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, 16 insertions, 10 deletions

diff --git a/Documentation/sysctl/fs.txt b/Documentation/sysctl/fs.txt
index 819caf8ca05f..58649bd4fcfc 100644
--- a/Documentation/sysctl/fs.txt
+++ b/Documentation/sysctl/fs.txt
@@ -56,26 +56,32 @@ of any kernel data structures.
 
 dentry-state:
 
-From linux/fs/dentry.c:
+From linux/include/linux/dcache.h:
 --------------------------------------------------------------
-struct {
+struct dentry_stat_t dentry_stat {
         int nr_dentry;
         int nr_unused;
         int age_limit;         /* age in seconds */
         int want_pages;        /* pages requested by system */
-        int dummy[2];
-} dentry_stat = {0, 0, 45, 0,};
--------------------------------------------------------------- 
-
-Dentries are dynamically allocated and deallocated, and
-nr_dentry seems to be 0 all the time. Hence it's safe to
-assume that only nr_unused, age_limit and want_pages are
-used. Nr_unused seems to be exactly what its name says.
+        int nr_negative;       /* # of unused negative dentries */
+        int dummy;             /* Reserved for future use */
+};
+--------------------------------------------------------------
+
+Dentries are dynamically allocated and deallocated.
+
+nr_dentry shows the total number of dentries allocated (active
++ unused). nr_unused shows the number of dentries that are not
+actively used, but are saved in the LRU list for future reuse.
+
 Age_limit is the age in seconds after which dcache entries
 can be reclaimed when memory is short and want_pages is
 nonzero when shrink_dcache_pages() has been called and the
 dcache isn't pruned yet.
 
+nr_negative shows the number of unused dentries that are also
+negative dentries which do not mapped to actual files.
+
 ==============================================================
 
 dquot-max & dquot-nr: