linux-kernel - Re: [PATCH v13 4/5] mm: support reporting free page blocks

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Date:   Thu, 03 Aug 2017 21:17:25 +0800
From:   Wei Wang <wei.w.wang@...el.com>
To:     Michal Hocko <mhocko@...nel.org>
CC:     linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
        linux-mm@...ck.org, mst@...hat.com, mawilcox@...rosoft.com,
        akpm@...ux-foundation.org, virtio-dev@...ts.oasis-open.org,
        david@...hat.com, cornelia.huck@...ibm.com,
        mgorman@...hsingularity.net, aarcange@...hat.com,
        amit.shah@...hat.com, pbonzini@...hat.com,
        liliang.opensource@...il.com, yang.zhang.wz@...il.com,
        quan.xu@...yun.com
Subject: Re: [PATCH v13 4/5] mm: support reporting free page blocks

On 08/03/2017 08:41 PM, Michal Hocko wrote:
> 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?

report_free_page_block() has handled all the possible cases after the 
lock is
dropped. For example, if the previous reported page has not been on the free
list, then the first node from the list of this order will be given. 
This is because
page allocation takes page blocks from the head to end, for example:

1,2,3,4,5,6
if the previous reported free block is 2, when we give 2 to the report 
function
to get the next page block, and find 1,2,3 have all gone, it will report 
4, which
is the head of the free list.

> 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.
>

The above lock is held for each free page block, instead of each migrate 
type, since
the report function only reports one page block each time.


Best,
Wei