Re: [PATCH] mm-add-vfree_atomic-fix

[Andy Lutomirski]

On Tue, Dec 13, 2016 at 2:12 AM, Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> [CC Andy]
>
> I've noticed the same
> http://lkml.kernel.org/r/20161209142820.GA4334@xxxxxxxxxxxxxx
> and also concluded same as you
>
> On Mon 12-12-16 17:46:21, Andrey Ryabinin wrote:
>> DEBUG_PREEMPT complains about using this_cpu_ptr() in preemptible:
>>       BUG: using smp_processor_id() in preemptible [00000000] code: iperf-300s-cs-l/277
>>       caller is debug_smp_processor_id+0x17/0x19
>>       CPU: 1 PID: 277 Comm: iperf-300s-cs-l Not tainted 4.9.0-rc8-00140-gcc639db #2
>>        ffffc900003f3cf0 ffffffff8123ae6f 0000000000000001 ffffffff818181da
>>        ffffc900003f3d20 ffffffff81252f41 0000000000012de0 00000000fffffdff
>>        ffff880009328f40 ffff88000592c400 ffffc900003f3d30 ffffffff81252f6a
>>       Call Trace:
>>        [<ffffffff8123ae6f>] dump_stack+0x9a/0xd0
>>        [<ffffffff81252f41>] check_preemption_disabled+0xdd/0xef
>>        [<ffffffff81252f6a>] debug_smp_processor_id+0x17/0x19
>>        [<ffffffff811796df>] __vfree_deferred+0x16/0x4c
>>        [<ffffffff8117b584>] vfree_atomic+0x22/0x24
>>        [<ffffffff81094f5d>] free_thread_stack+0xc2/0x106
>>        [<ffffffff810951be>] put_task_stack+0x4c/0x62
>>        [<ffffffff81095f81>] copy_process+0x7e0/0x16e8
>>        [<ffffffff8109702d>] _do_fork+0xbb/0x2d3
>>        [<ffffffff810465e8>] ? __do_page_fault+0x2e1/0x384
>>        [<ffffffff8112633f>] ? trace_hardirqs_off_caller+0x12/0x24
>>        [<ffffffff810972cb>] SyS_clone+0x19/0x1b
>>        [<ffffffff81003800>] do_syscall_64+0x143/0x173
>>        [<ffffffff81507289>] entry_SYSCALL64_slow_path+0x25/0x25
>>
>> Use raw_cpu_ptr() instead of this_cpu_ptr() to hide this warning.
>> It's fine because llist_add() implementation is lock-less, so it works even
>> if we adding to the list of some other cpu. schedule_work() is also preempt-safe.
>>
>> Reported-by: kernel test robot <ying.huang@xxxxxxxxxxxxxxx>
>> Signed-off-by: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
>
> Acked-by: Michal Hocko <mhocko@xxxxxxxx>

But not quite acked by me.  What happened to the vfree code that
causes vfree_deferred to be called in a preemptable context?  That
sounds like a bug.

(This code doesn't exist in Linus' tree.  What tree does this apply to.)

>
>> ---
>>  mm/vmalloc.c | 9 ++++++++-
>>  1 file changed, 8 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
>> index 43f0608..d8813963 100644
>> --- a/mm/vmalloc.c
>> +++ b/mm/vmalloc.c
>> @@ -1498,7 +1498,14 @@ static void __vunmap(const void *addr, int deallocate_pages)
>>
>>  static inline void __vfree_deferred(const void *addr)
>>  {
>> -     struct vfree_deferred *p = this_cpu_ptr(&vfree_deferred);
>> +     /*
>> +      * Use raw_cpu_ptr() because this can be called from preemptible
>> +      * context. Preemption is absolutely fine here, because llist_add()
>> +      * implementation is lockless, so it works even if we adding to list
>> +      * of the other cpu.
>> +      * schedule_work() should be fine with this too.
>> +      */
>> +     struct vfree_deferred *p = raw_cpu_ptr(&vfree_deferred);
>>
>>       if (llist_add((struct llist_node *)addr, &p->list))
>>               schedule_work(&p->wq);
>> --
>> 2.7.3
>
> --
> Michal Hocko
> SUSE Labs



-- 
Andy Lutomirski
AMA Capital Management, LLC