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@xxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Dave Chinner <dchinner@xxxxxxxxxx>
---
Andrew and mhocko: I have noted this while searching for Michal's problem.
Because we are now more or less in agreement that he is seeing an inode and
not a dentry problem, I unfortunately don't believe this is a fix for what
he is seeing - specially given his bisect results. But it is a clear bug
nevertheless, and this one I can even trigger myself by constantly flushing
the dcache for a sb while randomly touching dentries.
---
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 d3feea1..341d633 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -912,27 +912,29 @@ 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
@@ -942,10 +944,17 @@ shrink_dcache_list(
*/
void shrink_dcache_sb(struct super_block *sb)
{
- long disposed;
+ long freed;
- disposed = list_lru_dispose_all(&sb->s_dentry_lru, shrink_dcache_list);
- this_cpu_sub(nr_dentry_unused, disposed);
+ do {
+ LIST_HEAD(dispose);
+
+ freed = list_lru_walk(&sb->s_dentry_lru,
+ dentry_lru_isolate_shrink, &dispose, UINT_MAX);
+
+ 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 ff57503..3ce5417 100644
--- a/include/linux/list_lru.h
+++ b/include/linux/list_lru.h
@@ -128,21 +128,4 @@ 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 700d322..8447a56 100644
--- a/mm/list_lru.c
+++ b/mm/list_lru.c
@@ -118,48 +118,6 @@ restart:
}
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;
--
1.8.2.1
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