Re: [PATCH][RESEND] mm: Avoid possible deadlock caused by too_many_isolated()

[Simon Jeons <simon.jeons@gmail.com>]

On Mon, 2012-12-10 at 10:48 +0800, Fengguang Wu wrote: 
> Neil find that if too_many_isolated() returns true while performing
> direct reclaim we can end up waiting for other threads to complete their
> direct reclaim.  If those threads are allowed to enter the FS or IO to
> free memory, but this thread is not, then it is possible that those
> threads will be waiting on this thread and so we get a circular
> deadlock.
> 
> some task enters direct reclaim with GFP_KERNEL
>   => too_many_isolated() false
>     => vmscan and run into dirty pages
>       => pageout()
>         => take some FS lock
> 	  => fs/block code does GFP_NOIO allocation

Hi Fengguang,

GFP_NOIO allocation for what?

> 	    => enter direct reclaim again
> 	      => too_many_isolated() true
> 		  => waiting for others to progress, however the other
> 		     tasks may be circular waiting for the FS lock..
> 
> The fix is to let !__GFP_IO and !__GFP_FS direct reclaims enjoy higher
> priority than normal ones, by lowering the throttle threshold for the
> latter.
> 
> Allowing ~1/8 isolated pages in normal is large enough. For example,
> for a 1GB LRU list, that's ~128MB isolated pages, or 1k blocked tasks
> (each isolates 32 4KB pages), or 64 blocked tasks per logical CPU
> (assuming 16 logical CPUs per NUMA node). So it's not likely some CPU
> goes idle waiting (when it could make progress) because of this limit:
> there are much more sleeping reclaim tasks than the number of CPU, so
> the task may well be blocked by some low level queue/lock anyway.
> 
> Now !GFP_IOFS reclaims won't be waiting for GFP_IOFS reclaims to
> progress. They will be blocked only when there are too many concurrent
> !GFP_IOFS reclaims, however that's very unlikely because the IO-less

Why you said that direct reclaim is IO-less?

> direct reclaims is able to progress much more faster, and they won't
> deadlock each other. The threshold is raised high enough for them, so
> that there can be sufficient parallel progress of !GFP_IOFS reclaims.
> 
> CC: Torsten Kaiser <just.for.lkml@googlemail.com>
> Tested-by: NeilBrown <neilb@suse.de>
> Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
> Acked-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
> Acked-by: Rik van Riel <riel@redhat.com>
> Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
> ---
>  mm/vmscan.c |    7 +++++++
>  1 file changed, 7 insertions(+)
> 
> --- linux-next.orig/mm/vmscan.c	2012-12-10 10:43:06.474928860 +0800
> +++ linux-next/mm/vmscan.c	2012-12-10 10:43:09.022928920 +0800
> @@ -1202,6 +1202,13 @@ static int too_many_isolated(struct zone
>  		isolated = zone_page_state(zone, NR_ISOLATED_ANON);
>  	}
>  
> +	/*
> +	 * GFP_NOIO/GFP_NOFS callers are allowed to isolate more pages, so that
> +	 * they won't get blocked by normal ones and form circular deadlock.
> +	 */
> +	if ((sc->gfp_mask & GFP_IOFS) == GFP_IOFS)
> +		inactive >>= 3;
> +
>  	return isolated > inactive;
>  }
>  
> 
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