Commit 4e717f5c authored by Glauber Costa's avatar Glauber Costa Committed by Al Viro
Browse files

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: default avatarGlauber Costa <glommer@openvz.org>
Cc: Michal Hocko <mhocko@suse.cz>
Acked-by: default avatarDave Chinner <dchinner@redhat.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
parent 6a4f496f

fs/dcache.c

+29 −20
Original line number Diff line number Diff line
@@ -956,26 +956,28 @@ long prune_dcache_sb(struct super_block *sb, unsigned long nr_to_scan) 
	return freed;

}



static enum lru_status dentry_lru_isolate_shrink(struct list_head *item,

						spinlock_t *lru_lock, void *arg)

{

	struct list_head *freeable = arg;

	struct dentry	*dentry = container_of(item, struct dentry, d_lru);



	/*

 * 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.

	 * we are inverting the lru lock/dentry->d_lock here,

	 * so use a trylock. If we fail to get the lock, just skip

	 * it

	 */

static void

shrink_dcache_list(

	struct list_head *dispose)

{

	struct dentry *dentry;

	if (!spin_trylock(&dentry->d_lock))

		return LRU_SKIP;



	rcu_read_lock();

	list_for_each_entry_rcu(dentry, dispose, d_lru) {

		spin_lock(&dentry->d_lock);

	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;

}

	rcu_read_unlock();

	shrink_dentry_list(dispose);

}





/**

 * shrink_dcache_sb - shrink dcache for a 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);

include/linux/list_lru.h

+0 −17
Original line number Diff line number Diff line
@@ -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 */

mm/list_lru.c

+0 −42
Original line number Diff line number Diff line
@@ -112,48 +112,6 @@ 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;

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