Re: [PATCH v2 4/8] mm/isolation: close the two race problems related to pageblock isolation

[Joonsoo Kim <iamjoonsoo.kim@lge.com>]

On Tue, Aug 12, 2014 at 05:17:45AM +0000, Minchan Kim wrote:
> On Wed, Aug 06, 2014 at 04:18:33PM +0900, Joonsoo Kim wrote:
> > 2. #1 requires IPI for synchronization and we can't hold the zone lock
> > during processing IPI. In this time, some pages could be moved from buddy
> > list to pcp list on page allocation path and later it could be moved again
> > from pcp list to buddy list. In this time, this page would be on isolate
> > pageblock, so, the hook is required on free_pcppages_bulk() to prevent
> > misplacement. To remove this possibility, disabling and draining pcp
> > list is needed during isolation. It guaratees that there is no page on pcp
> > list on all cpus while isolation, so misplacement problem can't happen.
> > 
> > Note that this doesn't fix freepage counting problem. To fix it,
> > we need more logic. Following patches will do it.
> 
> I hope to revise description in next spin. It's very hard to parse for
> stupid me.

Okay. I will do it.

> 
> > 
> > Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> > ---
> >  mm/internal.h       |    2 ++
> >  mm/page_alloc.c     |   27 ++++++++++++++++++++-------
> >  mm/page_isolation.c |   45 +++++++++++++++++++++++++++++++++------------
> >  3 files changed, 55 insertions(+), 19 deletions(-)
> > 
> > diff --git a/mm/internal.h b/mm/internal.h
> > index a1b651b..81b8884 100644
> > --- a/mm/internal.h
> > +++ b/mm/internal.h
> > @@ -108,6 +108,8 @@ extern pmd_t *mm_find_pmd(struct mm_struct *mm, unsigned long address);
> >  /*
> >   * in mm/page_alloc.c
> >   */
> > +extern void zone_pcp_disable(struct zone *zone);
> > +extern void zone_pcp_enable(struct zone *zone);
> 
> Nit: Some of pcp functions has prefix zone but others don't.
> Which is better? If function has param zone as first argument,
> I think it's clear unless the function don't have prefix zone.

Okay.

> 
> >  extern void __free_pages_bootmem(struct page *page, unsigned int order);
> >  extern void prep_compound_page(struct page *page, unsigned long order);
> >  #ifdef CONFIG_MEMORY_FAILURE
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 3e1e344..4517b1d 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -726,11 +726,7 @@ static void free_pcppages_bulk(struct zone *zone, int count,
> >  			/* MIGRATE_MOVABLE list may include MIGRATE_RESERVEs */
> >  			__free_one_page(page, page_to_pfn(page), zone, 0, mt);
> >  			trace_mm_page_pcpu_drain(page, 0, mt);
> > -			if (likely(!is_migrate_isolate_page(page))) {
> > -				__mod_zone_page_state(zone, NR_FREE_PAGES, 1);
> > -				if (is_migrate_cma(mt))
> > -					__mod_zone_page_state(zone, NR_FREE_CMA_PAGES, 1);
> > -			}
> > +			__mod_zone_freepage_state(zone, 1, mt);
> >  		} while (--to_free && --batch_free && !list_empty(list));
> >  	}
> >  	spin_unlock(&zone->lock);
> > @@ -789,8 +785,8 @@ static void __free_pages_ok(struct page *page, unsigned int order)
> >  	if (!free_pages_prepare(page, order))
> >  		return;
> >  
> > -	migratetype = get_pfnblock_migratetype(page, pfn);
> >  	local_irq_save(flags);
> > +	migratetype = get_pfnblock_migratetype(page, pfn);
> 
> Could you add some comment about page-isolated locking rule in somewhere?
> I think it's valuable to add it in code rather than description.

Will do.

> In addition, as your description, get_pfnblock_migratetype should be
> protected by irq_disabled. Then, it would be better to add a comment or
> VM_BUG_ON check with irq_disabled in get_pfnblock_migratetype but I think
> get_pfnblock_migratetype might be called for other purpose in future.
> In that case, it's not necessary to disable irq so we could introduce
> "get_freeing_page_migratetype" with irq disabled check and use it.

