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

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Date:   Thu, 03 Aug 2017 20:11:58 +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 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