Re: [PATCH 2/3] Do not unconditionally treat zones that failzone_reclaim() as full

[Mel Gorman]

On Thu, Jun 11, 2009 at 09:48:53AM -0400, Christoph Lameter wrote:
> It needs to be mentioned that this fixes a bug introduced in 2.6.19.
> Possibly a portion of this code needs to be backported to stable.
> 

Andrew has sucked up the patch already so I can't patch it. Andrew, there
is a further note below on the patch if you'd like to pick it up.

On the stable front, I'm think that patches 1 and 2 should being considered
-stable candidates. Patch 1 is certainly needed because it fixes up the
malloc() stall and should be picked up by distro kernels as well. This patch
closes another obvious hole albeit one harder to trigger.

Ideally patch 3 would also be in -stable so distro kernels will suck it up
as it will help identify this problem in the field if it occurs again but
I'm not sure what the -stable policy is on such things are.

> On Thu, 11 Jun 2009, Mel Gorman wrote:
> 
> > On NUMA machines, the administrator can configure zone_reclaim_mode that
> > is a more targetted form of direct reclaim. On machines with large NUMA
> > distances for example, a zone_reclaim_mode defaults to 1 meaning that clean
> > unmapped pages will be reclaimed if the zone watermarks are not being
> > met. The problem is that zone_reclaim() failing at all means the zone
> > gets marked full.
> >
> > This can cause situations where a zone is usable, but is being skipped
> > because it has been considered full. Take a situation where a large tmpfs
> > mount is occuping a large percentage of memory overall. The pages do not
> > get cleaned or reclaimed by zone_reclaim(), but the zone gets marked full
> > and the zonelist cache considers them not worth trying in the future.
> >
> > This patch makes zone_reclaim() return more fine-grained information about
> > what occured when zone_reclaim() failued. The zone only gets marked full if
> > it really is unreclaimable. If it's a case that the scan did not occur or
> > if enough pages were not reclaimed with the limited reclaim_mode, then the
> > zone is simply skipped.
> >
> > There is a side-effect to this patch. Currently, if zone_reclaim()
> > successfully reclaimed SWAP_CLUSTER_MAX, an allocation attempt would
> > go ahead. With this patch applied, zone watermarks are rechecked after
> > zone_reclaim() does some work.

(Noted by Christoph)

This bug was introduced by commit 9276b1bc96a132f4068fdee00983c532f43d3a26
way back in 2.6.19 when the zonelist_cache was introduced. It was not intended
that zone_reclaim() aggressively consider the zone to be full when it failed
as full direct reclaim can still be an option. Due to the age of the bug,
it should be considered a -stable candidate.

> >
> > Signed-off-by: Mel Gorman <mel@xxxxxxxxx>
> > Reviewed-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
> > Reviewed-by: Rik van Riel <riel@xxxxxxxxxx>
> > Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
> > ---
> >  mm/internal.h   |    4 ++++
> >  mm/page_alloc.c |   26 ++++++++++++++++++++++----
> >  mm/vmscan.c     |   11 ++++++-----
> >  3 files changed, 32 insertions(+), 9 deletions(-)
> >
> > diff --git a/mm/internal.h b/mm/internal.h
> > index f02c750..f290c4d 100644
> > --- a/mm/internal.h
> > +++ b/mm/internal.h
> > @@ -259,4 +259,8 @@ int __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
> >  		     unsigned long start, int len, int flags,
> >  		     struct page **pages, struct vm_area_struct **vmas);
> >
> > +#define ZONE_RECLAIM_NOSCAN	-2
> > +#define ZONE_RECLAIM_FULL	-1
> > +#define ZONE_RECLAIM_SOME	0
> > +#define ZONE_RECLAIM_SUCCESS	1
> >  #endif
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index d35e753..667ffbb 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -1477,15 +1477,33 @@ zonelist_scan:
> >  		BUILD_BUG_ON(ALLOC_NO_WATERMARKS < NR_WMARK);
> >  		if (!(alloc_flags & ALLOC_NO_WATERMARKS)) {
> >  			unsigned long mark;
> > +			int ret;
> > +
> >  			mark = zone->watermark[alloc_flags & ALLOC_WMARK_MASK];
> > -			if (!zone_watermark_ok(zone, order, mark,
> > -				    classzone_idx, alloc_flags)) {
> > -				if (!zone_reclaim_mode ||
> > -				    !zone_reclaim(zone, gfp_mask, order))
> > +			if (zone_watermark_ok(zone, order, mark,
> > +				    classzone_idx, alloc_flags))
> > +				goto try_this_zone;
> > +
> > +			if (zone_reclaim_mode == 0)
> > +				goto this_zone_full;
> > +
> > +			ret = zone_reclaim(zone, gfp_mask, order);
> > +			switch (ret) {
> > +			case ZONE_RECLAIM_NOSCAN:
> > +				/* did not scan */
> > +				goto try_next_zone;
> > +			case ZONE_RECLAIM_FULL:
> > +				/* scanned but unreclaimable */
> > +				goto this_zone_full;
> > +			default:
> > +				/* did we reclaim enough */
> > +				if (!zone_watermark_ok(zone, order, mark,
> > +						classzone_idx, alloc_flags))
> >  					goto this_zone_full;
> >  			}
> >  		}
> >
> > +try_this_zone:
> >  		page = buffered_rmqueue(preferred_zone, zone, order,
> >  						gfp_mask, migratetype);
> >  		if (page)
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index d832ba8..7b8eb3f 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -2465,16 +2465,16 @@ int zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
> >  	 */
> >  	if (zone_pagecache_reclaimable(zone) <= zone->min_unmapped_pages &&
> >  	    zone_page_state(zone, NR_SLAB_RECLAIMABLE) <= zone->min_slab_pages)
> > -		return 0;
> > +		return ZONE_RECLAIM_FULL;
> >
> >  	if (zone_is_all_unreclaimable(zone))
> > -		return 0;
> > +		return ZONE_RECLAIM_FULL;
> >
> >  	/*
> >  	 * Do not scan if the allocation should not be delayed.
> >  	 */
> >  	if (!(gfp_mask & __GFP_WAIT) || (current->flags & PF_MEMALLOC))
> > -			return 0;
> > +		return ZONE_RECLAIM_NOSCAN;
> >
> >  	/*
> >  	 * Only run zone reclaim on the local zone or on zones that do not
> > @@ -2484,10 +2484,11 @@ int zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
> >  	 */
> >  	node_id = zone_to_nid(zone);
> >  	if (node_state(node_id, N_CPU) && node_id != numa_node_id())
> > -		return 0;
> > +		return ZONE_RECLAIM_NOSCAN;
> >
> >  	if (zone_test_and_set_flag(zone, ZONE_RECLAIM_LOCKED))
> > -		return 0;
> > +		return ZONE_RECLAIM_NOSCAN;
> > +
> >  	ret = __zone_reclaim(zone, gfp_mask, order);
> >  	zone_clear_flag(zone, ZONE_RECLAIM_LOCKED);
> >
> >
> 

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