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Michal Hocko

2017-Aug-03 11:28 UTC

[PATCH v13 4/5] mm: support reporting free page blocks

On Thu 03-08-17 19:27:19, Wei Wang wrote:> On 08/03/2017 06:44 PM, Michal Hocko wrote:
> >On Thu 03-08-17 18:42:15, Wei Wang wrote:
> >>On 08/03/2017 05:11 PM, Michal Hocko wrote:
> >>>On Thu 03-08-17 14:38:18, Wei Wang wrote:
> >[...]
> >>>>+static int report_free_page_block(struct zone *zone,
unsigned int order,
> >>>>+				  unsigned int migratetype, struct page **page)
> >>>This is just too ugly and wrong actually. Never provide struct
page
> >>>pointers outside of the zone->lock. What I've had in
mind was to simply
> >>>walk free lists of the suitable order and call the callback for
each one.
> >>>Something as simple as
> >>>
> >>>	for (i = 0; i < MAX_NR_ZONES; i++) {
> >>>		struct zone *zone = &pgdat->node_zones[i];
> >>>
> >>>		if (!populated_zone(zone))
> >>>			continue;
> >>>		spin_lock_irqsave(&zone->lock, flags);
> >>>		for (order = min_order; order < MAX_ORDER; ++order) {
> >>>			struct free_area *free_area =
&zone->free_area[order];
> >>>			enum migratetype mt;
> >>>			struct page *page;
> >>>
> >>>			if (!free_area->nr_pages)
> >>>				continue;
> >>>
> >>>			for_each_migratetype_order(order, mt) {
> >>>				list_for_each_entry(page,
> >>>						&free_area->free_list[mt], lru) {
> >>>
> >>>					pfn = page_to_pfn(page);
> >>>					visit(opaque2, prn, 1<<order);
> >>>				}
> >>>			}
> >>>		}
> >>>
> >>>		spin_unlock_irqrestore(&zone->lock, flags);
> >>>	}
> >>>
> >>>[...]
> >>
> >>I think the above would take the lock for too long time. That's
why we
> >>prefer to take one free page block each time, and taking it one by
one
> >>also doesn't make a difference, in terms of the performance
that we
> >>need.
> >I think you should start with simple approach and impove incrementally
> >if this turns out to be not optimal. I really detest taking struct
pages
> >outside of the lock. You never know what might happen after the lock is
> >dropped. E.g. can you race with the memory hotremove?
> 
> 
> The caller won't use pages returned from the function, so I think there
> shouldn't be an issue or race if the returned pages are used (i.e. not
free
> anymore) or simply gone due to hotremove.
No, this is just too error prone. Consider that struct page pointer
itself could get invalid in the meantime. Please always keep robustness
in mind first. Optimizations are nice but it is even not clear whether
the simple variant will cause any problems.
-- 
Michal Hocko
SUSE Labs

Wei Wang

2017-Aug-03 12:11 UTC

head link

[PATCH v13 4/5] mm: support reporting free page blocks

On 08/03/2017 07:28 PM, Michal Hocko wrote:> On Thu 03-08-17 19:27:19, Wei Wang wrote:
>> On 08/03/2017 06:44 PM, Michal Hocko wrote:
>>> On Thu 03-08-17 18:42:15, Wei Wang wrote:
>>>> On 08/03/2017 05:11 PM, Michal Hocko wrote:
>>>>> On Thu 03-08-17 14:38:18, Wei Wang wrote:
>>> [...]
>>>>>> +static int report_free_page_block(struct zone *zone,
unsigned int order,
>>>>>> +				  unsigned int migratetype, struct page **page)
>>>>> This is just too ugly and wrong actually. Never provide
struct page
>>>>> pointers outside of the zone->lock. What I've had in
mind was to simply
>>>>> walk free lists of the suitable order and call the callback
for each one.
>>>>> Something as simple as
>>>>>
>>>>> 	for (i = 0; i < MAX_NR_ZONES; i++) {
>>>>> 		struct zone *zone = &pgdat->node_zones[i];
>>>>>
>>>>> 		if (!populated_zone(zone))
>>>>> 			continue;
>>>>> 		spin_lock_irqsave(&zone->lock, flags);
>>>>> 		for (order = min_order; order < MAX_ORDER; ++order) {
>>>>> 			struct free_area *free_area =
&zone->free_area[order];
>>>>> 			enum migratetype mt;
>>>>> 			struct page *page;
>>>>>
>>>>> 			if (!free_area->nr_pages)
>>>>> 				continue;
>>>>>
>>>>> 			for_each_migratetype_order(order, mt) {
>>>>> 				list_for_each_entry(page,
>>>>> 						&free_area->free_list[mt], lru) {
>>>>>
>>>>> 					pfn = page_to_pfn(page);
>>>>> 					visit(opaque2, prn, 1<<order);
>>>>> 				}
>>>>> 			}
>>>>> 		}
>>>>>
>>>>> 		spin_unlock_irqrestore(&zone->lock, flags);
>>>>> 	}
>>>>>
>>>>> [...]
>>>> I think the above would take the lock for too long time.
That's why we
>>>> prefer to take one free page block each time, and taking it one
by one
>>>> also doesn't make a difference, in terms of the performance
that we
>>>> need.
>>> I think you should start with simple approach and impove
incrementally
>>> if this turns out to be not optimal. I really detest taking struct
pages
>>> outside of the lock. You never know what might happen after the
lock is
>>> dropped. E.g. can you race with the memory hotremove?
>>
>> The caller won't use pages returned from the function, so I think
there
>> shouldn't be an issue or race if the returned pages are used (i.e.
not free
>> anymore) or simply gone due to hotremove.
> No, this is just too error prone. Consider that struct page pointer
> itself could get invalid in the meantime. Please always keep robustness
> in mind first. Optimizations are nice but it is even not clear whether
> the simple variant will cause any problems.

