Michal Hocko <[email protected]> writes: > On Wed 08-11-17 17:16:19, Vitaly Kuznetsov wrote: >> Michal Hocko <[email protected]> writes: >> >> > On Wed 08-11-17 16:39:49, Vitaly Kuznetsov wrote: >> >> Michal Hocko <[email protected]> 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. >> > >> >> But how? When we boot we may have very little memory and later on we >> hotplug a lot so we may not even be able to ballon all possible memory >> without running out of memory. > > Just add more memory and make part of it unusable and return it back to > the host via standard ballooning means.
We don't have control over how much memory host gives us and we have no way to return anything to the host. > > How realistic is that the host gives only such a small amount of memory > btw? It happens all the time, Hyper-V host will gradually increase guest's memory when Dynamic Memory is enabled. Moreover, there's a manual interface when host's admin can hotplug memory to guests (starting WS2016) with 2M granularity. -- Vitaly
