On Thu, Jun 18, 2020 at 12:31 AM Jan Beulich <jbeul...@suse.com> wrote: > > On 17.06.2020 18:19, Tamas K Lengyel wrote: > > While forking VMs running a small RTOS system (Zephyr) a Xen crash has been > > observed due to a mm-lock order violation while copying the HVM CPU context > > from the parent. This issue has been identified to be due to > > hap_update_paging_modes first getting a lock on the gfn using get_gfn. This > > call also creates a shared entry in the fork's memory map for the cr3 gfn. > > The > > function later calls hap_update_cr3 while holding the paging_lock, which > > results in the lock-order violation in vmx_load_pdptrs when it tries to > > unshare > > the above entry when it grabs the page with the P2M_UNSHARE flag set. > > > > Since vmx_load_pdptrs only reads from the page its usage of P2M_UNSHARE was > > unnecessary to start with. Using P2M_ALLOC is the appropriate flag to ensure > > the p2m is properly populated and to avoid the lock-order violation we > > observed. > > Using P2M_ALLOC is not going to address the original problem though > afaict: You may hit the mem_sharing_fork_page() path that way, and > via nominate_page() => __grab_shared_page() => mem_sharing_page_lock() > you'd run into a lock order violation again.
Note that the nominate_page you see in that path is for the parent VM. The paging lock is not taken for the parent VM thus nominate_page succeeds without any issues any time fork_page is called. There is no nominate_page called for the client domain as there is nothing to nominate when plugging a hole. Tamas
