fs/dcache: Track & report number of negative dentries

The current dentry number tracking code doesn't distinguish between
positive & negative dentries.  It just reports the total number of
dentries in the LRU lists.

As excessive number of negative dentries can have an impact on system
performance, it will be wise to track the number of positive and
negative dentries separately.

This patch adds tracking for the total number of negative dentries in
the system LRU lists and reports it in the 5th field in the
/proc/sys/fs/dentry-state file.  The number, however, does not include
negative dentries that are in flight but not in the LRU yet as well as
those in the shrinker lists which are on the way out anyway.

The number of positive dentries in the LRU lists can be roughly found by
subtracting the number of negative dentries from the unused count.

Matthew Wilcox had confirmed that since the introduction of the
dentry_stat structure in 2.1.60, the dummy array was there, probably for
future extension.  They were not replacements of pre-existing fields.
So no sane applications that read the value of /proc/sys/fs/dentry-state
will do dummy thing if the last 2 fields of the sysctl parameter are not
zero.  IOW, it will be safe to use one of the dummy array entry for
negative dentry count.

Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 32 insertions, 0 deletions

diff --git a/fs/dcache.c b/fs/dcache.c
index 44e5652b2664..aac41adf4743 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -119,6 +119,7 @@ struct dentry_stat_t dentry_stat = {
 
 static DEFINE_PER_CPU(long, nr_dentry);
 static DEFINE_PER_CPU(long, nr_dentry_unused);
+static DEFINE_PER_CPU(long, nr_dentry_negative);
 
 #if defined(CONFIG_SYSCTL) && defined(CONFIG_PROC_FS)
 
@@ -152,11 +153,22 @@ static long get_nr_dentry_unused(void)
 	return sum < 0 ? 0 : sum;
 }
 
+static long get_nr_dentry_negative(void)
+{
+	int i;
+	long sum = 0;
+
+	for_each_possible_cpu(i)
+		sum += per_cpu(nr_dentry_negative, i);
+	return sum < 0 ? 0 : sum;
+}
+
 int proc_nr_dentry(struct ctl_table *table, int write, void __user *buffer,
 		   size_t *lenp, loff_t *ppos)
 {
 	dentry_stat.nr_dentry = get_nr_dentry();
 	dentry_stat.nr_unused = get_nr_dentry_unused();
+	dentry_stat.nr_negative = get_nr_dentry_negative();
 	return proc_doulongvec_minmax(table, write, buffer, lenp, ppos);
 }
 #endif
@@ -317,6 +329,8 @@ static inline void __d_clear_type_and_inode(struct dentry *dentry)
 	flags &= ~(DCACHE_ENTRY_TYPE | DCACHE_FALLTHRU);
 	WRITE_ONCE(dentry->d_flags, flags);
 	dentry->d_inode = NULL;
+	if (dentry->d_flags & DCACHE_LRU_LIST)
+		this_cpu_inc(nr_dentry_negative);
 }
 
 static void dentry_free(struct dentry *dentry)
@@ -371,6 +385,11 @@ static void dentry_unlink_inode(struct dentry * dentry)
  * The per-cpu "nr_dentry_unused" counters are updated with
  * the DCACHE_LRU_LIST bit.
  *
+ * The per-cpu "nr_dentry_negative" counters are only updated
+ * when deleted from or added to the per-superblock LRU list, not
+ * from/to the shrink list. That is to avoid an unneeded dec/inc
+ * pair when moving from LRU to shrink list in select_collect().
+ *
  * These helper functions make sure we always follow the
  * rules. d_lock must be held by the caller.
  */
@@ -380,6 +399,8 @@ static void d_lru_add(struct dentry *dentry)
 	D_FLAG_VERIFY(dentry, 0);
 	dentry->d_flags |= DCACHE_LRU_LIST;
 	this_cpu_inc(nr_dentry_unused);
+	if (d_is_negative(dentry))
+		this_cpu_inc(nr_dentry_negative);
 	WARN_ON_ONCE(!list_lru_add(&dentry->d_sb->s_dentry_lru, &dentry->d_lru));
 }
 
@@ -388,6 +409,8 @@ static void d_lru_del(struct dentry *dentry)
 	D_FLAG_VERIFY(dentry, DCACHE_LRU_LIST);
 	dentry->d_flags &= ~DCACHE_LRU_LIST;
 	this_cpu_dec(nr_dentry_unused);
+	if (d_is_negative(dentry))
+		this_cpu_dec(nr_dentry_negative);
 	WARN_ON_ONCE(!list_lru_del(&dentry->d_sb->s_dentry_lru, &dentry->d_lru));
 }
 
@@ -418,6 +441,8 @@ static void d_lru_isolate(struct list_lru_one *lru, struct dentry *dentry)
 	D_FLAG_VERIFY(dentry, DCACHE_LRU_LIST);
 	dentry->d_flags &= ~DCACHE_LRU_LIST;
 	this_cpu_dec(nr_dentry_unused);
+	if (d_is_negative(dentry))
+		this_cpu_dec(nr_dentry_negative);
 	list_lru_isolate(lru, &dentry->d_lru);
 }
 
@@ -426,6 +451,8 @@ static void d_lru_shrink_move(struct list_lru_one *lru, struct dentry *dentry,
 {
 	D_FLAG_VERIFY(dentry, DCACHE_LRU_LIST);
 	dentry->d_flags |= DCACHE_SHRINK_LIST;
+	if (d_is_negative(dentry))
+		this_cpu_dec(nr_dentry_negative);
 	list_lru_isolate_move(lru, &dentry->d_lru, list);
 }
 
@@ -1816,6 +1843,11 @@ static void __d_instantiate(struct dentry *dentry, struct inode *inode)
 	WARN_ON(d_in_lookup(dentry));
 
 	spin_lock(&dentry->d_lock);
+	/*
+	 * Decrement negative dentry count if it was in the LRU list.
+	 */
+	if (dentry->d_flags & DCACHE_LRU_LIST)
+		this_cpu_dec(nr_dentry_negative);
 	hlist_add_head(&dentry->d_u.d_alias, &inode->i_dentry);
 	raw_write_seqcount_begin(&dentry->d_seq);
 	__d_set_inode_and_type(dentry, inode, add_flags);