Re: [PATCH v29 1/4] mm: support reporting free page blocks

[Wei Wang]

On 03/27/2018 05:22 AM, Andrew Morton wrote:
> On Mon, 26 Mar 2018 10:39:51 +0800 Wei Wang <wei.w.wang@xxxxxxxxx> 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.
> >
> > ...
> >
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -4912,6 +4912,102 @@ void show_free_areas(unsigned int filter, nodemask_t *nodemask)
> >   	show_swap_cache_info();
> >   }
> >   
> > +/*
> > + * Walk through a free page list and report the found pfn range via the
> > + * callback.
> > + *
> > + * Return 0 if it completes the reporting. Otherwise, return the non-zero
> > + * value returned from the callback.
> > + */
> > +static int walk_free_page_list(void *opaque,
> > +			       struct zone *zone,
> > +			       int order,
> > +			       enum migratetype mt,
> > +			       int (*report_pfn_range)(void *,
> > +						       unsigned long,
> > +						       unsigned long))
> > +{
> > +	struct page *page;
> > +	struct list_head *list;
> > +	unsigned long pfn, flags;
> > +	int ret = 0;
> > +
> > +	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);
> > +		ret = report_pfn_range(opaque, pfn, 1 << order);
> > +		if (ret)
> > +			break;
> > +	}
> > +	spin_unlock_irqrestore(&zone->lock, flags);
> > +
> > +	return ret;
> > +}
> > +
> > +/**
> > + * 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_pfn_range: the callback to report the pfn range of the free pages
> > + *
> > + * If the callback returns a non-zero value, stop iterating the list of free
> > + * page blocks. Otherwise, continue to report.
> > + *
> > + * Please note that there are no locking guarantees for the callback and
> > + * that the reported pfn range might be freed or disappear after the
> > + * callback returns so the caller has to be very careful how it is used.
> > + *
> > + * The callback itself must not sleep or perform any operations which would
> > + * require any memory allocations directly (not even GFP_NOWAIT/GFP_ATOMIC)
> > + * or via any lock dependency. It is generally advisable to implement
> > + * the callback as simple as possible and defer any heavy lifting to a
> > + * different context.
> > + *
> > + * There is no guarantee that each free range will be reported only once
> > + * during one walk_free_mem_block invocation.
> > + *
> > + * pfn_to_page on the given range is strongly discouraged and if there is
> > + * an absolute need for that make sure to contact MM people to discuss
> > + * potential problems.
> > + *
> > + * The function itself might sleep so it cannot be called from atomic
> > + * contexts.
> I don't see how walk_free_mem_block() can sleep.

OK, it would be better to remove this sentence for the current version. 
But I think we could probably keep it if we decide to add cond_resched() 
below.

> > + * In general low orders tend to be very volatile and so it makes more
> > + * sense to query larger ones first for various optimizations which like
> > + * ballooning etc... This will reduce the overhead as well.
> > + *
> > + * Return 0 if it completes the reporting. Otherwise, return the non-zero
> > + * value returned from the callback.
> > + */
> > +int walk_free_mem_block(void *opaque,
> > +			int min_order,
> > +			int (*report_pfn_range)(void *opaque,
> > +			unsigned long pfn,
> > +			unsigned long num))
> > +{
> > +	struct zone *zone;
> > +	int order;
> > +	enum migratetype mt;
> > +	int ret;
> > +
> > +	for_each_populated_zone(zone) {
> > +		for (order = MAX_ORDER - 1; order >= min_order; order--) {
> > +			for (mt = 0; mt < MIGRATE_TYPES; mt++) {
> > +				ret = walk_free_page_list(opaque, zone,
> > +							  order, mt,
> > +							  report_pfn_range);
> > +				if (ret)
> > +					return ret;
> > +			}
==>
> > +		}
> > +	}
> > +
> > +	return 0;
> > +}
> > +EXPORT_SYMBOL_GPL(walk_free_mem_block);
> This looks like it could take a long time.  Will we end up needing to
> add cond_resched() in there somewhere?

OK. How about adding cond_resched at the above place "==>" (i.e. every 
order)?


Best,
Wei