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

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Date:   Thu, 03 Aug 2017 19:27:19 +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 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.


Best,
Wei