Re: [PATCH v13 4/5] mm: support reporting free page blocks

[Wei Wang]

On 08/03/2017 07:28 PM, Michal Hocko wrote:
> On Thu 03-08-17 19:27:19, Wei Wang wrote:
> > On 08/03/2017 06:44 PM, Michal Hocko wrote:
> > > On Thu 03-08-17 18:42:15, Wei Wang wrote:
> > > > On 08/03/2017 05:11 PM, Michal Hocko wrote:
> > > > > On Thu 03-08-17 14:38:18, Wei Wang wrote:
> > > [...]
> > > > > > +static int report_free_page_block(struct zone *zone, unsigned int order,
> > > > > > +				  unsigned int migratetype, struct page **page)
> > > > > This is just too ugly and wrong actually. Never provide struct page
> > > > > pointers outside of the zone->lock. What I've had in mind was to simply
> > > > > walk free lists of the suitable order and call the callback for each one.
> > > > > Something as simple as
> > > > >
> > > > > 	for (i = 0; i < MAX_NR_ZONES; i++) {
> > > > > 		struct zone *zone = &pgdat->node_zones[i];
> > > > >
> > > > > 		if (!populated_zone(zone))
> > > > > 			continue;
> > > > > 		spin_lock_irqsave(&zone->lock, flags);
> > > > > 		for (order = min_order; order < MAX_ORDER; ++order) {
> > > > > 			struct free_area *free_area = &zone->free_area[order];
> > > > > 			enum migratetype mt;
> > > > > 			struct page *page;
> > > > >
> > > > > 			if (!free_area->nr_pages)
> > > > > 				continue;
> > > > >
> > > > > 			for_each_migratetype_order(order, mt) {
> > > > > 				list_for_each_entry(page,
> > > > > 						&free_area->free_list[mt], lru) {
> > > > >
> > > > > 					pfn = page_to_pfn(page);
> > > > > 					visit(opaque2, prn, 1<<order);
> > > > > 				}
> > > > > 			}
> > > > > 		}
> > > > >
> > > > > 		spin_unlock_irqrestore(&zone->lock, flags);
> > > > > 	}
> > > > >
> > > > > [...]
> > > > I think the above would take the lock for too long time. That's why we
> > > > prefer to take one free page block each time, and taking it one by one
> > > > also doesn't make a difference, in terms of the performance that we
> > > > need.
> > > I think you should start with simple approach and impove incrementally
> > > if this turns out to be not optimal. I really detest taking struct pages
> > > outside of the lock. You never know what might happen after the lock is
> > > dropped. E.g. can you race with the memory hotremove?
> >
> > The caller won't use pages returned from the function, so I think there
> > shouldn't be an issue or race if the returned pages are used (i.e. not free
> > anymore) or simply gone due to hotremove.
> No, this is just too error prone. Consider that struct page pointer
> itself could get invalid in the meantime. Please always keep robustness
> in mind first. Optimizations are nice but it is even not clear whether
> the simple variant will cause any problems.


how about this:

for_each_populated_zone(zone) {
              for_each_migratetype_order_decend(min_order, order, type) {
                    do {
     =>                  spin_lock_irqsave(&zone->lock, flags);
                        ret = report_free_page_block(zone, order, type,
                             &page)) {
                               pfn = page_to_pfn(page);
                               nr_pages = 1 << order;
                               visit(opaque1, pfn, nr_pages);
                         }
     => spin_unlock_irqrestore(&zone->lock, flags);
                    } while (!ret)
}

In this way, we can still keep the lock granularity at one free page block
while having the struct page operated under the lock.



Best,
Wei