Linux Virtualization - Jan 2018 - [PATCH v24 1/2] mm: support reporting free page blocks

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 at intel.com>
> > > Signed-off-by: Liang Li <liang.z.li at intel.com>
> > > Cc: Michal Hocko <mhocko at kernel.org>
> > > Cc: Michael S. Tsirkin <mst at redhat.com>
> > > Acked-by: Michal Hocko <mhocko at 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.
> > 
> > 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.

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.
> 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?
> 
> 
> > 
> > 
> > > +
> > > +/**
> > > + * 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
> 
> The first one here has actually been achieved by v25, where
> walk_free_mem_block returns 0 on completing the reporting, or a non-zero
> value which is returned from the callback.
> So the caller will detect errors via letting the callback to return
> something.
> 
> Best,
> Wei
> 
>

On Fri, Jan 26, 2018 at 05:00:09PM +0200, 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 at intel.com>
> > > > Signed-off-by: Liang Li <liang.z.li at intel.com>
> > > > Cc: Michal Hocko <mhocko at kernel.org>
> > > > Cc: Michael S. Tsirkin <mst at redhat.com>
> > > > Acked-by: Michal Hocko <mhocko at 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.
> > > 
> > > 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.
I've been thinking about it. How about the following scheme:
1. register balloon to get a (new) callback when free list runs empty
2. take pages off the free list, add them to the balloon specific list
3. report to host
4. readd to free list at tail
5. if callback triggers, interrupt balloon reporting to host,
   and readd to free list at tail


This needs some thought wrt what happens when there are
multiple users of this API, but looks like it will work.

-- 
MST

On Saturday, January 27, 2018 5:44 AM, Michael S. Tsirkin
wrote:
> On Fri, Jan 26, 2018 at 05:00:09PM +0200, 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:
> > > 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.
> 
> I've been thinking about it. How about the following scheme:
> 1. register balloon to get a (new) callback when free list runs empty 2.
take
> pages off the free list, add them to the balloon specific list 3. report to
host 4.
> readd to free list at tail 5. if callback triggers, interrupt balloon
reporting to
> host,
>    and readd to free list at tail
So in step 2, when we walk through the free page list, take each block, and add
them to the balloon specific list, is this performed under the mm lock? If it is
still under the lock, then what would be the difference compared to walking
through the free list, and add each block to virtqueue?

Best,
Wei

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 at intel.com>
> > > > Signed-off-by: Liang Li <liang.z.li at intel.com>
> > > > Cc: Michal Hocko <mhocko at kernel.org>
> > > > Cc: Michael S. Tsirkin <mst at redhat.com>
> > > > Acked-by: Michal Hocko <mhocko at 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.
> > >
> > > 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