Re: [PATCH] mm: fix special case -1 order check in compact_finished

[Minchan Kim]

Sorry for breaking thread.
I resend.

On Mon, May 30, 2011 at 02:38:31PM +0200, Michal Hocko wrote:
> 56de7263 (mm: compaction: direct compact when a high-order allocation
> fails) introduced a check for cc->order == -1 in compact_finished. We
> should continue compacting in that case because the request came from
> userspace and there is no particular order to compact for.
> 
> The check is, however, done after zone_watermark_ok which uses order as
> a right hand argument for shifts. Not only watermark check is pointless
> if we can break out without it but it also uses 1 << -1 which is not
> well defined (at least from C standard). Let's move the -1 check above
> zone_watermark_ok.
> 
> Signed-off-by: Michal Hocko <mhocko@xxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> ---
>  compaction.c |   14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
> Index: linus_tree/mm/compaction.c
> ===================================================================
> --- linus_tree.orig/mm/compaction.c	2011-05-30 14:19:58.000000000 +0200
> +++ linus_tree/mm/compaction.c	2011-05-30 14:20:40.000000000 +0200
> @@ -420,13 +420,6 @@ static int compact_finished(struct zone
>  	if (cc->free_pfn <= cc->migrate_pfn)
>  		return COMPACT_COMPLETE;
>  
> -	/* Compaction run is not finished if the watermark is not met */
> -	watermark = low_wmark_pages(zone);
> -	watermark += (1 << cc->order);
> -
> -	if (!zone_watermark_ok(zone, cc->order, watermark, 0, 0))
> -		return COMPACT_CONTINUE;
> -
>  	/*
>  	 * order == -1 is expected when compacting via
>  	 * /proc/sys/vm/compact_memory
> @@ -434,6 +427,13 @@ static int compact_finished(struct zone
>  	if (cc->order == -1)
>  		return COMPACT_CONTINUE;
>  
> +	/* Compaction run is not finished if the watermark is not met */
> +	watermark = low_wmark_pages(zone);
> +	watermark += (1 << cc->order);
> +
> +	if (!zone_watermark_ok(zone, cc->order, watermark, 0, 0))
> +		return COMPACT_CONTINUE;
> +
>  	/* Direct compactor: Is a suitable page free? */
>  	for (order = cc->order; order < MAX_ORDER; order++) {
>  		/* Job done if page is free of the right migratetype */

It looks good to me.
Let's think about another place, compaction_suitable.
It has same problem so we can move the check right before zone_watermark_ok.
As I look it more, I thought we need free pages for compaction so we would 
be better to give up early if we can't get enough free pages. But I changed
my mind. It's a totally user request and we can get free pages in migration
progress(ex, other big memory hogger might free his big rss). 
So my conclusion is that we should do *best effort* than early give up.
If you agree with me, how about resending patch with compaction_suitable fix?


> -- 
> Michal Hocko
> SUSE Labs
> SUSE LINUX s.r.o.
> Lihovarska 1060/12
> 190 00 Praha 9    
> Czech Republic
> 
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