[Devel] Re: [Q] missing unused dentry in prune_dcache()?

[David Howells]

Vasily Averin <vvs at sw.ru> wrote:

> Therefore I believe that my patch is optimal solution.

I'm not sure that prune_dcache() is particularly optimal.  If we're looking to
prune for a specific superblock, it may scan most of the dentry_unused list
several times, once for each dentry it eliminates.

Imagine the list with a million dentries on it.  Imagine further that all the
dentries you're trying to eliminate are up near the head end: you're going to
have to scan most of the list several times unnecessarily; if you're asked to
kill 128 dentries, you might wind up examining on the order of 100,000,000
dentries, 99% of which you scan 128 times.

I wonder if this could be improved by making the assumption that there won't be
any entries inserted tailwards of where we've just looked.  The problem is that
if dcache_lock is dropped, we've no way of keeping track of the current
position without inserting a marker into the list.

Now we could do the marker thing quite easily.  We'd have to insert a dummy
dcache entry, probably with d_sb pointing to some special location that is
recognised as saying "that dentry is a marker".

We could do something like the attached patch, for example.  Note that the
patch compiles, but I haven't tested it.  It also uses a big chunk of stack
space for the marker.  It ought to be safe enough with respect to the other
functions that touch that list - all of those deal with specific dentries or
look for dentries by superblock.

David
---
diff --git a/fs/dcache.c b/fs/dcache.c
index eab1bf4..a1cae74 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -395,18 +395,27 @@ static void prune_one_dentry(struct dent
  * This function may fail to free any resources if
  * all the dentries are in use.
  */
- 
+
 static void prune_dcache(int count, struct super_block *sb)
 {
+	struct dentry marker = {
+		.d_sb	= (struct super_block *) &prune_dcache,
+	};
+
+	struct list_head *tmp;
+
 	spin_lock(&dcache_lock);
+	list_add_tail(&marker.d_lru, &dentry_unused);
+
 	for (; count ; count--) {
 		struct dentry *dentry;
-		struct list_head *tmp;
 		struct rw_semaphore *s_umount;
 
 		cond_resched_lock(&dcache_lock);
 
-		tmp = dentry_unused.prev;
+		tmp = marker.d_lru.prev;
+		list_del_init(&marker.d_lru);
+
 		if (sb) {
 			/* Try to find a dentry for this sb, but don't try
 			 * too hard, if they aren't near the tail they will
@@ -418,9 +427,18 @@ static void prune_dcache(int count, stru
 				skip--;
 				tmp = tmp->prev;
 			}
+		} else {
+			/* We may not be the only pruner */
+			while (tmp != &dentry_unused) {
+				dentry = list_entry(tmp, struct dentry, d_lru);
+				if (dentry->d_sb !=
+				    (struct super_block *) &prune_dcache)
+					break;
+			}
 		}
 		if (tmp == &dentry_unused)
 			break;
+		list_add(&marker.d_lru, tmp);
 		list_del_init(tmp);
 		prefetch(dentry_unused.prev);
  		dentry_stat.nr_unused--;
@@ -439,7 +457,7 @@ static void prune_dcache(int count, stru
 		/* If the dentry was recently referenced, don't free it. */
 		if (dentry->d_flags & DCACHE_REFERENCED) {
 			dentry->d_flags &= ~DCACHE_REFERENCED;
- 			list_add(&dentry->d_lru, &dentry_unused);
+ 			list_add(&dentry->d_lru, &marker.d_lru);
  			dentry_stat.nr_unused++;
  			spin_unlock(&dentry->d_lock);
 			continue;
@@ -478,12 +496,10 @@ static void prune_dcache(int count, stru
 			up_read(s_umount);
 		}
 		spin_unlock(&dentry->d_lock);
-		/* Cannot remove the first dentry, and it isn't appropriate
-		 * to move it to the head of the list, so give up, and try
-		 * later
-		 */
-		break;
+		list_add(&dentry->d_lru, &marker.d_lru);
+		dentry_stat.nr_unused++;
 	}
+	list_del(&marker.d_lru);
 	spin_unlock(&dcache_lock);
 }