On Wed 08-11-17 16:39:49, Vitaly Kuznetsov wrote: > Michal Hocko <mhocko@xxxxxxxxxx> writes: > > > On Wed 08-11-17 14:01:55, Vitaly Kuznetsov wrote: > >> Hyper-V balloon driver needs to hotplug memory in smaller chunks and to > >> workaround Linux's 128Mb allignment requirement so it does a trick: partly > >> populated 128Mb blocks are added and then a custom online_page_callback > >> hook checks if the particular page is 'backed' during onlining, in case it > >> is not backed it is left in Reserved state. When the host adds more pages > >> to the block we bring them online from the driver (see > >> hv_bring_pgs_online()/hv_page_online_one() in drivers/hv/hv_balloon.c). > >> Eventually the whole block becomes fully populated and we hotplug the next > >> 128Mb. This all works for quite some time already. > > > > Why does HyperV needs to workaround the section size limit in the first > > place? We are allocation memmap for the whole section anyway so it won't > > save any memory. So the whole thing sounds rather dubious to me. > > > > Memory hotplug requirements in Windows are different, they have 2Mb > granularity, not 128Mb like we have in Linux x86. > > Imagine there's a request to add 32Mb of memory comming from the > Hyper-V host. What can we do? Don't add anything at all and wait till > we're suggested to add > 128Mb and then add a section or the current > approach. Use a different approach than memory hotplug. E.g. memory balloning. > >> What is not working is offlining of such partly populated blocks: > >> check_pages_isolated_cb() callback will not pass with a sinle Reserved page > >> and we end up with -EBUSY. However, there's no reason to fail offlining in > >> this case: these pages are already offline, we may just skip them. Add the > >> appropriate workaround to test_pages_isolated(). > > > > How do you recognize pages reserved by other users. You cannot simply > > remove them, it would just blow up. > > > > I exepcted sumothing like that, thus RFC. Is there a way to detect pages > which were never onlined? E.g. it is Reserved and count == 0? That would be quite tricky. But I am not convinced that the whole thing makes any sense at all. We are in fact always creating the full section so onlining only a part of it sounds really dubious to me. -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
