Re: [PATCH kernel v8 3/4] mm: add inerface to offer info about unused pages

[Wei Wang <wei.w.wang@intel.com>]

On 03/30/2017 01:48 AM, Michael S. Tsirkin wrote:
> On Fri, Mar 17, 2017 at 02:55:33PM +0800, Wei Wang wrote:
>> On 03/17/2017 05:28 AM, Andrew Morton wrote:
>>> On Thu, 16 Mar 2017 15:08:46 +0800 Wei Wang <wei.w.wang@intel.com> wrote:
>>>
>>>> From: Liang Li <liang.z.li@intel.com>
>>>>
>>>> This patch adds a function to provides a snapshot of the present system
>>>> unused pages. An important usage of this function is to provide the
>>>> unsused pages to the Live migration thread, which skips the transfer of
>>>> thoses unused pages. Newly used pages can be re-tracked by the dirty
>>>> page logging mechanisms.
>>> I don't think this will be useful for anything other than
>>> virtio-balloon.  I guess it would be better to keep this code in the
>>> virtio-balloon driver if possible, even though that's rather a layering
>>> violation :( What would have to be done to make that possible?  Perhaps
>>> we can put some *small* helpers into page_alloc.c to prevent things
>>> from becoming too ugly.
>> The patch description was too narrowed and may have caused some
>> confusion, sorry about that. This function is aimed to be generic. I
>> agree with the description suggested by Michael.
>>
>> Since the main body of the function is related to operating on the
>> free_list. I think it is better to have them located here.
>> Small helpers may be less efficient and thereby causing some
>> performance loss as well.
>> I think one improvement we can make is to remove the "chunk format"
>> related things from this function. The function can generally offer the
>> base pfn to the caller's recording buffer. Then it will be the caller's
>> responsibility to format the pfn if they need.
> Sounds good at a high level, but we'd have to see the implementation
> to judge it properly.
>
>>>> --- a/mm/page_alloc.c
>>>> +++ b/mm/page_alloc.c
>>>> @@ -4498,6 +4498,120 @@ void show_free_areas(unsigned int filter)
>>>>    	show_swap_cache_info();
>>>>    }
>>>> +static int __record_unused_pages(struct zone *zone, int order,
>>>> +				 __le64 *buf, unsigned int size,
>>>> +				 unsigned int *offset, bool part_fill)
>>>> +{
>>>> +	unsigned long pfn, flags;
>>>> +	int t, ret = 0;
>>>> +	struct list_head *curr;
>>>> +	__le64 *chunk;
>>>> +
>>>> +	if (zone_is_empty(zone))
>>>> +		return 0;
>>>> +
>>>> +	spin_lock_irqsave(&zone->lock, flags);
>>>> +
>>>> +	if (*offset + zone->free_area[order].nr_free > size && !part_fill) {
>>>> +		ret = -ENOSPC;
>>>> +		goto out;
>>>> +	}
>>>> +	for (t = 0; t < MIGRATE_TYPES; t++) {
>>>> +		list_for_each(curr, &zone->free_area[order].free_list[t]) {
>>>> +			pfn = page_to_pfn(list_entry(curr, struct page, lru));
>>>> +			chunk = buf + *offset;
>>>> +			if (*offset + 2 > size) {
>>>> +				ret = -ENOSPC;
>>>> +				goto out;
>>>> +			}
>>>> +			/* Align to the chunk format used in virtio-balloon */
>>>> +			*chunk = cpu_to_le64(pfn << 12);
>>>> +			*(chunk + 1) = cpu_to_le64((1 << order) << 12);
>>>> +			*offset += 2;
>>>> +		}
>>>> +	}
>>>> +
>>>> +out:
>>>> +	spin_unlock_irqrestore(&zone->lock, flags);
>>>> +
>>>> +	return ret;
>>>> +}
>>> This looks like it could disable interrupts for a long time.  Too long?
>> What do you think if we give "budgets" to the above function?
>> For example, budget=1000, and there are 2000 nodes on the list.
>> record() returns with "incomplete" status in the first round, along with the
>> status info, "*continue_node".
>>
>> *continue_node: pointer to the starting node of the leftover. If
>> *continue_node
>> has been used at the time of the second call (i.e. continue_node->next ==
>> NULL),
>> which implies that the previous 1000 nodes have been used, then the record()
>> function can simply start from the head of the list.
>>
>> It is up to the caller whether it needs to continue the second round
>> when getting "incomplete".
> It might be cleaner to add APIs to
> 	- start iteration
> 	- do one step
> 	- end iteration
>
> caller can then iterate without too many issues
>

OK. I will re-implement it with this simple one - get only one node(page 
block) from the list in each call, and check if the time would increase 
a lot in comparison to v8.

Best,
Wei

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