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

[Wang, Wei W]

On Friday, January 26, 2018 11:00 PM, Michael S. Tsirkin wrote:
> On Fri, Jan 26, 2018 at 11:29:15AM +0800, Wei Wang wrote:
> > On 01/25/2018 09:41 PM, Michael S. Tsirkin wrote:
> > > 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@xxxxxxxxx>
> > > > Signed-off-by: Liang Li <liang.z.li@xxxxxxxxx>
> > > > Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> > > > Cc: Michael S. Tsirkin <mst@xxxxxxxxxx>
> > > > Acked-by: Michal Hocko <mhocko@xxxxxxxxxx>
> > > > ---
> > > >   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.
> > >
> > > 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.
> >
> >
> > I'm not sure how this would help. For example, we have a list of 2M
> > free page blocks:
> > A-->B-->C-->D-->E-->F-->G--H
> >
> > After reporting A and B, and put them to the end and exit, when the
> > caller comes back,
> > 1) if the list remains unchanged, then it will be
> > C-->D-->E-->F-->G-->H-->A-->B
> 
> Right. So here we can just scan until we see A, right?  It's a harder question
> what to do if A and only A has been consumed.  We don't want B to be sent
> twice ideally. OTOH maybe that isn't a big deal if it's only twice. Host might
> know page is already gone - how about host gives us a hint after using the
> buffer?
> 
> > 2) If worse, all the blocks have been split into smaller blocks and
> > used after the caller comes back.
> >
> > where could we continue?
> 
> I'm not sure. But an alternative appears to be to hold a lock and just block
> whoever wanted to use any pages.  Yes we are sending hints faster but
> apparently something wanted these pages, and holding the lock is interfering
> with this something.
> 
> >
> > The reason to think about "restart" is the worry about the virtqueue
> > is full, right? But we've agreed that losing some hints to report
> > isn't important, and in practice, the virtqueue won't be full as the
> > host side is faster.
> 
> It would be more convincing if we sent e.g. higher order pages first. As it is - it
> won't take long to stuff ring full of 4K pages and it seems highly unlikely that
> host won't ever be scheduled out.

Yes, actually we've already sent higher order pages first, please check this patch, we have:

for (order = MAX_ORDER - 1; order >= min_order; order--)
--> start from high order blocks. 

> 
> Can we maybe agree on what kind of benchmark makes sense for this work?
> I'm concerned that we are laser focused on just how long does it take to
> migrate ignoring e.g. slowdowns after migration.

Testing the time of linux compilation during migration? or what benchmark do you have in mind? 
We can compare how long it takes during legacy live migration and live migration with this feature?

If you really worry about this, we could also provide an configure option, e.g. under /sys/, for the guest to decide whether to enable or disable reporting free page hints to the host at any time. If disabled, in the balloon driver we skip the calling of walk_free_mem_block().


> > I'm concerned that actions on the free list may cause more controversy
> > though it might be safe to do from some aspect, and would be hard to
> > end debating. If possible, we could go with the most prudent approach
> > for now, and have more discussions in future improvement patches. What
> > would you think?
> 
> Well I'm not 100% about restartability. But keeping pages freed by host near
> head of the list looks kind of wrong.
> Try to float a patch on top for the rotation and see what happens?

I didn't get it, "pages freed by host", what does that mean?

Best,
Wei