Re: [PATCH 1/1] mm: Prevent dereferencing non-allocated per_cpu variables

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 27 Nov 2007 23:16:28 +0100
Andi Kleen <andi@firstfloor.org> wrote:

> On Tue, Nov 27, 2007 at 01:50:53PM -0800, travis@sgi.com wrote:
> > Change loops controlled by 'for (i = 0; i < NR_CPUS; i++)' to use
> > 'for_each_possible_cpu(i)' when there's a _remote possibility_ of
> > dereferencing a non-allocated per_cpu variable involved.
> > 
> > All files except mm/vmstat.c are x86 arch.
> > 
> > Based on 2.6.24-rc3-mm1 .
> > 
> > Thanks to pageexec@freemail.hu for pointing this out.
> 
> Looks good to me. 2.6.24 candidate.

hm.  Has anyone any evidence that we're actually touching
not-possible-cpu's memory here?

Also, the sum_vm_events() change looks buggy - it assumes that
cpu_possible_map has no gaps in it.  But that change is unneeded because
sum_vm_events() is only ever passed cpu_online_map and I'm hoping that we
don't usually online not-possible CPUs.

--- a/mm/vmstat.c~mm-prevent-dereferencing-non-allocated-per_cpu-variables-fix
+++ a/mm/vmstat.c
@@ -27,12 +27,12 @@ static void sum_vm_events(unsigned long 
 	memset(ret, 0, NR_VM_EVENT_ITEMS * sizeof(unsigned long));
 
 	cpu = first_cpu(*cpumask);
-	while (cpu < NR_CPUS && cpu_possible(cpu)) {
+	while (cpu < NR_CPUS) {
 		struct vm_event_state *this = &per_cpu(vm_event_states, cpu);
 
 		cpu = next_cpu(cpu, *cpumask);
 
-		if (cpu < NR_CPUS && cpu_possible(cpu))
+		if (cpu < NR_CPUS)
 			prefetch(&per_cpu(vm_event_states, cpu));
 
 
_