Re: [patch 2/2]vmscan: correctly detect GFP_ATOMIC allocation failure

[David Rientjes <rientjes@google.com>]

On Sat, 8 Oct 2011, Shaohua Li wrote:

> has_under_min_watermark_zone is used to detect if there is GFP_ATOMIC allocation
> failure risk. For a high end_zone, if any zone below or equal to it has min
> matermark ok, we have no risk. But current logic is any zone has min watermark
> not ok, then we have risk. This is wrong to me.
> 
> Signed-off-by: Shaohua Li <shaohua.li@intel.com>
> ---
>  mm/vmscan.c |    7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> Index: linux/mm/vmscan.c
> ===================================================================
> --- linux.orig/mm/vmscan.c	2011-09-27 15:09:29.000000000 +0800
> +++ linux/mm/vmscan.c	2011-09-27 15:14:45.000000000 +0800
> @@ -2463,7 +2463,7 @@ loop_again:
>  
>  	for (priority = DEF_PRIORITY; priority >= 0; priority--) {
>  		unsigned long lru_pages = 0;
> -		int has_under_min_watermark_zone = 0;
> +		int has_under_min_watermark_zone = 1;

bool

>  
>  		/* The swap token gets in the way of swapout... */
>  		if (!priority)
> @@ -2594,9 +2594,10 @@ loop_again:
>  				 * means that we have a GFP_ATOMIC allocation
>  				 * failure risk. Hurry up!
>  				 */
> -				if (!zone_watermark_ok_safe(zone, order,
> +				if (has_under_min_watermark_zone &&
> +					    zone_watermark_ok_safe(zone, order,
>  					    min_wmark_pages(zone), end_zone, 0))
> -					has_under_min_watermark_zone = 1;
> +					has_under_min_watermark_zone = 0;
>  			} else {
>  				/*
>  				 * If a zone reaches its high watermark,

Ignore checking the min watermark for a moment and consider if all zones 
are above the high watermark (a situation where kswapd does not need to 
do aggressive reclaim), then has_under_min_watermark_zone doesn't get 
cleared and never actually stalls on congestion_wait().  Notice this is 
congestion_wait() and not wait_iff_congested(), so the clearing of 
ZONE_CONGESTED doesn't prevent this.

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