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

[Konstantin Khlebnikov <koct9i@gmail.com>]

On Tue, Feb 24, 2015 at 10:11 PM, Johannes Weiner <hannes@cmpxchg.org> 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@suse.cz>
>> ---
>>
>> 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.

>
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