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

[Johannes Weiner <hannes@cmpxchg.org>]

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()?