Re: [PATCH v15 4/5] mm: support reporting free page blocks

[Wei Wang]

On 08/28/2017 09:33 PM, Michal Hocko wrote:
> On Mon 28-08-17 18:08:32, Wei Wang wrote:
> > This patch adds support to walk through the free page blocks in the
> > system and report them via a callback function. Some page blocks may
> > leave the free list after zone->lock is released, so it is the caller's
> > responsibility to either detect or prevent the use of such pages.
> >
> > One use example of this patch is to accelerate live migration by skipping
> > the transfer of free pages reported from the guest. A popular method used
> > by the hypervisor to track which part of memory is written during live
> > migration is to write-protect all the guest memory. So, those pages that
> > are reported as free pages but are written after the report function
> > returns will be captured by the hypervisor, and they will be added to the
> > next round of memory transfer.
> OK, looks much better. I still have few nits.
>
> > +extern void walk_free_mem_block(void *opaque,
> > +				int min_order,
> > +				bool (*report_page_block)(void *, unsigned long,
> > +							  unsigned long));
> > +
> please add names to arguments of the prototype
>
> >   /*
> >    * Free reserved pages within range [PAGE_ALIGN(start), end & PAGE_MASK)
> >    * into the buddy system. The freed pages will be poisoned with pattern
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 6d00f74..81eedc7 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -4762,6 +4762,71 @@ void show_free_areas(unsigned int filter, nodemask_t *nodemask)
> >   	show_swap_cache_info();
> >   }
> >   
> > +/**
> > + * walk_free_mem_block - Walk through the free page blocks in the system
> > + * @opaque: the context passed from the caller
> > + * @min_order: the minimum order of free lists to check
> > + * @report_page_block: the callback function to report free page blocks
> page_block has meaning in the core MM which doesn't strictly match its
> usage here. Moreover we are reporting pfn ranges rather than struct page
> range. So report_pfn_range would suit better.
>
> [...]
> > +	for_each_populated_zone(zone) {
> > +		for (order = MAX_ORDER - 1; order >= min_order; order--) {
> > +			for (mt = 0; !stop && mt < MIGRATE_TYPES; mt++) {
> > +				spin_lock_irqsave(&zone->lock, flags);
> > +				list = &zone->free_area[order].free_list[mt];
> > +				list_for_each_entry(page, list, lru) {
> > +					pfn = page_to_pfn(page);
> > +					stop = report_page_block(opaque, pfn,
> > +								 1 << order);
> > +					if (stop)
> > +						break;
> 					if (stop) {
> 						spin_unlock_irqrestore(&zone->lock, flags);
> 						return;
> 					}
>
> would be both easier and less error prone. E.g. You wouldn't pointlessly
> iterate over remaining orders just to realize there is nothing to be
> done for those...

Yes, that's better, thanks. I will take other suggestions as well.

Best,
Wei