how about this:

for_each_populated_zone(zone) {
               for_each_migratetype_order_decend(min_order, order, type) {
                     do {
      =>                  spin_lock_irqsave(&zone->lock, flags);
                         ret = report_free_page_block(zone, order, type,
                              &page)) {
                                pfn = page_to_pfn(page);
                                nr_pages = 1 << order;
                                visit(opaque1, pfn, nr_pages);
                          }
      => spin_unlock_irqrestore(&zone->lock, flags);
                     } while (!ret)
}

In this way, we can still keep the lock granularity at one free page block
while having the struct page operated under the lock.



Best,
Wei

Michal Hocko

2017-Aug-03 12:41 UTC

head link

[PATCH v13 4/5] mm: support reporting free page blocks

On Thu 03-08-17 20:11:58, Wei Wang wrote:> On 08/03/2017 07:28 PM, Michal Hocko wrote:
> >On Thu 03-08-17 19:27:19, Wei Wang wrote:
> >>On 08/03/2017 06:44 PM, Michal Hocko wrote:
> >>>On Thu 03-08-17 18:42:15, Wei Wang wrote:
> >>>>On 08/03/2017 05:11 PM, Michal Hocko wrote:
> >>>>>On Thu 03-08-17 14:38:18, Wei Wang wrote:
> >>>[...]
> >>>>>>+static int report_free_page_block(struct zone
*zone, unsigned int order,
> >>>>>>+				  unsigned int migratetype, struct page
**page)
> >>>>>This is just too ugly and wrong actually. Never provide
struct page
> >>>>>pointers outside of the zone->lock. What I've
had in mind was to simply
> >>>>>walk free lists of the suitable order and call the
callback for each one.
> >>>>>Something as simple as
> >>>>>
> >>>>>	for (i = 0; i < MAX_NR_ZONES; i++) {
> >>>>>		struct zone *zone = &pgdat->node_zones[i];
> >>>>>
> >>>>>		if (!populated_zone(zone))
> >>>>>			continue;
> >>>>>		spin_lock_irqsave(&zone->lock, flags);
> >>>>>		for (order = min_order; order < MAX_ORDER;
++order) {
> >>>>>			struct free_area *free_area =
&zone->free_area[order];
> >>>>>			enum migratetype mt;
> >>>>>			struct page *page;
> >>>>>
> >>>>>			if (!free_area->nr_pages)
> >>>>>				continue;
> >>>>>
> >>>>>			for_each_migratetype_order(order, mt) {
> >>>>>				list_for_each_entry(page,
> >>>>>						&free_area->free_list[mt], lru) {
> >>>>>
> >>>>>					pfn = page_to_pfn(page);
> >>>>>					visit(opaque2, prn, 1<<order);
> >>>>>				}
> >>>>>			}
> >>>>>		}
> >>>>>
> >>>>>		spin_unlock_irqrestore(&zone->lock, flags);
> >>>>>	}
> >>>>>
> >>>>>[...]
> >>>>I think the above would take the lock for too long time.
That's why we
> >>>>prefer to take one free page block each time, and taking it
one by one
> >>>>also doesn't make a difference, in terms of the
performance that we
> >>>>need.
> >>>I think you should start with simple approach and impove
incrementally
> >>>if this turns out to be not optimal. I really detest taking
struct pages
> >>>outside of the lock. You never know what might happen after the
lock is
> >>>dropped. E.g. can you race with the memory hotremove?
> >>
> >>The caller won't use pages returned from the function, so I
think there
> >>shouldn't be an issue or race if the returned pages are used
(i.e. not free
> >>anymore) or simply gone due to hotremove.
> >No, this is just too error prone. Consider that struct page pointer
> >itself could get invalid in the meantime. Please always keep robustness
> >in mind first. Optimizations are nice but it is even not clear whether
> >the simple variant will cause any problems.
> 
> 
> how about this:
> 
> for_each_populated_zone(zone) {
>               for_each_migratetype_order_decend(min_order, order, type) {
>                     do {
>      =>                  spin_lock_irqsave(&zone->lock, flags);
>                         ret = report_free_page_block(zone, order, type,
>                              &page)) {
>                                pfn = page_to_pfn(page);
>                                nr_pages = 1 << order;
>                                visit(opaque1, pfn, nr_pages);
>                          }
>      => spin_unlock_irqrestore(&zone->lock, flags);
>                     } while (!ret)
> }
> 
> In this way, we can still keep the lock granularity at one free page block
> while having the struct page operated under the lock.
How can you continue iteration of free_list after the lock has been
dropped? If you want to keep the lock held for each migrate type then
why not. Just push the lock inside for_each_migratetype_order loop from
my example.

-- 
Michal Hocko
SUSE Labs

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