Re: [PATCH 12/12] oom: give current access to memory reserves if it has been killed

[KOSAKI Motohiro]

> From: David Rientjes <rientjes@xxxxxxxxxx>
> 
> It's possible to livelock the page allocator if a thread has
> mm->mmap_sem and fails to make forward progress because the
> oom killer selects another thread sharing the same ->mm to
> kill that cannot exit until the semaphore is dropped.
> 
> The oom killer will not kill multiple tasks at the same time; each oom
> killed task must exit before another task may be killed.  Thus, if one
> thread is holding mm->mmap_sem and cannot allocate memory, all threads
> sharing the same ->mm are blocked from exiting as well.  In the oom kill
> case, that means the thread holding mm->mmap_sem will never free
> additional memory since it cannot get access to memory reserves and the
> thread that depends on it with access to memory reserves cannot exit
> because it cannot acquire the semaphore.  Thus, the page allocators
> livelocks.
> 
> When the oom killer is called and current happens to have a pending
> SIGKILL, this patch automatically gives it access to memory reserves and
> returns.  Upon returning to the page allocator, its allocation will
> hopefully succeed so it can quickly exit and free its memory.  If not,
> the page allocator will fail the allocation if it is not __GFP_NOFAIL.
> 
> Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> Signed-off-by: David Rientjes <rientjes@xxxxxxxxxx>
> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
> ---
>  mm/oom_kill.c |   10 ++++++++++
>  1 files changed, 10 insertions(+), 0 deletions(-)
> 
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index 67b5fa5..ad85e1b 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -638,6 +638,16 @@ void out_of_memory(struct zonelist *zonelist, gfp_t gfp_mask,
>  	}
>  
>  	/*
> +	 * If current has a pending SIGKILL, then automatically select it.  The
> +	 * goal is to allow it to allocate so that it may quickly exit and free
> +	 * its memory.
> +	 */
> +	if (fatal_signal_pending(current)) {
> +		set_tsk_thread_flag(current, TIF_MEMDIE);
> +		return;
> +	}

Self NAK this.
We have no gurantee that current is oom killable. Oh, here is
out_of_memory(), sigh.



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