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.com
Signed-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>

1 files changed, 9 insertions, 2 deletions

diff --git a/include/linux/percpu_counter.h b/include/linux/percpu_counter.h
index 8cb7c071bd5c..3a44dd1e33d2 100644
--- a/include/linux/percpu_counter.h
+++ b/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 */