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

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.

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

> 
> 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.
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
> 
> --
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> the body to majordomo at kvack.org.  For more info on Linux MM,
> see: http://www.linux-mm.org/ .
> Don't email: <a href=mailto:"dont at kvack.org"> email
at kvack.org </a>
>

On Thu, Jan 25, 2018 at 09:56:01AM -0500, Pankaj Gupta
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.
> 
> 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
The main difference between Wei's and Nitesh's patches add hints to page
alloc/free path. It's this more risky performance-wise: you need to
enable it at guest boot, not just at migration time.
Maybe the overhead isn't big, unfortunately no one posted any
numbers yet.


-- 
MST

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

2) If worse, all the blocks have been split into smaller blocks and used 
after the caller comes back.

where could we continue?


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.

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?


>
>
>> +
>> +/**
>> + * 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 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
> 
>