super: fix calculation of shrinkable objects for small numbers

The sysctl knob sysctl_vfs_cache_pressure is used to determine which
percentage of the shrinkable objects in our cache we should actively try
to shrink.

It works great in situations in which we have many objects (at least more
than 100), because the aproximation errors will be negligible.  But if
this is not the case, specially when total_objects < 100, we may end up
concluding that we have no objects at all (total / 100 = 0, if total <
100).

This is certainly not the biggest killer in the world, but may matter in
very low kernel memory situations.

Signed-off-by: Glauber Costa <glommer@openvz.org>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Mel Gorman <mgorman@suse.de>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: "Theodore Ts'o" <tytso@mit.edu>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Cc: Arve Hjønnevåg <arve@android.com>
Cc: Carlos Maiolino <cmaiolino@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Chuck Lever <chuck.lever@oracle.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: David Rientjes <rientjes@google.com>
Cc: Gleb Natapov <gleb@redhat.com>
Cc: Greg Thelen <gthelen@google.com>
Cc: J. Bruce Fields <bfields@redhat.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Jerome Glisse <jglisse@redhat.com>
Cc: John Stultz <john.stultz@linaro.org>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Kent Overstreet <koverstreet@google.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Steven Whitehouse <swhiteho@redhat.com>
Cc: Thomas Hellstrom <thellstrom@vmware.com>
Cc: Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

8 files changed, 18 insertions, 15 deletions

diff --git a/fs/gfs2/glock.c b/fs/gfs2/glock.c
index 722329cac98f..b782bb56085d 100644
--- a/fs/gfs2/glock.c
+++ b/fs/gfs2/glock.c
@@ -1462,7 +1462,7 @@ static int gfs2_shrink_glock_memory(struct shrinker *shrink,
                 gfs2_scan_glock_lru(sc->nr_to_scan);
         }
 
-        return (atomic_read(&lru_count) / 100) * sysctl_vfs_cache_pressure;
+        return vfs_pressure_ratio(atomic_read(&lru_count));
 }
 
 static struct shrinker glock_shrinker = {
diff --git a/fs/gfs2/quota.c b/fs/gfs2/quota.c
index 3768c2f40e43..d550a5d6a05f 100644
--- a/fs/gfs2/quota.c
+++ b/fs/gfs2/quota.c
@@ -114,7 +114,7 @@ int gfs2_shrink_qd_memory(struct shrinker *shrink, struct shrink_control *sc)
         spin_unlock(&qd_lru_lock);
 
 out:
-        return (atomic_read(&qd_lru_count) * sysctl_vfs_cache_pressure) / 100;
+        return vfs_pressure_ratio(atomic_read(&qd_lru_count));
 }
 
 static u64 qd2index(struct gfs2_quota_data *qd)
diff --git a/fs/mbcache.c b/fs/mbcache.c
index 8c32ef3ba88e..5eb04767cb29 100644
--- a/fs/mbcache.c
+++ b/fs/mbcache.c
@@ -189,7 +189,7 @@ mb_cache_shrink_fn(struct shrinker *shrink, struct shrink_control *sc)
         list_for_each_entry_safe(entry, tmp, &free_list, e_lru_list) {
                 __mb_cache_entry_forget(entry, gfp_mask);
         }
-        return (count / 100) * sysctl_vfs_cache_pressure;
+        return vfs_pressure_ratio(count);
 }
 
 
diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c
index e79bc6ce828e..813ef2571545 100644
--- a/fs/nfs/dir.c
+++ b/fs/nfs/dir.c
@@ -2046,7 +2046,7 @@ remove_lru_entry:
         }
         spin_unlock(&nfs_access_lru_lock);
         nfs_access_free_list(&head);
-        return (atomic_long_read(&nfs_access_nr_entries) / 100) * sysctl_vfs_cache_pressure;
+        return vfs_pressure_ratio(atomic_long_read(&nfs_access_nr_entries));
 }
 
 static void __nfs_access_zap_cache(struct nfs_inode *nfsi, struct list_head *head)
diff --git a/fs/quota/dquot.c b/fs/quota/dquot.c
index 9a702e193538..13eee847605c 100644
--- a/fs/quota/dquot.c
+++ b/fs/quota/dquot.c
@@ -719,9 +719,8 @@ static int shrink_dqcache_memory(struct shrinker *shrink,
                 prune_dqcache(nr);
                 spin_unlock(&dq_list_lock);
         }
-        return ((unsigned)
-                percpu_counter_read_positive(&dqstats.counter[DQST_FREE_DQUOTS])
-                /100) * sysctl_vfs_cache_pressure;
+        return vfs_pressure_ratio(
+        percpu_counter_read_positive(&dqstats.counter[DQST_FREE_DQUOTS]));
 }
 
 static struct shrinker dqcache_shrinker = {
diff --git a/fs/super.c b/fs/super.c
index f6961ea84c56..63b6863bac7b 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -82,13 +82,13 @@ static int prune_super(struct shrinker *shrink, struct shrink_control *sc)
                 int     inodes;
 
                 /* proportion the scan between the caches */
-                dentries = (sc->nr_to_scan * sb->s_nr_dentry_unused) /
-                                                        total_objects;
-                inodes = (sc->nr_to_scan * sb->s_nr_inodes_unused) /
-                                                        total_objects;
+                dentries = mult_frac(sc->nr_to_scan, sb->s_nr_dentry_unused,
+                                                        total_objects);
+                inodes = mult_frac(sc->nr_to_scan, sb->s_nr_inodes_unused,
+                                                        total_objects);
                 if (fs_objects)
-                        fs_objects = (sc->nr_to_scan * fs_objects) /
-                                                        total_objects;
+                        fs_objects = mult_frac(sc->nr_to_scan, fs_objects,
+                                                        total_objects);
                 /*
                  * prune the dcache first as the icache is pinned by it, then
                  * prune the icache, followed by the filesystem specific caches
@@ -104,7 +104,7 @@ static int prune_super(struct shrinker *shrink, struct shrink_control *sc)
                                 sb->s_nr_inodes_unused + fs_objects;
         }
 
-        total_objects = (total_objects / 100) * sysctl_vfs_cache_pressure;
+        total_objects = vfs_pressure_ratio(total_objects);
         drop_super(sb);
         return total_objects;
 }
diff --git a/fs/xfs/xfs_qm.c b/fs/xfs/xfs_qm.c
index 6218a0aeeeea..956da2e1c7af 100644
--- a/fs/xfs/xfs_qm.c
+++ b/fs/xfs/xfs_qm.c
@@ -1722,7 +1722,7 @@ xfs_qm_shake(
         }
 
 out:
-        return (qi->qi_lru_count / 100) * sysctl_vfs_cache_pressure;
+        return vfs_pressure_ratio(qi->qi_lru_count);
 }
 
 /*
diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index 844a1ef387e4..59066e0b4ff1 100644
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -395,4 +395,8 @@ static inline bool d_mountpoint(const struct dentry *dentry)
 
 extern int sysctl_vfs_cache_pressure;
 
+static inline unsigned long vfs_pressure_ratio(unsigned long val)
+{
+        return mult_frac(val, sysctl_vfs_cache_pressure, 100);
+}
 #endif  /* __LINUX_DCACHE_H */