Re: [PATCH 3/3] mm: page allocator: Drain per-cpu lists after direct reclaim allocation fails

[Minchan Kim <minchan.kim@gmail.com>]

On Mon, Aug 16, 2010 at 10:42:13AM +0100, Mel Gorman wrote:
> When under significant memory pressure, a process enters direct reclaim
> and immediately afterwards tries to allocate a page. If it fails and no
> further progress is made, it's possible the system will go OOM. However,
> on systems with large amounts of memory, it's possible that a significant
> number of pages are on per-cpu lists and inaccessible to the calling
> process. This leads to a process entering direct reclaim more often than
> it should increasing the pressure on the system and compounding the problem.
> 
> This patch notes that if direct reclaim is making progress but
> allocations are still failing that the system is already under heavy
> pressure. In this case, it drains the per-cpu lists and tries the
> allocation a second time before continuing.
> 
> Signed-off-by: Mel Gorman <mel@csn.ul.ie>
> ---
>  mm/page_alloc.c |   19 +++++++++++++++++--
>  1 files changed, 17 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 67a2ed0..a8651a4 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1844,6 +1844,7 @@ __alloc_pages_direct_reclaim(gfp_t gfp_mask, unsigned int order,
>  	struct page *page = NULL;
>  	struct reclaim_state reclaim_state;
>  	struct task_struct *p = current;
> +	bool drained = false;
>  
>  	cond_resched();
>  
> @@ -1865,11 +1866,25 @@ __alloc_pages_direct_reclaim(gfp_t gfp_mask, unsigned int order,
>  	if (order != 0)
>  		drain_all_pages();
>  

Nitpick: 

How about removing above condition and drain_all_pages?
If get_page_from_freelist fails, we do drain_all_pages at last. 
It can remove double calling of drain_all_pagse in case of order > 0.
In addition, if the VM can't reclaim anythings, we don't need to drain
in case of order > 0. 


> -	if (likely(*did_some_progress))
> -		page = get_page_from_freelist(gfp_mask, nodemask, order,
> +	if (unlikely(!(*did_some_progress)))
> +		return NULL;
> +
> +retry:
> +	page = get_page_from_freelist(gfp_mask, nodemask, order,
>  					zonelist, high_zoneidx,
>  					alloc_flags, preferred_zone,
>  					migratetype);
> +
> +	/*
> +	 * If an allocation failed after direct reclaim, it could be because
> +	 * pages are pinned on the per-cpu lists. Drain them and try again
> +	 */
> +	if (!page && !drained) {
> +		drain_all_pages();
> +		drained = true;
> +		goto retry;
> +	}
> +
>  	return page;
>  }
>  
> -- 
> 1.7.1
> 
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-- 
Kind regards,
Minchan Kim

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