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SubjectRe: [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)
>> show_swap_cache_info();
>> }
>>
>> +/**
>> + * 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);
> return;
> }
>
> 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.

Best,
Wei



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