Messages in this thread | | | Date | Wed, 27 Jun 2018 11:00:05 +0800 | From | Wei Wang <> | Subject | Re: [PATCH v34 2/4] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT |
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On 06/27/2018 10:41 AM, Michael S. Tsirkin wrote: > On Wed, Jun 27, 2018 at 09:24:18AM +0800, Wei Wang wrote: >> On 06/26/2018 09:34 PM, Michael S. Tsirkin wrote: >>> On Tue, Jun 26, 2018 at 08:27:44PM +0800, Wei Wang wrote: >>>> On 06/26/2018 11:56 AM, Michael S. Tsirkin wrote: >>>>> On Tue, Jun 26, 2018 at 11:46:35AM +0800, Wei Wang wrote: >>>>> >>>>>>>> + if (!arrays) >>>>>>>> + return NULL; >>>>>>>> + >>>>>>>> + for (i = 0; i < max_array_num; i++) { >>>>>>> So we are getting a ton of memory here just to free it up a bit later. >>>>>>> Why doesn't get_from_free_page_list get the pages from free list for us? >>>>>>> We could also avoid the 1st allocation then - just build a list >>>>>>> of these. >>>>>> That wouldn't be a good choice for us. If we check how the regular >>>>>> allocation works, there are many many things we need to consider when pages >>>>>> are allocated to users. >>>>>> For example, we need to take care of the nr_free >>>>>> counter, we need to check the watermark and perform the related actions. >>>>>> Also the folks working on arch_alloc_page to monitor page allocation >>>>>> activities would get a surprise..if page allocation is allowed to work in >>>>>> this way. >>>>>> >>>>> mm/ code is well positioned to handle all this correctly. >>>> I'm afraid that would be a re-implementation of the alloc functions, >>> A re-factoring - you can share code. The main difference is locking. >>> >>>> and >>>> that would be much more complex than what we have. I think your idea of >>>> passing a list of pages is better. >>>> >>>> Best, >>>> Wei >>> How much memory is this allocating anyway? >>> >> For every 2TB memory that the guest has, we allocate 4MB. > Hmm I guess I'm missing something, I don't see it: > > > + max_entries = max_free_page_blocks(ARRAY_ALLOC_ORDER); > + entries_per_page = PAGE_SIZE / sizeof(__le64); > + entries_per_array = entries_per_page * (1 << ARRAY_ALLOC_ORDER); > + max_array_num = max_entries / entries_per_array + > + !!(max_entries % entries_per_array); > > Looks like you always allocate the max number?
Yes. We allocated the max number and then free what's not used. For example, a 16TB guest, we allocate Four 4MB buffers and pass the 4 buffers to get_from_free_page_list. If it uses 3, then the remaining 1 "4MB buffer" will end up being freed.
For today's guests, max_array_num is usually 1.
Best, Wei
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