On Thu 03-08-17 21:17:25, Wei Wang wrote:> On 08/03/2017 08:41 PM, Michal Hocko wrote: > >On Thu 03-08-17 20:11:58, Wei Wang wrote: > >>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. > >How can you continue iteration of free_list after the lock has been > >dropped? > > report_free_page_block() has handled all the possible cases after the lock > is > dropped. For example, if the previous reported page has not been on the free > list, then the first node from the list of this order will be given. This is > because > page allocation takes page blocks from the head to end, for example: > > 1,2,3,4,5,6 > if the previous reported free block is 2, when we give 2 to the report > function > to get the next page block, and find 1,2,3 have all gone, it will report 4, > which > is the head of the free list.As I've said earlier. Start simple optimize incrementally with some numbers to justify a more subtle code. -- Michal Hocko SUSE Labs
On Thursday, August 3, 2017 9:51 PM, Michal Hocko:> As I've said earlier. Start simple optimize incrementally with some numbers to > justify a more subtle code. > --OK. Let's start with the simple implementation as you suggested. Best, Wei
Michael S. Tsirkin
2017-Aug-03 21:02 UTC
[PATCH v13 4/5] mm: support reporting free page blocks
On Thu, Aug 03, 2017 at 03:20:09PM +0000, Wang, Wei W wrote:> On Thursday, August 3, 2017 9:51 PM, Michal Hocko: > > As I've said earlier. Start simple optimize incrementally with some numbers to > > justify a more subtle code. > > -- > > OK. Let's start with the simple implementation as you suggested. > > Best, > WeiThe tricky part is when you need to drop the lock and then restart because the device is busy. Would it maybe make sense to rotate the list so that new head will consist of pages not yet sent to device? -- MST
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