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Re: [Q] missing unused dentry in prune_dcache()? [message #7850 is a reply to message #7844] Fri, 27 October 2006 10:42 Go to previous messageGo to previous message
David Howells is currently offline  David Howells
Messages: 44
Registered: October 2006
Member
Vasily Averin <vvs@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);
}

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