From 4e717f5c1083995c334ced639cc77a75e9972567 Mon Sep 17 00:00:00 2001
From: Glauber Costa <glommer@gmail.com>
Date: Wed, 28 Aug 2013 10:18:03 +1000
Subject: [PATCH] list_lru: remove special case function list_lru_dispose_all.
The list_lru implementation has one function, list_lru_dispose_all, with
only one user (the dentry code). At first, such function appears to make
sense because we are really not interested in the result of isolating each
dentry separately - all of them are going away anyway. However, it's
implementation is buggy in the following way:
When we call list_lru_dispose_all in fs/dcache.c, we scan all dentries
marking them with DCACHE_SHRINK_LIST. However, this is done without the
nlru->lock taken. The imediate result of that is that someone else may
add or remove the dentry from the LRU at the same time. When list_lru_del
happens in that scenario we will see an element that is not yet marked
with DCACHE_SHRINK_LIST (even though it will be in the future) and
obviously remove it from an lru where the element no longer is. Since
list_lru_dispose_all will in effect count down nlru's nr_items and
list_lru_del will do the same, this will lead to an imbalance.
The solution for this would not be so simple: we can obviously just keep
the lru_lock taken, but then we have no guarantees that we will be able to
acquire the dentry lock (dentry->d_lock). To properly solve this, we need
a communication mechanism between the lru and dentry code, so they can
coordinate this with each other.
Such mechanism already exists in the form of the list_lru_walk_cb
callback. So it is possible to construct a dcache-side prune function
that does the right thing only by calling list_lru_walk in a loop until no
more dentries are available.
With only one user, plus the fact that a sane solution for the problem
would involve boucing between dcache and list_lru anyway, I see little
justification to keep the special case list_lru_dispose_all in tree.
Signed-off-by: Glauber Costa <glommer@openvz.org>
Cc: Michal Hocko <mhocko@suse.cz>
Acked-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
---
fs/dcache.c | 49 ++++++++++++++++++++++++----------------
include/linux/list_lru.h | 17 --------------
mm/list_lru.c | 42 ----------------------------------
3 files changed, 29 insertions(+), 79 deletions(-)
diff --git a/fs/dcache.c b/fs/dcache.c
index 38a4a03499a28..d74b5bdff7f95 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -956,27 +956,29 @@ long prune_dcache_sb(struct super_block *sb, unsigned long nr_to_scan)
return freed;
}
-/*
- * Mark all the dentries as on being the dispose list so we don't think they are
- * still on the LRU if we try to kill them from ascending the parent chain in
- * try_prune_one_dentry() rather than directly from the dispose list.
- */
-static void
-shrink_dcache_list(
- struct list_head *dispose)
+static enum lru_status dentry_lru_isolate_shrink(struct list_head *item,
+ spinlock_t *lru_lock, void *arg)
{
- struct dentry *dentry;
+ struct list_head *freeable = arg;
+ struct dentry *dentry = container_of(item, struct dentry, d_lru);
- rcu_read_lock();
- list_for_each_entry_rcu(dentry, dispose, d_lru) {
- spin_lock(&dentry->d_lock);
- dentry->d_flags |= DCACHE_SHRINK_LIST;
- spin_unlock(&dentry->d_lock);
- }
- rcu_read_unlock();
- shrink_dentry_list(dispose);
+ /*
+ * we are inverting the lru lock/dentry->d_lock here,
+ * so use a trylock. If we fail to get the lock, just skip
+ * it
+ */
+ if (!spin_trylock(&dentry->d_lock))
+ return LRU_SKIP;
+
+ dentry->d_flags |= DCACHE_SHRINK_LIST;
+ list_move_tail(&dentry->d_lru, freeable);
+ this_cpu_dec(nr_dentry_unused);
+ spin_unlock(&dentry->d_lock);
+
+ return LRU_REMOVED;
}
+
/**
* shrink_dcache_sb - shrink dcache for a superblock
* @sb: superblock
@@ -986,10 +988,17 @@ shrink_dcache_list(
*/
void shrink_dcache_sb(struct super_block *sb)
{
- long disposed;
+ long freed;
+
+ do {
+ LIST_HEAD(dispose);
+
+ freed = list_lru_walk(&sb->s_dentry_lru,
+ dentry_lru_isolate_shrink, &dispose, UINT_MAX);
- disposed = list_lru_dispose_all(&sb->s_dentry_lru, shrink_dcache_list);
- this_cpu_sub(nr_dentry_unused, disposed);
+ this_cpu_sub(nr_dentry_unused, freed);
+ shrink_dentry_list(&dispose);
+ } while (freed > 0);
}
EXPORT_SYMBOL(shrink_dcache_sb);
diff --git a/include/linux/list_lru.h b/include/linux/list_lru.h
index 2fe13e1a809aa..4d02ad3badab5 100644
--- a/include/linux/list_lru.h
+++ b/include/linux/list_lru.h
@@ -137,21 +137,4 @@ list_lru_walk(struct list_lru *lru, list_lru_walk_cb isolate,
}
return isolated;
}
-
-typedef void (*list_lru_dispose_cb)(struct list_head *dispose_list);
-/**
- * list_lru_dispose_all: forceably flush all elements in an @lru
- * @lru: the lru pointer
- * @dispose: callback function to be called for each lru list.
- *
- * This function will forceably isolate all elements into the dispose list, and
- * call the @dispose callback to flush the list. Please note that the callback
- * should expect items in any state, clean or dirty, and be able to flush all of
- * them.
- *
- * Return value: how many objects were freed. It should be equal to all objects
- * in the list_lru.
- */
-unsigned long
-list_lru_dispose_all(struct list_lru *lru, list_lru_dispose_cb dispose);
#endif /* _LRU_LIST_H */
diff --git a/mm/list_lru.c b/mm/list_lru.c
index 86cb55464f71a..f91c24188573a 100644
--- a/mm/list_lru.c
+++ b/mm/list_lru.c
@@ -112,48 +112,6 @@ list_lru_walk_node(struct list_lru *lru, int nid, list_lru_walk_cb isolate,
}
EXPORT_SYMBOL_GPL(list_lru_walk_node);
-static unsigned long list_lru_dispose_all_node(struct list_lru *lru, int nid,
- list_lru_dispose_cb dispose)
-{
- struct list_lru_node *nlru = &lru->node[nid];
- LIST_HEAD(dispose_list);
- unsigned long disposed = 0;
-
- spin_lock(&nlru->lock);
- while (!list_empty(&nlru->list)) {
- list_splice_init(&nlru->list, &dispose_list);
- disposed += nlru->nr_items;
- nlru->nr_items = 0;
- node_clear(nid, lru->active_nodes);
- spin_unlock(&nlru->lock);
-
- dispose(&dispose_list);
-
- spin_lock(&nlru->lock);
- }
- spin_unlock(&nlru->lock);
- return disposed;
-}
-
-unsigned long list_lru_dispose_all(struct list_lru *lru,
- list_lru_dispose_cb dispose)
-{
- unsigned long disposed;
- unsigned long total = 0;
- int nid;
-
- do {
- disposed = 0;
- for_each_node_mask(nid, lru->active_nodes) {
- disposed += list_lru_dispose_all_node(lru, nid,
- dispose);
- }
- total += disposed;
- } while (disposed != 0);
-
- return total;
-}
-
int list_lru_init(struct list_lru *lru)
{
int i;
--
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