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Date: Wed, 08 Feb 2012 16:14:46 +0100
From: Marek Szyprowski <m.szyprowski@...sung.com>
To: 'Mel Gorman' <mel@....ul.ie>
Cc: linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
linux-media@...r.kernel.org, linux-mm@...ck.org,
linaro-mm-sig@...ts.linaro.org,
'Michal Nazarewicz' <mina86@...a86.com>,
'Kyungmin Park' <kyungmin.park@...sung.com>,
'Russell King' <linux@....linux.org.uk>,
'Andrew Morton' <akpm@...ux-foundation.org>,
'KAMEZAWA Hiroyuki' <kamezawa.hiroyu@...fujitsu.com>,
'Daniel Walker' <dwalker@...eaurora.org>,
'Arnd Bergmann' <arnd@...db.de>,
'Jesse Barker' <jesse.barker@...aro.org>,
'Jonathan Corbet' <corbet@....net>,
'Shariq Hasnain' <shariq.hasnain@...aro.org>,
'Chunsang Jeong' <chunsang.jeong@...aro.org>,
'Dave Hansen' <dave@...ux.vnet.ibm.com>,
'Benjamin Gaignard' <benjamin.gaignard@...aro.org>,
'Rob Clark' <rob.clark@...aro.org>,
'Ohad Ben-Cohen' <ohad@...ery.com>
Subject: RE: [PATCH 11/15] mm: trigger page reclaim in alloc_contig_range() to
stabilize watermarks
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@...sung.com>
> > Signed-off-by: Kyungmin Park <kyungmin.park@...sung.com>
> > CC: Michal Nazarewicz <mina86@...a86.com>
> > Tested-by: Rob Clark <rob.clark@...aro.org>
> > Tested-by: Ohad Ben-Cohen <ohad@...ery.com>
> > Tested-by: Benjamin Gaignard <benjamin.gaignard@...aro.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
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