percpu_counter: extend _limited_add() to negative amounts

Though tmpfs does not need it, percpu_counter_limited_add() can be twice
as useful if it works sensibly with negative amounts (subs) - typically
decrements towards a limit of 0 or nearby: as suggested by Dave Chinner.

And in the course of that reworking, skip the percpu counter sum if it is
already obvious that the limit would be passed: as suggested by Tim Chen.

Extend the comment above __percpu_counter_limited_add(), defining the
behaviour with positive and negative amounts, allowing negative limits,
but not bothering about overflow beyond S64_MAX.

Link: https://lkml.kernel.org/r/8f86083b-c452-95d4-365b-f16a2e4ebcd4@google.comSigned-off-by: Hugh Dickins <hughd@google.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Carlos Maiolino <cem@kernel.org>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Chuck Lever <chuck.lever@oracle.com>
Cc: Darrick J. Wong <djwong@kernel.org>
Cc: Dave Chinner <dchinner@redhat.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Tim Chen <tim.c.chen@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/percpu_counter.h

@@ -198,14 +198,21 @@ static inline bool
 percpu_counter_limited_add(struct percpu_counter *fbc, s64 limit, s64 amount)
 {
 	unsigned long flags;
+	bool good = false;
 	s64 count;
 
+	if (amount == 0)
+		return true;
+
 	local_irq_save(flags);
 	count = fbc->count + amount;
-	if (count <= limit)
+	if ((amount > 0 && count <= limit) ||
+	    (amount < 0 && count >= limit)) {
 		fbc->count = count;
+		good = true;
+	}
 	local_irq_restore(flags);
-	return count <= limit;
+	return good;
 }
 
 /* non-SMP percpu_counter_add_local is the same with percpu_counter_add */

lib/percpu_counter.c

@@ -279,8 +279,16 @@ 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.
+ * Compare counter, and add amount if total is: less than or equal to limit if
+ * amount is positive, or greater than or equal to limit if amount is negative.
+ * Return true if amount is added, or false if total would be beyond the limit.
+ *
+ * Negative limit is allowed, but unusual.
+ * When negative amounts (subs) are given to percpu_counter_limited_add(),
+ * the limit would most naturally be 0 - but other limits are also allowed.
+ *
+ * Overflow beyond S64_MAX is not allowed for: counter, limit and amount
+ * are all assumed to be sane (far from S64_MIN and S64_MAX).
  */
 bool __percpu_counter_limited_add(struct percpu_counter *fbc,
 				  s64 limit, s64 amount, s32 batch)
@@ -288,10 +296,10 @@ bool __percpu_counter_limited_add(struct percpu_counter *fbc,
 	s64 count;
 	s64 unknown;
 	unsigned long flags;
-	bool good;
+	bool good = false;
 
-	if (amount > limit)
-		return false;
+	if (amount == 0)
+		return true;
 
 	local_irq_save(flags);
 	unknown = batch * num_online_cpus();
@@ -299,7 +307,8 @@ bool __percpu_counter_limited_add(struct percpu_counter *fbc,
 
 	/* Skip taking the lock when safe */
 	if (abs(count + amount) <= batch &&
-	    fbc->count + unknown <= limit) {
+	    ((amount > 0 && fbc->count + unknown <= limit) ||
+	     (amount < 0 && fbc->count - unknown >= limit))) {
 		this_cpu_add(*fbc->counters, amount);
 		local_irq_restore(flags);
 		return true;
@@ -309,7 +318,19 @@ bool __percpu_counter_limited_add(struct percpu_counter *fbc,
 	count = fbc->count + amount;
 
 	/* Skip percpu_counter_sum() when safe */
-	if (count + unknown > limit) {
+	if (amount > 0) {
+		if (count - unknown > limit)
+			goto out;
+		if (count + unknown <= limit)
+			good = true;
+	} else {
+		if (count + unknown < limit)
+			goto out;
+		if (count - unknown >= limit)
+			good = true;
+	}
+
+	if (!good) {
 		s32 *pcount;
 		int cpu;
 
@@ -317,15 +338,20 @@ bool __percpu_counter_limited_add(struct percpu_counter *fbc,
 			pcount = per_cpu_ptr(fbc->counters, cpu);
 			count += *pcount;
 		}
+		if (amount > 0) {
+			if (count > limit)
+				goto out;
+		} else {
+			if (count < limit)
+				goto out;
+		}
+		good = true;
 	}
 
-	good = count <= limit;
-	if (good) {
-		count = __this_cpu_read(*fbc->counters);
-		fbc->count += count + amount;
-		__this_cpu_sub(*fbc->counters, count);
-	}
-
+	count = __this_cpu_read(*fbc->counters);
+	fbc->count += count + amount;
+	__this_cpu_sub(*fbc->counters, count);
+out:
 	raw_spin_unlock(&fbc->lock);
 	local_irq_restore(flags);
 	return good;