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[PATCH 2.6.19-rc3] VFS: per-sb dentry lru list [message #7860 is a reply to message #7852] Fri, 27 October 2006 14:05 Go to previous messageGo to previous message
vaverin is currently offline  vaverin
Messages: 708
Registered: September 2005
Senior Member
From: Vasily Averin <vvs@sw.ru>

Virtuozzo/OpenVZ linux kernel team has discovered that umount/remount can last
for hours looping in shrink_dcache_sb() without much successes. Since during
shrinking s_umount semaphore is taken lots of other unrelated operations like
sync can stop working until shrink finished.

It happens due to very long unused dentries list (>1 million of dentries),
so that it takes shrink_dcache_sb() longer then 1/HZ seconds to get to the
required dentry and after freeing this single dentry it reschedules and restarts.

The proposed fix prevents this issue by using per-sb dentry LRU list. It
provides very quickly search for the unused dentries for given super block thus
forcing shrinking always making good progress.

It was well tested on 2.6.9-based Virtuozzo/OpenVZ kernels, but the port to the
latest mainstream kernel is not tested.

Signed-off-by: Kirill Korotaev <dev@openvz.org>
Signed-off-by: Vasily Averin <vvs@sw.ru>

--- linux-2.6.19-rc3/fs/dcache.c.shrsb 2006-10-27 10:45:11.000000000 +0400
+++ linux-2.6.19-rc3/fs/dcache.c 2006-10-27 15:49:54.000000000 +0400
@@ -173,6 +173,7 @@ repeat:
if (list_empty(&dentry->d_lru)) {
dentry->d_flags |= DCACHE_REFERENCED;
list_add(&dentry->d_lru, &dentry_unused);
+ list_add(&dentry->d_sb_lru, &dentry->d_sb->s_dentry_unused);
dentry_stat.nr_unused++;
}
spin_unlock(&dentry->d_lock);
@@ -190,6 +191,7 @@ kill_it: {
*/
if (!list_empty(&dentry->d_lru)) {
list_del(&dentry->d_lru);
+ list_del(&dentry->d_sb_lru);
dentry_stat.nr_unused--;
}
list_del(&dentry->d_u.d_child);
@@ -270,6 +272,7 @@ static inline struct dentry * __dget_loc
if (!list_empty(&dentry->d_lru)) {
dentry_stat.nr_unused--;
list_del_init(&dentry->d_lru);
+ list_del_init(&dentry->d_sb_lru);
}
return dentry;
}
@@ -398,6 +401,10 @@ static void prune_one_dentry(struct dent

static void prune_dcache(int count, struct super_block *sb)
{
+ struct list_head *lru_head;
+
+ lru_head = sb ? &sb->s_dentry_unused : &dentry_unused;
+
spin_lock(&dcache_lock);
for (; count ; count--) {
struct dentry *dentry;
@@ -406,25 +413,16 @@ static void prune_dcache(int count, stru

cond_resched_lock(&dcache_lock);

- tmp = dentry_unused.prev;
- 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
- * be moved down again soon
- */
- int skip = count;
- while (skip && tmp != &dentry_unused &&
- list_entry(tmp, struct dentry, d_lru)->d_sb != sb) {
- skip--;
- tmp = tmp->prev;
- }
- }
- if (tmp == &dentry_unused)
+ tmp = lru_head->prev;
+ if (tmp == lru_head)
break;
- list_del_init(tmp);
- prefetch(dentry_unused.prev);
+
+ prefetch(lru_head->prev);
dentry_stat.nr_unused--;
- dentry = list_entry(tmp, struct dentry, d_lru);
+ dentry = sb ? list_entry(tmp, struct dentry, d_sb_lru) :
+ list_entry(tmp, struct dentry, d_lru);
+ list_del_init(&dentry->d_lru);
+ list_del_init(&dentry->d_sb_lru);

spin_lock(&dentry->d_lock);
/*
@@ -440,6 +438,8 @@ static void prune_dcache(int count, stru
if (dentry->d_flags & DCACHE_REFERENCED) {
dentry->d_flags &= ~DCACHE_REFERENCED;
list_add(&dentry->d_lru, &dentry_unused);
+ list_add(&dentry->d_sb_lru,
+ &dentry->d_sb->s_dentry_unused);
dentry_stat.nr_unused++;
spin_unlock(&dentry->d_lock);
continue;
@@ -455,7 +455,8 @@ static void prune_dcache(int count, stru
* If this dentry is for "my" filesystem, then I can prune it
* without taking the s_umount lock (I already hold it).
*/
- if (sb && dentry->d_sb == sb) {
+ if (sb) {
+ BUG_ON(dentry->d_sb != sb);
prune_one_dentry(dentry);
continue;
}
@@ -480,6 +481,8 @@ static void prune_dcache(int count, stru
spin_unlock(&dentry->d_lock);
/* Inserting dentry to tail of the list leads to cycle */
list_add(&dentry->d_lru, &dentry_unused);
+ list_add(&dentry->d_sb_lru,
+ &dentry->d_sb->s_dentry_unused);
dentry_stat.nr_unused++;
}
spin_unlock(&dcache_lock);
@@ -509,31 +512,15 @@ static void prune_dcache(int count, stru

void shrink_dcache_sb(struct super_block * sb)
{
- struct list_head *tmp, *next;
- struct dentry *dentry;
-
- /*
- * Pass one ... move the dentries for the specified
- * superblock to the most recent end of the unused list.
- */
spin_lock(&dcache_lock);
- list_for_each_safe(tmp, next, &dentry_unused) {
- dentry = list_entry(tmp, struct dentry, d_lru);
- if (dentry->d_sb != sb)
- continue;
- list_move(tmp, &dentry_unused);
- }
+ while (!list_empty(&sb->s_dentry_unused)) {
+ struct dentry *dentry;

