/* Ref lookup also touches following */
 	struct lockref d_lockref;	/* per-dentry lock and refcount */
-- 
2.18.1