Re: [PATCH] oom killer: break from infinite loop

[Oleg Nesterov <oleg@redhat.com>]

On 03/28, David Rientjes wrote:
>
> The problem with this approach is that we could easily deplete all memory
> reserves if the oom killed task has an extremely large number of threads,
> there has always been only a single thread with TIF_MEMDIE set per cpuset
> or memcg; for systems that don't run with cpusets or memory controller,
> this has been limited to one thread with TIF_MEMDIE for the entire system.
>
> There's risk involved with suddenly allowing 1000 threads to have
> TIF_MEMDIE set and the chances of fully depleting all allowed zones is
> much higher if they allocate memory prior to exit, for example.
>
> An alternative is to fail allocations if they are failable and the
> allocating task has a pending SIGKILL.  It's better to preempt the oom
> killer since current is going to be exiting anyway and this avoids a
> needless kill.
>
> That's possible if it's guaranteed that __GFP_NOFAIL allocations with a
> pending SIGKILL are granted ALLOC_NO_WATERMARKS to prevent them from
> endlessly looping while making no progress.
>
> Comments?

Can't comment, I do not understand these subtleties.

But I'd like to note that fatal_signal_pending() can be true when the
process wasn't killed, but another thread does exit_group/exec.

I am not saying this is wrong, just I'd like to be sure this didn't
escape your attention.

> ---
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1610,13 +1610,21 @@ try_next_zone:
>  }
>
>  static inline int
> -should_alloc_retry(gfp_t gfp_mask, unsigned int order,
> +should_alloc_retry(struct task_struct *p, gfp_t gfp_mask, unsigned int order,
>  				unsigned long pages_reclaimed)
>  {
>  	/* Do not loop if specifically requested */
>  	if (gfp_mask & __GFP_NORETRY)
>  		return 0;
>
> +	/* Loop if specifically requested */
> +	if (gfp_mask & __GFP_NOFAIL)
> +		return 1;
> +
> +	/* Task is killed, fail the allocation if possible */
> +	if (fatal_signal_pending(p))
> +		return 0;
> +
>  	/*
>  	 * In this implementation, order <= PAGE_ALLOC_COSTLY_ORDER
>  	 * means __GFP_NOFAIL, but that may not be true in other
> @@ -1635,13 +1643,6 @@ should_alloc_retry(gfp_t gfp_mask, unsigned int order,
>  	if (gfp_mask & __GFP_REPEAT && pages_reclaimed < (1 << order))
>  		return 1;
>
> -	/*
> -	 * Don't let big-order allocations loop unless the caller
> -	 * explicitly requests that.
> -	 */
> -	if (gfp_mask & __GFP_NOFAIL)
> -		return 1;
> -
>  	return 0;
>  }
>
> @@ -1798,6 +1799,7 @@ gfp_to_alloc_flags(gfp_t gfp_mask)
>  	if (likely(!(gfp_mask & __GFP_NOMEMALLOC))) {
>  		if (!in_interrupt() &&
>  		    ((p->flags & PF_MEMALLOC) ||
> +		     (fatal_signal_pending(p) && (gfp_mask & __GFP_NOFAIL)) ||
>  		     unlikely(test_thread_flag(TIF_MEMDIE))))
>  			alloc_flags |= ALLOC_NO_WATERMARKS;
>  	}
> @@ -1812,6 +1814,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>  	int migratetype)
>  {
>  	const gfp_t wait = gfp_mask & __GFP_WAIT;
> +	const gfp_t nofail = gfp_mask & __GFP_NOFAIL;
>  	struct page *page = NULL;
>  	int alloc_flags;
>  	unsigned long pages_reclaimed = 0;
> @@ -1876,7 +1879,7 @@ rebalance:
>  		goto nopage;
>
>  	/* Avoid allocations with no watermarks from looping endlessly */
> -	if (test_thread_flag(TIF_MEMDIE) && !(gfp_mask & __GFP_NOFAIL))
> +	if (test_thread_flag(TIF_MEMDIE) && !nofail)
>  		goto nopage;
>
>  	/* Try direct reclaim and then allocating */
> @@ -1888,6 +1891,10 @@ rebalance:
>  	if (page)
>  		goto got_pg;
>
> +	/* Task is killed, fail the allocation if possible */
> +	if (fatal_signal_pending(p) && !nofail)
> +		goto nopage;
> +
>  	/*
>  	 * If we failed to make any progress reclaiming, then we are
>  	 * running out of options and have to consider going OOM
> @@ -1909,8 +1916,7 @@ rebalance:
>  			 * made, there are no other options and retrying is
>  			 * unlikely to help.
>  			 */
> -			if (order > PAGE_ALLOC_COSTLY_ORDER &&
> -						!(gfp_mask & __GFP_NOFAIL))
> +			if (order > PAGE_ALLOC_COSTLY_ORDER && !nofail)
>  				goto nopage;
>
>  			goto restart;
> @@ -1919,7 +1925,7 @@ rebalance:
>
>  	/* Check if we should retry the allocation */
>  	pages_reclaimed += did_some_progress;
> -	if (should_alloc_retry(gfp_mask, order, pages_reclaimed)) {
> +	if (should_alloc_retry(p, gfp_mask, order, pages_reclaimed)) {
>  		/* Wait for some write requests to complete then retry */
>  		congestion_wait(BLK_RW_ASYNC, HZ/50);
>  		goto rebalance;