Messages in this thread | | | Date | Thu, 15 Aug 2013 10:32:50 +0800 | From | Xishi Qiu <> | Subject | Re: [PATCH] mm: skip the page buddy block instead of one page |
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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
> 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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