Re: [RFC PATCH 2/2] mm, oom: do not trigger out_of_memory from the #PF

[Michal Hocko <mhocko@kernel.org>]

Does anybody see any problem with the patch or I can send it for the
inclusion?

On Fri 19-05-17 13:26:04, Michal Hocko wrote:
> From: Michal Hocko <mhocko@suse.com>
> 
> Any allocation failure during the #PF path will return with VM_FAULT_OOM
> which in turn results in pagefault_out_of_memory. This can happen for
> 2 different reasons. a) Memcg is out of memory and we rely on
> mem_cgroup_oom_synchronize to perform the memcg OOM handling or b)
> normal allocation fails.
> 
> The later is quite problematic because allocation paths already trigger
> out_of_memory and the page allocator tries really hard to not fail
> allocations. Anyway, if the OOM killer has been already invoked there
> is no reason to invoke it again from the #PF path. Especially when the
> OOM condition might be gone by that time and we have no way to find out
> other than allocate.
> 
> Moreover if the allocation failed and the OOM killer hasn't been
> invoked then we are unlikely to do the right thing from the #PF context
> because we have already lost the allocation context and restictions and
> therefore might oom kill a task from a different NUMA domain.
> 
> An allocation might fail also when the current task is the oom victim
> and there are no memory reserves left and we should simply bail out
> from the #PF rather than invoking out_of_memory.
> 
> This all suggests that there is no legitimate reason to trigger
> out_of_memory from pagefault_out_of_memory so drop it. Just to be sure
> that no #PF path returns with VM_FAULT_OOM without allocation print a
> warning that this is happening before we restart the #PF.
> 
> Signed-off-by: Michal Hocko <mhocko@suse.com>
> ---
>  mm/oom_kill.c | 23 ++++++++++-------------
>  1 file changed, 10 insertions(+), 13 deletions(-)
> 
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index 04c9143a8625..0f24bdfaadfd 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -1051,25 +1051,22 @@ bool out_of_memory(struct oom_control *oc)
>  }
>  
>  /*
> - * The pagefault handler calls here because it is out of memory, so kill a
> - * memory-hogging task. If oom_lock is held by somebody else, a parallel oom
> - * killing is already in progress so do nothing.
> + * The pagefault handler calls here because some allocation has failed. We have
> + * to take care of the memcg OOM here because this is the only safe context without
> + * any locks held but let the oom killer triggered from the allocation context care
> + * about the global OOM.
>   */
>  void pagefault_out_of_memory(void)
>  {
> -	struct oom_control oc = {
> -		.zonelist = NULL,
> -		.nodemask = NULL,
> -		.memcg = NULL,
> -		.gfp_mask = 0,
> -		.order = 0,
> -	};
> +	static DEFINE_RATELIMIT_STATE(pfoom_rs, DEFAULT_RATELIMIT_INTERVAL,
> +				      DEFAULT_RATELIMIT_BURST);
>  
>  	if (mem_cgroup_oom_synchronize(true))
>  		return;
>  
> -	if (!mutex_trylock(&oom_lock))
> +	if (fatal_signal_pending)
>  		return;
> -	out_of_memory(&oc);
> -	mutex_unlock(&oom_lock);
> +
> +	if (__ratelimit(&pfoom_rs))
> +		pr_warn("Huh VM_FAULT_OOM leaked out to the #PF handler. Retrying PF\n");
>  }
> -- 
> 2.11.0
> 

-- 
Michal Hocko
SUSE Labs

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