Linux Virtualization - Apr 2018 - [PATCH v29 1/4] mm: support reporting free page blocks

On Mon, 26 Mar 2018 10:39:51 +0800 Wei Wang <wei.w.wang at intel.com>
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.
> + * 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?
>  static void zoneref_set_zone(struct zone *zone, struct zoneref *zoneref)
>  {
>  	zoneref->zone = zone;
> -- 
> 2.7.4

On 03/27/2018 05:22 AM, Andrew Morton wrote:
> On Mon, 26 Mar 2018 10:39:51 +0800 Wei Wang <wei.w.wang at intel.com>
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

On Tue 27-03-18 14:23:51, Wei Wang wrote:
> On 03/27/2018 05:22 AM, Andrew Morton wrote:
> > On Mon, 26 Mar 2018 10:39:51 +0800 Wei Wang <wei.w.wang at
intel.com> 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.
The point of this sentence was to make any user aware that the function
might sleep from the very begining rather than chase existing callers
when we need to add cond_resched or sleep for any other reason. So I
would rather keep it.
-- 
Michal Hocko
SUSE Labs

On Mon, Mar 26, 2018 at 02:22:54PM -0700, Andrew Morton
wrote:
> On Mon, 26 Mar 2018 10:39:51 +0800 Wei Wang <wei.w.wang at intel.com>
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.
> 
> > + * 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?
Andrew, were your questions answered? If yes could I bother you for an ack on
this?

> >  static void zoneref_set_zone(struct zone *zone, struct zoneref
*zoneref)
> >  {
> >  	zoneref->zone = zone;
> > -- 
> > 2.7.4

On Tue, 10 Apr 2018 21:19:31 +0300 "Michael S. Tsirkin" <mst at
redhat.com> wrote:
> 
> Andrew, were your questions answered? If yes could I bother you for an ack
on this?
> 
Still not very happy that readers are told that "this function may
sleep" when it clearly doesn't do so.  If we wish to be able to change
it to sleep in the future then that should be mentioned.  And even put a
might_sleep() in there, to catch people who didn't read the comments...

Otherwise it looks OK.