Re: [PATCH 2/3 v3] mm: bug fix free page check in zone_watermark_ok

[Michal Hocko <mhocko@suse.cz>]

On Thu 12-07-12 11:50:48, Minchan Kim wrote:
> In __zone_watermark_ok, free and min are signed long type
> while z->lowmem_reserve[classzone_idx] is unsigned long type.
> So comparision of them could be wrong due to type conversion
> to unsigned although free_pages is minus value.

Agreed on that
but
> 
> It could return true instead of false in case of order-0 check
> so that kswapd could sleep forever. 

I am kind of lost here. How can we have negative free_pages with
order-0? It would need to come with a negative value because 
free_pages -= (1 << order) - 1;
won't make it negative.

> It means livelock because direct reclaimer loops forever until kswapd
> set zone->all_unreclaimable.
> 
> Aaditya reported this problem when he test my hotplug patch.
> 
> Reported-off-by: Aaditya Kumar <aaditya.kumar@ap.sony.com>
> Tested-by: Aaditya Kumar <aaditya.kumar@ap.sony.com>
> Signed-off-by: Aaditya Kumar <aaditya.kumar@ap.sony.com>
> Signed-off-by: Minchan Kim <minchan@kernel.org>

So you can add my Reviewed-by: Michal Hocko <mhocko@suse.cz>
but the changelog could be more clear.

> ---
> This patch isn't dependent with this series.
> It seems to be candidate for -stable but I'm not sure because of this part.
> So, pass the decision to akpm.
> 
> " - It must fix a real bug that bothers people (not a, "This could be a
>    problem..." type thing)."

I am wondering what Testted-by means if "This could be a problem..."
type thing)."

> 
>  mm/page_alloc.c |    3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index f17e6e4..627653c 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1594,6 +1594,7 @@ static bool __zone_watermark_ok(struct zone *z, int order, unsigned long mark,
>  {
>  	/* free_pages my go negative - that's OK */
>  	long min = mark;
> +	long lowmem_reserve = z->lowmem_reserve[classzone_idx];
>  	int o;
>  
>  	free_pages -= (1 << order) - 1;
> @@ -1602,7 +1603,7 @@ static bool __zone_watermark_ok(struct zone *z, int order, unsigned long mark,
>  	if (alloc_flags & ALLOC_HARDER)
>  		min -= min / 4;
>  
> -	if (free_pages <= min + z->lowmem_reserve[classzone_idx])
> +	if (free_pages <= min + lowmem_reserve)
>  		return false;
>  	for (o = 0; o < order; o++) {
>  		/* At the next order, this order's pages become unavailable */
> -- 
> 1.7.9.5
> 
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-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic

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