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

    On Friday, February 10, 2012 12:19 PM Mel Gorman wrote:

    > On Wed, Feb 08, 2012 at 04:14:46PM +0100, Marek Szyprowski wrote:
    > > > > <SNIP>
    > > > > +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):
    > >
    >
    > It's part of a larger series and the rest of that series is
    > controversial. That single patch can be split out obviously so feel free
    > to add it to your series and stick your Signed-off-by on the end of it.

    Ok, I will grab it from that series.

    > > 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);
    > > }
    >
    > This is better in that it is not vunerable to parallel updates of
    > min_free_kbytes. It would be slightly tidier to introduce something
    > like cma_wmark_pages() that returns min_cma_pages if CONFIG_CMA and 0
    > otherwise. Use the helper to get right of this ifdef CONFIG_CMA within
    > setup_per_zone_wmarks().

    Thanks for the hint. I will submit this change soon.

    > You'll still have the problem of kswapd not taking CMA pages properly into
    > account when deciding whether to reclaim or not though.

    This will only affect CMA clients, I hope we can fix this later once the patches
    get merged. I would really like to get it into v3.4, what means that it should be
    tested in next asap.

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




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