Re: How to handle TIF_MEMDIE stalls?

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Michal Hocko wrote:
> On Thu 19-02-15 13:29:14, Michal Hocko wrote:
> [...]
> > Something like the following.
> __GFP_HIGH doesn't seem to be sufficient so we would need something
> slightly else but the idea is still the same:
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 8d52ab18fe0d..2d224bbdf8e8 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2599,6 +2599,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>  	enum migrate_mode migration_mode = MIGRATE_ASYNC;
>  	bool deferred_compaction = false;
>  	int contended_compaction = COMPACT_CONTENDED_NONE;
> +	int oom = 0;
>  
>  	/*
>  	 * In the slowpath, we sanity check order to avoid ever trying to
> @@ -2635,6 +2636,15 @@ retry:
>  	alloc_flags = gfp_to_alloc_flags(gfp_mask);
>  
>  	/*
> +	 * __GFP_NOFAIL allocations cannot fail but yet the current context
> +	 * might be blocking resources needed by the OOM victim to terminate.
> +	 * Allow the caller to dive into memory reserves to succeed the
> +	 * allocation and break out from a potential deadlock.
> +	 */

We don't know how many callers will pass __GFP_NOFAIL. But if 1000
threads are doing the same operation which requires __GFP_NOFAIL
allocation with a lock held, wouldn't memory reserves deplete?

This heuristic can't continue if memory reserves depleted or
continuous pages of requested order cannot be found.

> +	if (oom > 10 && (gfp_mask & __GFP_NOFAIL))
> +		alloc_flags |= ALLOC_NO_WATERMARKS;
> +
> +	/*
>  	 * Find the true preferred zone if the allocation is unconstrained by
>  	 * cpusets.
>  	 */
> @@ -2759,6 +2769,8 @@ retry:
>  				goto got_pg;
>  			if (!did_some_progress)
>  				goto nopage;
> +
> +			oom++;
>  		}
>  		/* Wait for some write requests to complete then retry */
>  		wait_iff_congested(ac->preferred_zone, BLK_RW_ASYNC, HZ/50);
> -- 
> Michal Hocko
> SUSE Labs
> 

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