RE: [PATCH 11/15] mm: trigger page reclaim in alloc_contig_range() to stabilize watermarks

[Marek Szyprowski <m.szyprowski@samsung.com>]

Hello,

On Friday, February 03, 2012 3:04 PM Mel Gorman wrote:

> On Fri, Feb 03, 2012 at 01:18:54PM +0100, Marek Szyprowski wrote:
> > alloc_contig_range() performs memory allocation so it also should keep
> > track on keeping the correct level of memory watermarks. This commit adds
> > a call to *_slowpath style reclaim to grab enough pages to make sure that
> > the final collection of contiguous pages from freelists will not starve
> > the system.
> >
> > Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
> > Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
> > CC: Michal Nazarewicz <mina86@mina86.com>
> > Tested-by: Rob Clark <rob.clark@linaro.org>
> > Tested-by: Ohad Ben-Cohen <ohad@wizery.com>
> > Tested-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
> 
> I still do not intend to ack this patch and any damage is confined to
> CMA but I have a few comments anyway.
> 
> > ---
> >  mm/page_alloc.c |   47 +++++++++++++++++++++++++++++++++++++++++++++++
> >  1 files changed, 47 insertions(+), 0 deletions(-)
> >
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 983ccba..371a79f 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -5632,6 +5632,46 @@ static int __alloc_contig_migrate_range(unsigned long start, unsigned
> long end)
> >  	return ret > 0 ? 0 : ret;
> >  }
> >
> > +/*
> > + * Trigger memory pressure bump to reclaim some pages in order to be able to
> > + * allocate 'count' pages in single page units. Does similar work as
> > + *__alloc_pages_slowpath() function.
> > + */
> > +static int __reclaim_pages(struct zone *zone, gfp_t gfp_mask, int count)
> > +{
> > +	enum zone_type high_zoneidx = gfp_zone(gfp_mask);
> > +	struct zonelist *zonelist = node_zonelist(0, gfp_mask);
> > +	int did_some_progress = 0;
> > +	int order = 1;
> > +	unsigned long watermark;
> > +
> > +	/*
> > +	 * Increase level of watermarks to force kswapd do his job
> > +	 * to stabilize at new watermark level.
> > +	 */
> > +	min_free_kbytes += count * PAGE_SIZE / 1024;
> 
> There is a risk of overflow here although it is incredibly
> small. Still, a potentially nicer way of doing this was
> 
> count << (PAGE_SHIFT - 10)
> 
> > +	setup_per_zone_wmarks();
> > +
> 
> Nothing prevents two or more processes updating the wmarks at the same
> time which is racy and unpredictable. Today it is not much of a problem
> but CMA makes this path hotter than it was and you may see weirdness
> if two processes are updating zonelists at the same time. Swap-over-NFS
> actually starts with a patch that serialises setup_per_zone_wmarks()
> 
> You also potentially have a BIG problem here if this happens
> 
> min_free_kbytes = 32768
> Process a: min_free_kbytes  += 65536
> Process a: start direct reclaim
> echo 16374 > /proc/sys/vm/min_free_kbytes
> Process a: exit direct_reclaim
> Process a: min_free_kbytes -= 65536
> 
> min_free_kbytes now wraps negative and the machine hangs.
> 
> The damage is confined to CMA though so I am not going to lose sleep
> over it but you might want to consider at least preventing parallel
> updates to min_free_kbytes from proc.

Right. This approach was definitely too hacky. What do you think about replacing 
it with the following code (I assume that setup_per_zone_wmarks() serialization 
patch will be merged anyway so I skipped it here):

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 82f4fa5..bb9ae41 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -371,6 +371,13 @@ struct zone {
        /* see spanned/present_pages for more description */
        seqlock_t               span_seqlock;
 #endif
+#ifdef CONFIG_CMA
+       /*
+        * CMA needs to increase watermark levels during the allocation
+        * process to make sure that the system is not starved.
+        */
+       unsigned long           min_cma_pages;
+#endif
        struct free_area        free_area[MAX_ORDER];

 #ifndef CONFIG_SPARSEMEM
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 824fb37..1ca52f0 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5044,6 +5044,11 @@ void setup_per_zone_wmarks(void)

                zone->watermark[WMARK_LOW]  = min_wmark_pages(zone) + (tmp >> 2);
                zone->watermark[WMARK_HIGH] = min_wmark_pages(zone) + (tmp >> 1);
+#ifdef CONFIG_CMA
+               zone->watermark[WMARK_MIN] += zone->min_cma_pages;
+               zone->watermark[WMARK_LOW] += zone->min_cma_pages;
+               zone->watermark[WMARK_HIGH] += zone->min_cma_pages;
+#endif
                setup_zone_migrate_reserve(zone);
                spin_unlock_irqrestore(&zone->lock, flags);
        }
@@ -5625,13 +5630,15 @@ static int __reclaim_pages(struct zone *zone, gfp_t gfp_mask, int count)
        struct zonelist *zonelist = node_zonelist(0, gfp_mask);
        int did_some_progress = 0;
        int order = 1;
-       unsigned long watermark;
+       unsigned long watermark, flags;

        /*
         * Increase level of watermarks to force kswapd do his job
         * to stabilize at new watermark level.
         */
-       min_free_kbytes += count * PAGE_SIZE / 1024;
+       spin_lock_irqsave(&zone->lock, flags);
+       zone->min_cma_pages += count;
+       spin_unlock_irqrestore(&zone->lock, flags);
        setup_per_zone_wmarks();

        /* Obey watermarks as if the page was being allocated */
@@ -5648,7 +5655,9 @@ static int __reclaim_pages(struct zone *zone, gfp_t gfp_mask, int count)
        }

        /* Restore original watermark levels. */
-       min_free_kbytes -= count * PAGE_SIZE / 1024;
+       spin_lock_irqsave(&zone->lock, flags);
+       zone->min_cma_pages -= count;
+       spin_unlock_irqrestore(&zone->lock, flags);
        setup_per_zone_wmarks();

        return count;

Best regards
-- 
Marek Szyprowski
Samsung Poland R&D Center