[PATCH 3.18 096/108] perf/core: Fix impossible ring-buffer sizes warning

[Greg Kroah-Hartman]

3.18-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Ingo Molnar <mingo@xxxxxxxxxx>

commit 528871b456026e6127d95b1b2bd8e3a003dc1614 upstream.

The following commit:

  9dff0aa95a32 ("perf/core: Don't WARN() for impossible ring-buffer sizes")

results in perf recording failures with larger mmap areas:

  root@skl:/tmp# perf record -g -a
  failed to mmap with 12 (Cannot allocate memory)

The root cause is that the following condition is buggy:

	if (order_base_2(size) >= MAX_ORDER)
		goto fail;

The problem is that @size is in bytes and MAX_ORDER is in pages,
so the right test is:

	if (order_base_2(size) >= PAGE_SHIFT+MAX_ORDER)
		goto fail;

Fix it.

Reported-by: "Jin, Yao" <yao.jin@xxxxxxxxxxxxxxx>
Bisected-by: Borislav Petkov <bp@xxxxxxxxx>
Analyzed-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Julien Thierry <julien.thierry@xxxxxxx>
Cc: Mark Rutland <mark.rutland@xxxxxxx>
Cc: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Namhyung Kim <namhyung@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Fixes: 9dff0aa95a32 ("perf/core: Don't WARN() for impossible ring-buffer sizes")
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 kernel/events/ring_buffer.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/kernel/events/ring_buffer.c
+++ b/kernel/events/ring_buffer.c
@@ -282,7 +282,7 @@ struct ring_buffer *rb_alloc(int nr_page
 	size = sizeof(struct ring_buffer);
 	size += nr_pages * sizeof(void *);
 
-	if (order_base_2(size) >= MAX_ORDER)
+	if (order_base_2(size) >= PAGE_SHIFT+MAX_ORDER)
 		goto fail;
 
 	rb = kzalloc(size, GFP_KERNEL);