Re: [patch 07/11] mm: vmscan: convert unevictable page rescue scanner to per-memcg LRU lists

[Michal Hocko <mhocko@suse.cz>]

On Wed 21-09-11 15:47:51, Johannes Weiner wrote:
> On Wed, Sep 21, 2011 at 02:33:56PM +0200, Michal Hocko wrote:
> > On Mon 12-09-11 12:57:24, Johannes Weiner wrote:
> > > The global per-zone LRU lists are about to go away on memcg-enabled
> > > kernels, the unevictable page rescue scanner must be able to find its
> > > pages on the per-memcg LRU lists.
> > > 
> > > Signed-off-by: Johannes Weiner <jweiner@redhat.com>
> > 
> > The patch is correct but I guess the original implementation of
> > scan_zone_unevictable_pages is buggy (see bellow). This should be
> > addressed separatelly, though.
> > 
> > Reviewed-by: Michal Hocko <mhocko@suse.cz>
> 
> Thanks for your effort, Michal, I really appreciate it.

you're welcome. You've made really a good job so it is not that hard to
review.

> 
> > > @@ -3490,32 +3501,40 @@ void scan_mapping_unevictable_pages(struct address_space *mapping)
> > >  #define SCAN_UNEVICTABLE_BATCH_SIZE 16UL /* arbitrary lock hold batch size */
> > >  static void scan_zone_unevictable_pages(struct zone *zone)
> > >  {
> > > -	struct list_head *l_unevictable = &zone->lru[LRU_UNEVICTABLE].list;
> > > -	unsigned long scan;
> > > -	unsigned long nr_to_scan = zone_page_state(zone, NR_UNEVICTABLE);
> > > -
> > > -	while (nr_to_scan > 0) {
> > > -		unsigned long batch_size = min(nr_to_scan,
> > > -						SCAN_UNEVICTABLE_BATCH_SIZE);
> > > -
> > > -		spin_lock_irq(&zone->lru_lock);
> > > -		for (scan = 0;  scan < batch_size; scan++) {
> > > -			struct page *page = lru_to_page(l_unevictable);
> > > +	struct mem_cgroup *mem;
> > >  
> > > -			if (!trylock_page(page))
> > > -				continue;
> > > +	mem = mem_cgroup_iter(NULL, NULL, NULL);
> > > +	do {
> > > +		struct mem_cgroup_zone mz = {
> > > +			.mem_cgroup = mem,
> > > +			.zone = zone,
> > > +		};
> > > +		unsigned long nr_to_scan;
> > >  
> > > -			prefetchw_prev_lru_page(page, l_unevictable, flags);
> > > +		nr_to_scan = zone_nr_lru_pages(&mz, LRU_UNEVICTABLE);
> > > +		while (nr_to_scan > 0) {
> > > +			unsigned long batch_size;
> > > +			unsigned long scan;
> > >  
> > > -			if (likely(PageLRU(page) && PageUnevictable(page)))
> > > -				check_move_unevictable_page(page, zone);
> > > +			batch_size = min(nr_to_scan,
> > > +					 SCAN_UNEVICTABLE_BATCH_SIZE);
> > > +			spin_lock_irq(&zone->lru_lock);
> > > +			for (scan = 0; scan < batch_size; scan++) {
> > > +				struct page *page;
> > >  
> > > -			unlock_page(page);
> > > +				page = lru_tailpage(&mz, LRU_UNEVICTABLE);
> > > +				if (!trylock_page(page))
> > > +					continue;
> > 
> > We are not moving to the next page so we will try it again in the next
> > round while we already increased the scan count. In the end we will
> > missed some pages.
> 
> I guess this is about latency.  This code is only executed when the
> user requests so by writing to a proc-file, check the comment above
> scan_all_zones_unevictable_pages. I think at one point Lee wanted to
> move anon pages to the unevictable LRU when no swap is configured, but
> we have separate anon LRUs now that are not scanned without swap, and
> I think except for bugs there is no actual need to move these pages by
> hand, let alone reliably every single page.

OK, fair point. Probably not worth fixing (I will put it on my TODO list
with a low priority).
-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic

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