Okay.

> 
> Question. soft_offline_page doesn't have any lock
> for get_pageblock_migratetype. Is it okay?

Hmm... I think it is okay. But, I guess that it need to check
return value of set_migratetype_isolate().

> 
> >  	__count_vm_events(PGFREE, 1 << order);
> >  	set_freepage_migratetype(page, migratetype);
> >  	free_one_page(page_zone(page), page, pfn, order, migratetype);
> > @@ -1410,9 +1406,9 @@ void free_hot_cold_page(struct page *page, bool cold)
> >  	if (!free_pages_prepare(page, 0))
> >  		return;
> >  
> > +	local_irq_save(flags);
> >  	migratetype = get_pfnblock_migratetype(page, pfn);
> >  	set_freepage_migratetype(page, migratetype);
> > -	local_irq_save(flags);
> >  	__count_vm_event(PGFREE);
> >  
> >  	/*
> > @@ -6469,6 +6465,23 @@ void free_contig_range(unsigned long pfn, unsigned nr_pages)
> >  }
> >  #endif
> >  
> > +#ifdef CONFIG_MEMORY_ISOLATION
> > +void zone_pcp_disable(struct zone *zone)
> > +{
> > +	mutex_lock(&pcp_batch_high_lock);
> > +	pageset_update(zone, 1, 1);
> > +}
> > +
> > +void zone_pcp_enable(struct zone *zone)
> > +{
> > +	int high, batch;
> > +
> > +	pageset_get_values(zone, &high, &batch);
> > +	pageset_update(zone, high, batch);
> > +	mutex_unlock(&pcp_batch_high_lock);
> > +}
> > +#endif
> 
> Nit:
> It is used for only page_isolation.c so how about moving to page_isolation.c?

I'd like to leave pcp management code in page_alloc.c.

> > +
> >  #ifdef CONFIG_MEMORY_HOTPLUG
> >  /*
> >   * The zone indicated has a new number of managed_pages; batch sizes and percpu
> > diff --git a/mm/page_isolation.c b/mm/page_isolation.c
> > index 3100f98..439158d 100644
> > --- a/mm/page_isolation.c
> > +++ b/mm/page_isolation.c
> > @@ -16,9 +16,10 @@ int set_migratetype_isolate(struct page *page, bool skip_hwpoisoned_pages)
> >  	struct memory_isolate_notify arg;
> >  	int notifier_ret;
> >  	int ret = -EBUSY;
> > +	unsigned long nr_pages;
> > +	int migratetype;
> >  
> >  	zone = page_zone(page);
> > -
> 
> Unnecessary change.

Okay.

> 
> >  	spin_lock_irqsave(&zone->lock, flags);
> >  
> >  	pfn = page_to_pfn(page);
> > @@ -55,20 +56,32 @@ int set_migratetype_isolate(struct page *page, bool skip_hwpoisoned_pages)
> >  	 */
> >  
> >  out:
> > -	if (!ret) {
> > -		unsigned long nr_pages;
> > -		int migratetype = get_pageblock_migratetype(page);
> > +	if (ret) {
> > +		spin_unlock_irqrestore(&zone->lock, flags);
> > +		return ret;
> > +	}
> >  
> > -		set_pageblock_migratetype(page, MIGRATE_ISOLATE);
> > -		nr_pages = move_freepages_block(zone, page, MIGRATE_ISOLATE);
> > +	migratetype = get_pageblock_migratetype(page);
> > +	set_pageblock_migratetype(page, MIGRATE_ISOLATE);
> > +	spin_unlock_irqrestore(&zone->lock, flags);
> >  
> > -		__mod_zone_freepage_state(zone, -nr_pages, migratetype);
> > -	}
> > +	zone_pcp_disable(zone);
> 
> You pcp disable/enable per pageblock so that overhead would be severe.
> I believe your remaining patches will solve it. Anyway, let's add "
> XXX: should save pcp disable/enable" and you could remove the comment
> when your further patches handles it so reviewer could be happy with
> fact which author already know the problem and someone could solve
> the issue even though your furhter patches might reject.

Okay.

Thanks.

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