Hi Roman, > We're running a lot of KVM virtual machines on POWER8 hosts and > sometimes new VMs can't be started because there are no contiguous > regions for HPT because of CMA region fragmentation. > > The issue is covered in the LWN article: https://lwn.net/Articles/684611/ > The article points that you raised the problem on LSFMM 2016. However I > couldn't find a follow up article on the issue. > > Looking at the kernel commit log I've identified a few commits that > might reduce CMA fragmentaiton and overcome HPT allocation failure: > - bd2e75633c801 ("dma-contiguous: use fallback alloc_pages for single > pages") > - 678e174c4c16a ("powerpc/mm/iommu: allow migration of cma allocated > pages during mm_iommu_do_alloc") > - 9a4e9f3b2d739 ("mm: update get_user_pages_longterm to migrate pages > allocated from > CMA region") > - d7fefcc8de914 ("mm/cma: add PF flag to force non cma alloc") > > Are there any other commits that address the issue? What is the first > kernel version that shouldn't have the HPT allocation problem due to CMA > fragmentation?
I've had some success increasing the CMA allocation with the kvm_cma_resv_ratio boot parameter - see arch/powerpc/kvm/book3s_hv_builtin.c The default is 5%. In a support case in a former job we had a customer who increased this to I think 7 or 8% and saw the symptoms subside dramatically. HTH, Daniel > > Thank you, > Roman
