linux-kernel - Re: [PATCH] mm: disallow direct reclaim page writeback

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Date:	Tue, 13 Apr 2010 10:58:15 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	Dave Chinner <david@...morbit.com>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] mm: disallow direct reclaim page writeback

On Tue, Apr 13, 2010 at 10:17:58AM +1000, Dave Chinner wrote:
> From: Dave Chinner <dchinner@...hat.com>
> 
> When we enter direct reclaim we may have used an arbitrary amount of stack
> space, and hence enterring the filesystem to do writeback can then lead to
> stack overruns. This problem was recently encountered x86_64 systems with
> 8k stacks running XFS with simple storage configurations.
> 
> Writeback from direct reclaim also adversely affects background writeback. The
> background flusher threads should already be taking care of cleaning dirty
> pages, and direct reclaim will kick them if they aren't already doing work. If
> direct reclaim is also calling ->writepage, it will cause the IO patterns from
> the background flusher threads to be upset by LRU-order writeback from
> pageout() which can be effectively random IO. Having competing sources of IO
> trying to clean pages on the same backing device reduces throughput by
> increasing the amount of seeks that the backing device has to do to write back
> the pages.
> 

It's already known that the VM requesting specific pages be cleaned and
reclaimed is a bad IO pattern but unfortunately it is still required by
lumpy reclaim. This change would appear to break that although I haven't
tested it to be 100% sure.

Even without high-order considerations, this patch would appear to make
fairly large changes to how direct reclaim behaves. It would no longer
wait on page writeback for example so direct reclaim will return sooner
than it did potentially going OOM if there were a lot of dirty pages and
it made no progress during direct reclaim.

> Hence for direct reclaim we should not allow ->writepages to be entered at all.
> Set up the relevant scan_control structures to enforce this, and prevent
> sc->may_writepage from being set in other places in the direct reclaim path in
> response to other events.
> 

If an FS caller cannot re-enter the FS, it should be using GFP_NOFS
instead of GFP_KERNEL.

> Reported-by: John Berthels <john@...yo.com>
> Signed-off-by: Dave Chinner <dchinner@...hat.com>
> ---
>  mm/vmscan.c |   13 ++++++-------
>  1 files changed, 6 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index e0e5f15..5321ac4 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1826,10 +1826,8 @@ static unsigned long do_try_to_free_pages(struct zonelist *zonelist,
>  		 * writeout.  So in laptop mode, write out the whole world.
>  		 */
>  		writeback_threshold = sc->nr_to_reclaim + sc->nr_to_reclaim / 2;
> -		if (total_scanned > writeback_threshold) {
> +		if (total_scanned > writeback_threshold)
>  			wakeup_flusher_threads(laptop_mode ? 0 : total_scanned);
> -			sc->may_writepage = 1;
> -		}
>  
>  		/* Take a nap, wait for some writeback to complete */
>  		if (!sc->hibernation_mode && sc->nr_scanned &&
> @@ -1871,7 +1869,7 @@ unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
>  {
>  	struct scan_control sc = {
>  		.gfp_mask = gfp_mask,
> -		.may_writepage = !laptop_mode,
> +		.may_writepage = 0,
>  		.nr_to_reclaim = SWAP_CLUSTER_MAX,
>  		.may_unmap = 1,
>  		.may_swap = 1,
> @@ -1893,7 +1891,7 @@ unsigned long mem_cgroup_shrink_node_zone(struct mem_cgroup *mem,
>  						struct zone *zone, int nid)
>  {
>  	struct scan_control sc = {
> -		.may_writepage = !laptop_mode,
> +		.may_writepage = 0,
>  		.may_unmap = 1,
>  		.may_swap = !noswap,
>  		.swappiness = swappiness,
> @@ -1926,7 +1924,7 @@ unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *mem_cont,
>  {
>  	struct zonelist *zonelist;
>  	struct scan_control sc = {
> -		.may_writepage = !laptop_mode,
> +		.may_writepage = 0,
>  		.may_unmap = 1,
>  		.may_swap = !noswap,
>  		.nr_to_reclaim = SWAP_CLUSTER_MAX,
> @@ -2567,7 +2565,8 @@ static int __zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
>  	struct reclaim_state reclaim_state;
>  	int priority;
>  	struct scan_control sc = {
> -		.may_writepage = !!(zone_reclaim_mode & RECLAIM_WRITE),
> +		.may_writepage = (current_is_kswapd() &&
> +					(zone_reclaim_mode & RECLAIM_WRITE)),
>  		.may_unmap = !!(zone_reclaim_mode & RECLAIM_SWAP),
>  		.may_swap = 1,
>  		.nr_to_reclaim = max_t(unsigned long, nr_pages,
> -- 
> 1.6.5
> 

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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