[PATCH 3/3] perf: builtin-record: Document and check that mmap_pages must be a power of two.

[Nelson Elhage <nelhage@nelhage.com>]

Now that we automatically point users at it, let's provide them some
guidance so that they hopefully don't just get mysterious EINVAL's
from the kernel.

Signed-off-by: Nelson Elhage <nelhage@nelhage.com>
---
 tools/perf/Documentation/perf-record.txt |    2 +-
 tools/perf/builtin-record.c              |    3 +++
 tools/perf/util/util.h                   |   11 +++++++++++
 3 files changed, 15 insertions(+), 1 deletions(-)

diff --git a/tools/perf/Documentation/perf-record.txt b/tools/perf/Documentation/perf-record.txt
index 5a520f8..2937f7e 100644
--- a/tools/perf/Documentation/perf-record.txt
+++ b/tools/perf/Documentation/perf-record.txt
@@ -89,7 +89,7 @@ OPTIONS
 
 -m::
 --mmap-pages=::
-	Number of mmap data pages.
+	Number of mmap data pages. Must be a power of two.
 
 -g::
 --call-graph::
diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
index 45b23c1..275c78d 100644
--- a/tools/perf/builtin-record.c
+++ b/tools/perf/builtin-record.c
@@ -560,6 +560,9 @@ static int __cmd_record(int argc, const char **argv)
 	if (mmap_pages == UINT_MAX)
 		mmap_pages = (512 * 1024) / page_size;
 
+	if (!is_power_of_2(mmap_pages))
+		die("--mmap_pages/-m value must be a power of two.");
+
 	if (forks) {
 		child_pid = fork();
 		if (child_pid < 0) {
diff --git a/tools/perf/util/util.h b/tools/perf/util/util.h
index 0128906..37be34d 100644
--- a/tools/perf/util/util.h
+++ b/tools/perf/util/util.h
@@ -245,4 +245,15 @@ int readn(int fd, void *buf, size_t size);
 #define _STR(x) #x
 #define STR(x) _STR(x)
 
+/*
+ *  Determine whether some value is a power of two, where zero is
+ * *not* considered a power of two.
+ */
+
+static inline __attribute__((const))
+bool is_power_of_2(unsigned long n)
+{
+	return (n != 0 && ((n & (n - 1)) == 0));
+}
+
 #endif
-- 
1.7.4.41.g50b8f