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>
Orig-LKML-Reference: <1237981712.7972.812.camel@twins>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 5 insertions, 1 deletions

diff --git a/kernel/perf_counter.c b/kernel/perf_counter.c
index f3e1b27bc1b..95e02575546 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))