Re: [PATCH v24 1/2] mm: support reporting free page blocks

[Pankaj Gupta <pagupta@redhat.com>]

> 
> On Wed, Jan 24, 2018 at 06:42:41PM +0800, 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.
> > 
> > Signed-off-by: Wei Wang <wei.w.wang@intel.com>
> > Signed-off-by: Liang Li <liang.z.li@intel.com>
> > Cc: Michal Hocko <mhocko@kernel.org>
> > Cc: Michael S. Tsirkin <mst@redhat.com>
> > Acked-by: Michal Hocko <mhocko@kernel.org>
> > ---
> >  include/linux/mm.h |  6 ++++
> >  mm/page_alloc.c    | 91
> >  ++++++++++++++++++++++++++++++++++++++++++++++++++++++
> >  2 files changed, 97 insertions(+)
> > 
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index ea818ff..b3077dd 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -1938,6 +1938,12 @@ extern void free_area_init_node(int nid, unsigned
> > long * zones_size,
> >  		unsigned long zone_start_pfn, unsigned long *zholes_size);
> >  extern void free_initmem(void);
> >  
> > +extern void walk_free_mem_block(void *opaque,
> > +				int min_order,
> > +				bool (*report_pfn_range)(void *opaque,
> > +							 unsigned long pfn,
> > +							 unsigned long num));
> > +
> >  /*
> >   * 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 76c9688..705de22 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -4899,6 +4899,97 @@ 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 false if the callback requests to stop reporting. Otherwise,
> > + * return true.
> > + */
> > +static bool walk_free_page_list(void *opaque,
> > +				struct zone *zone,
> > +				int order,
> > +				enum migratetype mt,
> > +				bool (*report_pfn_range)(void *,
> > +							 unsigned long,
> > +							 unsigned long))
> > +{
> > +	struct page *page;
> > +	struct list_head *list;
> > +	unsigned long pfn, flags;
> > +	bool ret;
> > +
> > +	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;
> > +}
> 
> There are two issues with this API. One is that it is not
> restarteable: if you return false, you start from the
> beginning. So no way to drop lock, do something slow
> and then proceed.
> 
> Another is that you are using it to report free page hints. Presumably
> the point is to drop these pages - keeping them near head of the list
> and reusing the reported ones will just make everything slower
> invalidating the hint.

I think that's where patches[1] by 'Nitesh' will help: This patch-set
will send free page hints transparently to host and host decides to delete such 
pages.

If I compare with patchset by 'Wei', host gets/asks free page hints and ignore 
such pages during live migration. But as already discussed, if free pages are 
still in guest memory there is no point of traversing & getting all such pages
again.

[1] https://www.spinics.net/lists/kvm/msg159790.html

> 
> How about rotating these pages towards the end of the list?
> Probably not on each call, callect reported pages and then
> move them to tail when we exit.
> 
> Of course it's possible not all reporters want this.
> So maybe change the callback to return int:
>  0 - page reported, move page to end of free list
>  > 0 - page skipped, proceed
>  < 0 - stop processing
> 
> 
> > +
> > +/**
> > + * 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 false, 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.
> > + *
> > + * 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.
> > + */
> > +void walk_free_mem_block(void *opaque,
> > +			 int min_order,
> > +			 bool (*report_pfn_range)(void *opaque,
> > +						  unsigned long pfn,
> > +						  unsigned long num))
> > +{
> > +	struct zone *zone;
> > +	int order;
> > +	enum migratetype mt;
> > +	bool 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;
> > +			}
> > +		}
> > +	}
> > +}
> > +EXPORT_SYMBOL_GPL(walk_free_mem_block);
> > +
> 
> I think callers need a way to
> 1. distinguish between completion and exit on error
> 2. restart from where we stopped
> 
> So I would both accept and return the current zone
> and a special value to mean "complete"
> 
> >  static void zoneref_set_zone(struct zone *zone, struct zoneref *zoneref)
> >  {
> >  	zoneref->zone = zone;
> > --
> > 2.7.4
> 
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