Re: [PATCH] mm: skip the page buddy block instead of one page

[Xishi Qiu]

On 2013/8/15 10:44, Minchan Kim wrote:

> Hi Xishi,
> 
> On Thu, Aug 15, 2013 at 10:32:50AM +0800, Xishi Qiu wrote:
>> On 2013/8/15 2:00, Mel Gorman wrote:
>>
>>>>> Even if the page is still page buddy, there is no guarantee that it's
>>>>> the same page order as the first read. It could have be currently
>>>>> merging with adjacent buddies for example. There is also a really
>>>>> small race that a page was freed, allocated with some number stuffed
>>>>> into page->private and freed again before the second PageBuddy check.
>>>>> It's a bit of a hand grenade. How much of a performance benefit is there
>>>>
>>>> 1. Just worst case is skipping pageblock_nr_pages
>>>
>>> No, the worst case is that page_order returns a number that is
>>> completely garbage and low_pfn goes off the end of the zone
>>>
>>>> 2. Race is really small
>>>> 3. Higher order page allocation customer always have graceful fallback.
>>>>
>>
>> Hi Minchan, 
>> I think in this case, we may get the wrong value from page_order(page).
>>
>> 1. page is in page buddy
>>
>>> if (PageBuddy(page)) {
>>
>> 2. someone allocated the page, and set page->private to another value
>>
>>> 	int nr_pages = (1 << page_order(page)) - 1;
>>
>> 3. someone freed the page
>>
>>> 	if (PageBuddy(page)) {
>>
>> 4. we will skip wrong pages
> 
> So, what's the result by that?
> As I said, it's just skipping (pageblock_nr_pages -1) at worst case

Hi Minchan,
I mean if the private is set to a large number, it will skip 2^private 
pages, not (pageblock_nr_pages -1). I find somewhere will use page->private, 
such as fs. Here is the comment about parivate.
/* Mapping-private opaque data:
 * usually used for buffer_heads
 * if PagePrivate set; used for
 * swp_entry_t if PageSwapCache;
 * indicates order in the buddy
 * system if PG_buddy is set.
 */
Thanks,
Xishi Qiu

> and the case you mentioned is right academically and I and Mel
> already pointed out that. But how often could that happen in real
> practice? I believe such is REALLY REALLY rare.
> So, as Mel said, if you have some workloads to see the benefit
> from this patch, I think we could accept the patch.
> Could you try and respin with the number?
> I guess big contigous memory range or memory-hotplug which are
> full of free pages in embedded CPU which is rather slower than server
> or desktop side could have benefit.
> 
> Thanks.
> 
>>
>>> 		nr_pages = min(nr_pages, MAX_ORDER_NR_PAGES - 1);
>>> 		low_pfn += nr_pages;
>>> 		continue;
>>> 	}
>>> }
>>>
>>> It's still race-prone meaning that it really should be backed by some
>>> performance data justifying it.
>>>
>>
>>
>>
> 



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