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Subject: Re: [PATCH v15 4/5] mm: support reporting free page blocks

On 08/28/2017 09:33 PM, Michal Hocko wrote:
On Mon 28-08-17 18:08:32, Wei Wang wrote:
This patch adds support to walk through the free page blocks in the
system and report them via a callback function. Some page blocks may
leave the free list after zone->lock is released, so it is the caller's
responsibility to either detect or prevent the use of such pages.

One use example of this patch is to accelerate live migration by skipping
the transfer of free pages reported from the guest. A popular method used
by the hypervisor to track which part of memory is written during live
migration is to write-protect all the guest memory. So, those pages that
are reported as free pages but are written after the report function
returns will be captured by the hypervisor, and they will be added to the
next round of memory transfer.
OK, looks much better. I still have few nits.

+extern void walk_free_mem_block(void *opaque,
+				int min_order,
+				bool (*report_page_block)(void *, unsigned long,
+							  unsigned long));
please add names to arguments of the prototype

   * Free reserved pages within range [PAGE_ALIGN(start), end & PAGE_MASK)
   * into the buddy system. The freed pages will be poisoned with pattern
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 6d00f74..81eedc7 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4762,6 +4762,71 @@ void show_free_areas(unsigned int filter, nodemask_t *nodemask)
+ * walk_free_mem_block - Walk through the free page blocks in the system
+ * @opaque: the context passed from the caller
+ * @min_order: the minimum order of free lists to check
+ * @report_page_block: the callback function to report free page blocks
page_block has meaning in the core MM which doesn't strictly match its
usage here. Moreover we are reporting pfn ranges rather than struct page
range. So report_pfn_range would suit better.

+	for_each_populated_zone(zone) {
+		for (order = MAX_ORDER - 1; order >= min_order; order--) {
+			for (mt = 0; !stop && mt < MIGRATE_TYPES; mt++) {
+				spin_lock_irqsave(&zone->lock, flags);
+				list = &zone->free_area[order].free_list[mt];
+				list_for_each_entry(page, list, lru) {
+					pfn = page_to_pfn(page);
+					stop = report_page_block(opaque, pfn,
+								 1 << order);
+					if (stop)
+						break;
					if (stop) {
						spin_unlock_irqrestore(&zone->lock, flags);

would be both easier and less error prone. E.g. You wouldn't pointlessly
iterate over remaining orders just to realize there is nothing to be
done for those...

Yes, that's better, thanks. I will take other suggestions as well.


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