[tip:perfcounters/core] perf_counter: allow and require one-page mmap on counting counters

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

Commit-ID:  bb0ad4e22511e7caa715362945ad0766c7cca37e
Gitweb:     http://git.kernel.org/tip/bb0ad4e22511e7caa715362945ad0766c7cca37e
Author:     Peter Zijlstra <a.p.zijlstra@chello.nl>
AuthorDate: Wed, 25 Mar 2009 12:48:31 +0100
Committer:  Ingo Molnar <mingo@elte.hu>
CommitDate: Wed, 25 Mar 2009 13:02:54 +0100

perf_counter: allow and require one-page mmap on counting counters

A brainfart stopped single page mmap()s working. The rest of the code
should be perfectly fine with not having any data pages.

Reported-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1237981712.7972.812.camel@twins>
Signed-off-by: Ingo Molnar <mingo@elte.hu>


---
 kernel/perf_counter.c |    6 +++++-
 1 files changed, 5 insertions(+), 1 deletions(-)

diff --git a/kernel/perf_counter.c b/kernel/perf_counter.c
index f3e1b27..95e0257 100644
--- a/kernel/perf_counter.c
+++ b/kernel/perf_counter.c
@@ -1369,7 +1369,11 @@ static int perf_mmap(struct file *file, struct vm_area_struct *vma)
 	vma_size = vma->vm_end - vma->vm_start;
 	nr_pages = (vma_size / PAGE_SIZE) - 1;
 
-	if (nr_pages == 0 || !is_power_of_2(nr_pages))
+	/*
+	 * If we have data pages ensure they're a power-of-two number, so we
+	 * can do bitmasks instead of modulo.
+	 */
+	if (nr_pages != 0 && !is_power_of_2(nr_pages))
 		return -EINVAL;
 
 	if (vma_size != PAGE_SIZE * (1 + nr_pages))