On 25.11.21 12:27, Eric Ren wrote: > Hi David, Hi Eric, > > As the subject, I'm wondering if we can make virtio_mm plug/unplug with > futher more fine granularity like > > 2MB? That's actually very high on my todo list. See "Hot(un)plug Granularity" under: https://virtio-mem.gitlab.io/developer-guide.html > > > I gave it try as below, and it works. > > 1. Revert this patch: aac65321ba69("mm/memory_hotplug: simplify page > onlining"); > > 2. Tell mm hotplug core invoke online page callback with order 9 with > changes as below; > > > ``` > > diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c > index 5873563a3518..22a9636402b1 100644 > --- a/mm/memory_hotplug.c > +++ b/mm/memory_hotplug.c > @@ -622,7 +622,9 @@ static int online_pages_range(unsigned long > start_pfn, unsigned long nr_pages, > * them as being online/belonging to this zone ("present"). > */ > for (pfn = start_pfn; pfn < end_pfn; pfn += 1ul << order) { > - order = min(MAX_ORDER - 1, get_order(PFN_PHYS(end_pfn - > pfn))); > + /* 2MB callback */ > + order = min(MAX_ORDER - 2, get_order(PFN_PHYS(end_pfn - > pfn))); > /* __free_pages_core() wants pfns to be aligned to the > order */ > ``` > > 3. in virtio_mem driver, change subblock size to 2MB. > > > Thought the hack testing works, I'm feeling not safe. Yes, it's not safe in general when unplugging memory. When plugging memory, some changes to virtio_mem_online_page_cb() are required. It has to traverse all pageblocks part of the passed range -- so that avoids the mm/memory_hotplug.c change above. But for memory unplug, alloc_contig_range() and pageblock isolation needs changes (below). > > > Could you please help give some insights on these questions? > > 1. Is 4M a hard limitation? No, the target is 2M. It will improve virtio-mem capability on ZONE_NORMAL significantly. > > 2. What troubles we can foresee if using 2MB as granularity? alloc_contig_range() needs to be taught to handle pageblock granularity (2M) correctly. Otherwise unplug on ZONE_NORMAL will be mostly broken. Fortunately, this is WIP, but still needs to fix some things: https://lkml.kernel.org/r/20211115193725.737539-1-zi.yan@xxxxxxxx -- Thanks, David / dhildenb
