[PATCH] fs: allow for fs-specific objects to be pruned as part of pruning inodes

[Theodore Ts'o <tytso@mit.edu>]

The VFS's prune_super() function allows for the file system to prune
file-system specific objects.  Ext4 would like to use this to prune
parts of the inode's extent cache.  The object lifetime rules used by
ext4 is somewhat different from the those of the dentry and inode in
the VFS.  Ext4's extent cache objects can be pruned without removing
the inode; however if an inode is pruned, all of the extent cache
objects associated with the inode are immediately removed.

To accomodate this rule, we measure the number of fs-specific objects
before the dentry and inodes are pruned, and then measure them again
afterwards.  If the number of fs-specific objects have decreased, we
credit that decrease as part of the shrink operation, so that we do
not end up removing too many fs-specific objects.

In the case where fs-specific objects are not removed when inodes are
removed, this will not change the behavior of prune_super() in any
appreciable way.  (Currently the only other user of this facility is
XFS, and this change should not affect XFS's usage of this facility
for this reason.)

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: linux-fsdevel@vger.kernel.org
---
 fs/super.c | 22 +++++++++++++++++++---
 1 file changed, 19 insertions(+), 3 deletions(-)

diff --git a/fs/super.c b/fs/super.c
index 12f1237..fb57bd2 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -80,6 +80,7 @@ static int prune_super(struct shrinker *shrink, struct shrink_control *sc)
 	if (sc->nr_to_scan) {
 		int	dentries;
 		int	inodes;
+		int	fs_to_scan = 0;
 
 		/* proportion the scan between the caches */
 		dentries = (sc->nr_to_scan * sb->s_nr_dentry_unused) /
@@ -87,7 +88,7 @@ static int prune_super(struct shrinker *shrink, struct shrink_control *sc)
 		inodes = (sc->nr_to_scan * sb->s_nr_inodes_unused) /
 							total_objects;
 		if (fs_objects)
-			fs_objects = (sc->nr_to_scan * fs_objects) /
+			fs_to_scan = (sc->nr_to_scan * fs_objects) /
 							total_objects;
 		/*
 		 * prune the dcache first as the icache is pinned by it, then
@@ -96,8 +97,23 @@ static int prune_super(struct shrinker *shrink, struct shrink_control *sc)
 		prune_dcache_sb(sb, dentries);
 		prune_icache_sb(sb, inodes);
 
-		if (fs_objects && sb->s_op->free_cached_objects) {
-			sb->s_op->free_cached_objects(sb, fs_objects);
+		/*
+		 * If as a result of pruning the icache, we released some
+		 * of the fs_objects, give credit to the fact and
+		 * reduce the number of fs objects that we should try
+		 * to release.
+		 */
+		if (fs_to_scan) {
+			int fs_objects_now = sb->s_op->nr_cached_objects(sb);
+
+			if (fs_objects_now < fs_objects)
+				fs_to_scan -= fs_objects - fs_objects_now;
+			if (fs_to_scan < 0)
+				fs_to_scan = 0;
+		}
+
+		if (fs_to_scan && sb->s_op->free_cached_objects) {
+			sb->s_op->free_cached_objects(sb, fs_to_scan);
 			fs_objects = sb->s_op->nr_cached_objects(sb);
 		}
 		total_objects = sb->s_nr_dentry_unused +
-- 
1.7.12.rc0.22.gcdd159b