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Subject[patch 02/22] fix quadratic behavior of shrink_dcache_parent()
From: Miklos Szeredi <mszeredi@suse.cz>

Changes:
o dput already checks dentry == NULL, so remove check
from prune_one_dentry()


The time shrink_dcache_parent() takes, grows quadratically with the
depth of the tree under 'parent'. This starts to get noticable at
about 10,000.

These kinds of depths don't occur normally, and filesystems which
invoke shrink_dcache_parent() via d_invalidate() seem to have other
depth dependent timings, so it's not even easy to expose this problem.

However with FUSE it's easy to create a deep tree and d_invalidate()
will also get called. This can make a syscall hang for a very long
time.

This is the original discovery of the problem by Russ Cox:

http://article.gmane.org/gmane.comp.file-systems.fuse.devel/3826

The following patch fixes the quadratic behavior, by optionally
allowing prune_dcache() to prune ancestors of a dentry in one go,
instead of doing it one at a time.

Common code in dput() and prune_one_dentry() is extracted into a new
helper function d_kill().

shrink_dcache_parent() as well as shrink_dcache_sb() are converted to
use the ancestry-pruner option. Only for shrink_dcache_memory() is
this behavior not desirable, so it keeps using the old algorithm.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
---
Index: linux/fs/dcache.c
===================================================================
--- linux.orig/fs/dcache.c 2007-02-27 14:40:56.000000000 +0100
+++ linux/fs/dcache.c 2007-02-27 14:41:07.000000000 +0100
@@ -121,6 +121,28 @@ static void dentry_iput(struct dentry *
}
}

+/**
+ * d_kill - kill dentry and return parent
+ * @dentry: dentry to kill
+ *
+ * Called with dcache_lock and d_lock, releases both. The dentry must
+ * already be unhashed and removed from the LRU.
+ *
+ * If this is the root of the dentry tree, return NULL.
+ */
+static struct dentry *d_kill(struct dentry *dentry)
+{
+ struct dentry *parent;
+
+ list_del(&dentry->d_u.d_child);
+ dentry_stat.nr_dentry--; /* For d_free, below */
+ /*drops the locks, at that point nobody can reach this dentry */
+ dentry_iput(dentry);
+ parent = dentry->d_parent;
+ d_free(dentry);
+ return dentry == parent ? NULL : parent;
+}
+
/*
* This is dput
*
@@ -189,28 +211,17 @@ repeat:

unhash_it:
__d_drop(dentry);
-
-kill_it: {
- struct dentry *parent;
-
- /* If dentry was on d_lru list
- * delete it from there
- */
- if (!list_empty(&dentry->d_lru)) {
- list_del(&dentry->d_lru);
- dentry_stat.nr_unused--;
- }
- list_del(&dentry->d_u.d_child);
- dentry_stat.nr_dentry--; /* For d_free, below */
- /*drops the locks, at that point nobody can reach this dentry */
- dentry_iput(dentry);
- parent = dentry->d_parent;
- d_free(dentry);
- if (dentry == parent)
- return;
- dentry = parent;
- goto repeat;
+kill_it:
+ /* If dentry was on d_lru list
+ * delete it from there
+ */
+ if (!list_empty(&dentry->d_lru)) {
+ list_del(&dentry->d_lru);
+ dentry_stat.nr_unused--;
}
+ dentry = d_kill(dentry);
+ if (dentry)
+ goto repeat;
}

/**
@@ -371,22 +382,40 @@ restart:
* Throw away a dentry - free the inode, dput the parent. This requires that
* the LRU list has already been removed.
*
+ * If prune_parents is true, try to prune ancestors as well.
+ *
* Called with dcache_lock, drops it and then regains.
* Called with dentry->d_lock held, drops it.
*/
-static void prune_one_dentry(struct dentry * dentry)
+static void prune_one_dentry(struct dentry * dentry, int prune_parents)
{
- struct dentry * parent;
-
__d_drop(dentry);
- list_del(&dentry->d_u.d_child);
- dentry_stat.nr_dentry--; /* For d_free, below */
- dentry_iput(dentry);
- parent = dentry->d_parent;
- d_free(dentry);
- if (parent != dentry)
- dput(parent);
+ dentry = d_kill(dentry);
+ if (!prune_parents) {
+ dput(dentry);
+ spin_lock(&dcache_lock);
+ return;
+ }
+
+ /*
+ * Prune ancestors. Locking is simpler than in dput(),
+ * because dcache_lock needs to be taken anyway.
+ */
spin_lock(&dcache_lock);
+ while (dentry) {
+ if (!atomic_dec_and_lock(&dentry->d_count, &dentry->d_lock))
+ return;
+
+ if (dentry->d_op && dentry->d_op->d_delete)
+ dentry->d_op->d_delete(dentry);
+ if (!list_empty(&dentry->d_lru)) {
+ list_del(&dentry->d_lru);
+ dentry_stat.nr_unused--;
+ }
+ __d_drop(dentry);
+ dentry = d_kill(dentry);
+ spin_lock(&dcache_lock);
+ }
}

/**
@@ -394,6 +423,7 @@ static void prune_one_dentry(struct dent
* @count: number of entries to try and free
* @sb: if given, ignore dentries for other superblocks
* which are being unmounted.
+ * @prune_parents: if true, try to prune ancestors as well in one go
*
* Shrink the dcache. This is done when we need
* more memory, or simply when we need to unmount
@@ -404,7 +434,7 @@ static void prune_one_dentry(struct dent
* all the dentries are in use.
*/

-static void prune_dcache(int count, struct super_block *sb)
+static void prune_dcache(int count, struct super_block *sb, int prune_parents)
{
spin_lock(&dcache_lock);
for (; count ; count--) {
@@ -464,7 +494,7 @@ static void prune_dcache(int count, stru
* without taking the s_umount lock (I already hold it).
*/
if (sb && dentry->d_sb == sb) {
- prune_one_dentry(dentry);
+ prune_one_dentry(dentry, prune_parents);
continue;
}
/*
@@ -479,7 +509,7 @@ static void prune_dcache(int count, stru
s_umount = &dentry->d_sb->s_umount;
if (down_read_trylock(s_umount)) {
if (dentry->d_sb->s_root != NULL) {
- prune_one_dentry(dentry);
+ prune_one_dentry(dentry, prune_parents);
up_read(s_umount);
continue;
}
@@ -550,7 +580,7 @@ repeat:
spin_unlock(&dentry->d_lock);
continue;
}
- prune_one_dentry(dentry);
+ prune_one_dentry(dentry, 1);
cond_resched_lock(&dcache_lock);
goto repeat;
}
@@ -829,7 +859,7 @@ void shrink_dcache_parent(struct dentry
int found;

while ((found = select_parent(parent)) != 0)
- prune_dcache(found, parent->d_sb);
+ prune_dcache(found, parent->d_sb, 1);
}

/*
@@ -849,7 +879,7 @@ static int shrink_dcache_memory(int nr,
if (nr) {
if (!(gfp_mask & __GFP_FS))
return -1;
- prune_dcache(nr, NULL);
+ prune_dcache(nr, NULL, 0);
}
return (dentry_stat.nr_unused / 100) * sysctl_vfs_cache_pressure;
}
--
-
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