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Re: [Xen-devel] [PATCH v5 1/2] memory_hotplug: Free pages as higher order

On 2018-10-10 21:00, Vlastimil Babka wrote:
On 10/5/18 10:10 AM, Arun KS wrote:
When free pages are done with higher order, time spend on
coalescing pages by buddy allocator can be reduced. With
section size of 256MB, hot add latency of a single section
shows improvement from 50-60 ms to less than 1 ms, hence
improving the hot add latency by 60%. Modify external
providers of online callback to align with the change.

Signed-off-by: Arun KS <arunks@xxxxxxxxxxxxxx>


@@ -655,26 +655,44 @@ void __online_page_free(struct page *page)

-static void generic_online_page(struct page *page)
+static int generic_online_page(struct page *page, unsigned int order)
-       __online_page_set_limits(page);

This is now not called anymore, although the xen/hv variants still do
it. The function seems empty these days, maybe remove it as a followup

-       __online_page_increment_counters(page);
-       __online_page_free(page);
+       __free_pages_core(page, order);
+       totalram_pages += (1UL << order);
+       if (PageHighMem(page))
+               totalhigh_pages += (1UL << order);

__online_page_increment_counters() would have used
adjust_managed_page_count() which would do the changes under
managed_page_count_lock. Are we safe without the lock? If yes, there
should perhaps be a comment explaining why.

Looks unsafe without managed_page_count_lock. I think better have a similar implementation of free_boot_core() in memory_hotplug.c like we had in version 1 of patch. And use adjust_managed_page_count() instead of page_zone(page)->managed_pages += nr_pages;


-static void generic_online_page(struct page *page)
+static int generic_online_page(struct page *page, unsigned int order)
-       __online_page_set_limits(page);
-       __online_page_increment_counters(page);
-       __online_page_free(page);
+       unsigned long nr_pages = 1 << order;
+       struct page *p = page;
+       for (loop = 0 ; loop < nr_pages ; loop++, p++) {
+               __ClearPageReserved(p);
+               set_page_count(p, 0);
+       }
+       adjust_managed_page_count(page, nr_pages);
+       set_page_refcounted(page);
+       __free_pages(page, order);
+       return 0;


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