Re: [PATCH] mm, oom: do not fail __GFP_NOFAIL allocation if oom killer is disbaled

[Konstantin Khlebnikov]

On Wed, Feb 25, 2015 at 1:09 AM, Konstantin Khlebnikov <koct9i@xxxxxxxxx> wrote:
> On Tue, Feb 24, 2015 at 10:11 PM, Johannes Weiner <hannes@xxxxxxxxxxx> wrote:
>> On Tue, Feb 24, 2015 at 07:19:24PM +0100, Michal Hocko wrote:
>>> Tetsuo Handa has pointed out that __GFP_NOFAIL allocations might fail
>>> after OOM killer is disabled if the allocation is performed by a
>>> kernel thread. This behavior was introduced from the very beginning by
>>> 7f33d49a2ed5 (mm, PM/Freezer: Disable OOM killer when tasks are frozen).
>>> This means that the basic contract for the allocation request is broken
>>> and the context requesting such an allocation might blow up unexpectedly.
>>>
>>> There are basically two ways forward.
>>> 1) move oom_killer_disable after kernel threads are frozen. This has a
>>>    risk that the OOM victim wouldn't be able to finish because it would
>>>    depend on an already frozen kernel thread. This would be really
>>>    tricky to debug.
>>> 2) do not fail GFP_NOFAIL allocation no matter what and risk a potential
>>>    Freezable kernel threads will loop and fail the suspend. Incidental
>>>    allocations after kernel threads are frozen will at least dump a
>>>    warning - if we are lucky and the serial console is still active of
>>>    course...
>>>
>>> This patch implements the later option because it is safer. We would see
>>> warnings rather than allocation failures for the kernel threads which
>>> would blow up otherwise and have a higher chances to identify
>>> __GFP_NOFAIL users from deeper pm code.
>>>
>>> Signed-off-by: Michal Hocko <mhocko@xxxxxxx>
>>> ---
>>>
>>> We haven't seen any bug reports
>>>
>>>  mm/oom_kill.c | 8 ++++++++
>>>  1 file changed, 8 insertions(+)
>>>
>>> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
>>> index 642f38cb175a..ea8b443cd871 100644
>>> --- a/mm/oom_kill.c
>>> +++ b/mm/oom_kill.c
>>> @@ -772,6 +772,10 @@ out:
>>>               schedule_timeout_killable(1);
>>>  }
>>>
>>> +static DEFINE_RATELIMIT_STATE(oom_disabled_rs,
>>> +             DEFAULT_RATELIMIT_INTERVAL,
>>> +             DEFAULT_RATELIMIT_BURST);
>>> +
>>>  /**
>>>   * out_of_memory -  tries to invoke OOM killer.
>>>   * @zonelist: zonelist pointer
>>> @@ -792,6 +796,10 @@ bool out_of_memory(struct zonelist *zonelist, gfp_t gfp_mask,
>>>       if (!oom_killer_disabled) {
>>>               __out_of_memory(zonelist, gfp_mask, order, nodemask, force_kill);
>>>               ret = true;
>>> +     } else if (gfp_mask & __GFP_NOFAIL) {
>>> +             if (__ratelimit(&oom_disabled_rs))
>>> +                     WARN(1, "Unable to make forward progress for __GFP_NOFAIL because OOM killer is disbaled\n");
>>> +             ret = true;
>>
>> I'm fine with keeping the allocation looping, but is that message
>> helpful?  It seems completely useless to the user encountering it.  Is
>> it going to help kernel developers when we get a bug report with it?
>>
>> WARN_ON_ONCE()?
>
> maybe panic() ?
>
> If somebody turns off oom-killer it seems he's pretty sure that he has
> enough memory.

Ah, that's used in freeze/suspend code. I thought that some kind of
sysctl for brave sysadmins.

>
>>
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