Re: [PATCH] perf_counter: allow and require one-page mmap on counting counters

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

On Wed, 2009-03-25 at 19:15 +1100, Paul Mackerras wrote:
> Impact: bug fix
> 
> Currently the mmap code requires that the length of the mmap be at least
> two pages.  That is fine for sampling counters, but for counting
> counters the second and subsequent pages are just wasted, since counting
> counters don't generate events.
> 
> This changes the code to require that the mmap be one page in length
> for counting counters, and applies the existing check to sampling
> counters.

Does the below work for you Paul?

---
Subject: perf_counter: allow one-page mmap on counters
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
Date: Wed Mar 25 12:44:16 CET 2009

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>
---
 kernel/perf_counter.c |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

Index: linux-2.6/kernel/perf_counter.c
===================================================================
--- linux-2.6.orig/kernel/perf_counter.c
+++ linux-2.6/kernel/perf_counter.c
@@ -1369,7 +1369,11 @@ static int perf_mmap(struct file *file, 
 	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))