- /*
- * Pass two ... free the dentries for this superblock.
- */
-repeat:
- list_for_each_safe(tmp, next, &dentry_unused) {
- dentry = list_entry(tmp, struct dentry, d_lru);
- if (dentry->d_sb != sb)
- continue;
+ dentry = list_entry((&sb->s_dentry_unused)->next,
+ struct dentry, d_sb_lru);
dentry_stat.nr_unused--;
- list_del_init(tmp);
+ list_del_init(&dentry->d_lru);
+ list_del_init(&dentry->d_sb_lru);
spin_lock(&dentry->d_lock);
if (atomic_read(&dentry->d_count)) {
spin_unlock(&dentry->d_lock);
@@ -541,7 +528,6 @@ repeat:
}
prune_one_dentry(dentry);
cond_resched_lock(&dcache_lock);
- goto repeat;
}
spin_unlock(&dcache_lock);
}
@@ -563,6 +549,7 @@ static void shrink_dcache_for_umount_sub
if (!list_empty(&dentry->d_lru)) {
dentry_stat.nr_unused--;
list_del_init(&dentry->d_lru);
+ list_del_init(&dentry->d_sb_lru);
}
__d_drop(dentry);
spin_unlock(&dcache_lock);
@@ -580,6 +567,7 @@ static void shrink_dcache_for_umount_sub
if (!list_empty(&loop->d_lru)) {
dentry_stat.nr_unused--;
list_del_init(&loop->d_lru);
+ list_del_init(&loop->d_sb_lru);
}

__d_drop(loop);
@@ -766,6 +754,7 @@ resume:
if (!list_empty(&dentry->d_lru)) {
dentry_stat.nr_unused--;
list_del_init(&dentry->d_lru);
+ list_del_init(&dentry->d_sb_lru);
}
/*
* move only zero ref count dentries to the end
@@ -773,6 +762,8 @@ resume:
*/
if (!atomic_read(&dentry->d_count)) {
list_add_tail(&dentry->d_lru, &dentry_unused);
+ list_add_tail(&dentry->d_sb_lru,
+ &dentry->d_sb->s_dentry_unused);
dentry_stat.nr_unused++;
found++;
}
@@ -892,6 +883,7 @@ struct dentry *d_alloc(struct dentry * p
#endif
INIT_HLIST_NODE(&dentry->d_hash);
INIT_LIST_HEAD(&dentry->d_lru);
+ INIT_LIST_HEAD(&dentry->d_sb_lru);
INIT_LIST_HEAD(&dentry->d_subdirs);
INIT_LIST_HEAD(&dentry->d_alias);

--- linux-2.6.19-rc3/fs/super.c.shrsb 2006-10-24 10:29:13.000000000 +0400
+++ linux-2.6.19-rc3/fs/super.c 2006-10-27 15:36:33.000000000 +0400
@@ -69,6 +69,7 @@ static struct super_block *alloc_super(s
INIT_LIST_HEAD(&s->s_io);
INIT_LIST_HEAD(&s->s_files);
INIT_LIST_HEAD(&s->s_instances);
+ INIT_LIST_HEAD(&s->s_dentry_unused);
INIT_HLIST_HEAD(&s->s_anon);
INIT_LIST_HEAD(&s->s_inodes);
init_rwsem(&s->s_umount);
--- linux-2.6.19-rc3/include/linux/dcache.h.shrsb 2006-10-24 10:29:14.000000000
+0400
+++ linux-2.6.19-rc3/include/linux/dcache.h 2006-10-27 15:36:33.000000000 +0400
@@ -94,6 +94,7 @@ struct dentry {
struct qstr d_name;

struct list_head d_lru; /* LRU list */
+ struct list_head d_sb_lru; /* per-sb LRU list */
/*
* d_child and d_rcu can share memory
*/
--- linux-2.6.19-rc3/include/linux/fs.h.shrsb 2006-10-24 10:29:14.000000000 +0400
+++ linux-2.6.19-rc3/include/linux/fs.h 2006-10-27 15:36:33.000000000 +0400
@@ -939,6 +939,7 @@ struct super_block {
struct list_head s_dirty; /* dirty inodes */
struct list_head s_io; /* parked for writeback */
struct hlist_head s_anon; /* anonymous dentries for (nfs) exporting */
+ struct list_head s_dentry_unused;
struct list_head s_files;

struct block_device *s_bdev;

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