3 files changed, 81 insertions, 5 deletions

diff --git a/include/linux/percpu_counter.h b/include/linux/percpu_counter.h
index d01351b1526f..8cb7c071bd5c 100644
--- a/include/linux/percpu_counter.h
+++ b/include/linux/percpu_counter.h
@@ -57,6 +57,8 @@ void percpu_counter_add_batch(struct percpu_counter *fbc, s64 amount,
 			      s32 batch);
 s64 __percpu_counter_sum(struct percpu_counter *fbc);
 int __percpu_counter_compare(struct percpu_counter *fbc, s64 rhs, s32 batch);
+bool __percpu_counter_limited_add(struct percpu_counter *fbc, s64 limit,
+				  s64 amount, s32 batch);
 void percpu_counter_sync(struct percpu_counter *fbc);
 
 static inline int percpu_counter_compare(struct percpu_counter *fbc, s64 rhs)
@@ -69,6 +71,13 @@ static inline void percpu_counter_add(struct percpu_counter *fbc, s64 amount)
 	percpu_counter_add_batch(fbc, amount, percpu_counter_batch);
 }
 
+static inline bool
+percpu_counter_limited_add(struct percpu_counter *fbc, s64 limit, s64 amount)
+{
+	return __percpu_counter_limited_add(fbc, limit, amount,
+					    percpu_counter_batch);
+}
+
 /*
  * With percpu_counter_add_local() and percpu_counter_sub_local(), counts
  * are accumulated in local per cpu counter and not in fbc->count until
@@ -185,6 +194,20 @@ percpu_counter_add(struct percpu_counter *fbc, s64 amount)
 	local_irq_restore(flags);
 }
 
+static inline bool
+percpu_counter_limited_add(struct percpu_counter *fbc, s64 limit, s64 amount)
+{
+	unsigned long flags;
+	s64 count;
+
+	local_irq_save(flags);
+	count = fbc->count + amount;
+	if (count <= limit)
+		fbc->count = count;
+	local_irq_restore(flags);
+	return count <= limit;
+}
+
 /* non-SMP percpu_counter_add_local is the same with percpu_counter_add */
 static inline void
 percpu_counter_add_local(struct percpu_counter *fbc, s64 amount)
diff --git a/lib/percpu_counter.c b/lib/percpu_counter.c
index 9073430dc865..58a3392f471b 100644
--- a/lib/percpu_counter.c
+++ b/lib/percpu_counter.c
@@ -278,6 +278,59 @@ int __percpu_counter_compare(struct percpu_counter *fbc, s64 rhs, s32 batch)
 }
 EXPORT_SYMBOL(__percpu_counter_compare);
 
+/*
+ * Compare counter, and add amount if the total is within limit.
+ * Return true if amount was added, false if it would exceed limit.
+ */
+bool __percpu_counter_limited_add(struct percpu_counter *fbc,
+				  s64 limit, s64 amount, s32 batch)
+{
+	s64 count;
+	s64 unknown;
+	unsigned long flags;
+	bool good;
+
+	if (amount > limit)
+		return false;
+
+	local_irq_save(flags);
+	unknown = batch * num_online_cpus();
+	count = __this_cpu_read(*fbc->counters);
+
+	/* Skip taking the lock when safe */
+	if (abs(count + amount) <= batch &&
+	    fbc->count + unknown <= limit) {
+		this_cpu_add(*fbc->counters, amount);
+		local_irq_restore(flags);
+		return true;
+	}
+
+	raw_spin_lock(&fbc->lock);
+	count = fbc->count + amount;
+
+	/* Skip percpu_counter_sum() when safe */
+	if (count + unknown > limit) {
+		s32 *pcount;
+		int cpu;
+
+		for_each_cpu_or(cpu, cpu_online_mask, cpu_dying_mask) {
+			pcount = per_cpu_ptr(fbc->counters, cpu);
+			count += *pcount;
+		}
+	}
+
+	good = count <= limit;
+	if (good) {
+		count = __this_cpu_read(*fbc->counters);
+		fbc->count += count + amount;
+		__this_cpu_sub(*fbc->counters, count);
+	}
+
+	raw_spin_unlock(&fbc->lock);
+	local_irq_restore(flags);
+	return good;
+}
+
 static int __init percpu_counter_startup(void)
 {
 	int ret;
diff --git a/mm/shmem.c b/mm/shmem.c
index 269cd3c1110f..61b170324e5c 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -217,15 +217,15 @@ static int shmem_inode_acct_blocks(struct inode *inode, long pages)
 
 	might_sleep();	/* when quotas */
 	if (sbinfo->max_blocks) {
-		if (percpu_counter_compare(&sbinfo->used_blocks,
-					   sbinfo->max_blocks - pages) > 0)
+		if (!percpu_counter_limited_add(&sbinfo->used_blocks,
+						sbinfo->max_blocks, pages))
 			goto unacct;
 
 		err = dquot_alloc_block_nodirty(inode, pages);
-		if (err)
+		if (err) {
+			percpu_counter_sub(&sbinfo->used_blocks, pages);
 			goto unacct;
-
-		percpu_counter_add(&sbinfo->used_blocks, pages);
+		}
 	} else {
 		err = dquot_alloc_block_nodirty(inode, pages);
 		if (err)