Messages in this thread | | | Date | Thu, 03 Aug 2017 20:11:58 +0800 | From | Wei Wang <> | Subject | Re: [PATCH v13 4/5] mm: support reporting free page blocks |
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On 08/03/2017 07:28 PM, Michal Hocko wrote: > On Thu 03-08-17 19:27:19, Wei Wang wrote: >> On 08/03/2017 06:44 PM, Michal Hocko wrote: >>> On Thu 03-08-17 18:42:15, Wei Wang wrote: >>>> On 08/03/2017 05:11 PM, Michal Hocko wrote: >>>>> On Thu 03-08-17 14:38:18, Wei Wang wrote: >>> [...] >>>>>> +static int report_free_page_block(struct zone *zone, unsigned int order, >>>>>> + unsigned int migratetype, struct page **page) >>>>> This is just too ugly and wrong actually. Never provide struct page >>>>> pointers outside of the zone->lock. What I've had in mind was to simply >>>>> walk free lists of the suitable order and call the callback for each one. >>>>> Something as simple as >>>>> >>>>> for (i = 0; i < MAX_NR_ZONES; i++) { >>>>> struct zone *zone = &pgdat->node_zones[i]; >>>>> >>>>> if (!populated_zone(zone)) >>>>> continue; >>>>> spin_lock_irqsave(&zone->lock, flags); >>>>> for (order = min_order; order < MAX_ORDER; ++order) { >>>>> struct free_area *free_area = &zone->free_area[order]; >>>>> enum migratetype mt; >>>>> struct page *page; >>>>> >>>>> if (!free_area->nr_pages) >>>>> continue; >>>>> >>>>> for_each_migratetype_order(order, mt) { >>>>> list_for_each_entry(page, >>>>> &free_area->free_list[mt], lru) { >>>>> >>>>> pfn = page_to_pfn(page); >>>>> visit(opaque2, prn, 1<<order); >>>>> } >>>>> } >>>>> } >>>>> >>>>> spin_unlock_irqrestore(&zone->lock, flags); >>>>> } >>>>> >>>>> [...] >>>> I think the above would take the lock for too long time. That's why we >>>> prefer to take one free page block each time, and taking it one by one >>>> also doesn't make a difference, in terms of the performance that we >>>> need. >>> I think you should start with simple approach and impove incrementally >>> if this turns out to be not optimal. I really detest taking struct pages >>> outside of the lock. You never know what might happen after the lock is >>> dropped. E.g. can you race with the memory hotremove? >> >> The caller won't use pages returned from the function, so I think there >> shouldn't be an issue or race if the returned pages are used (i.e. not free >> anymore) or simply gone due to hotremove. > No, this is just too error prone. Consider that struct page pointer > itself could get invalid in the meantime. Please always keep robustness > in mind first. Optimizations are nice but it is even not clear whether > the simple variant will cause any problems.
how about this:
for_each_populated_zone(zone) { for_each_migratetype_order_decend(min_order, order, type) { do { => spin_lock_irqsave(&zone->lock, flags); ret = report_free_page_block(zone, order, type, &page)) { pfn = page_to_pfn(page); nr_pages = 1 << order; visit(opaque1, pfn, nr_pages); } => spin_unlock_irqrestore(&zone->lock, flags); } while (!ret) }
In this way, we can still keep the lock granularity at one free page block while having the struct page operated under the lock.
Best, Wei
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