On 18.06.2020 14:39, Tamas K Lengyel wrote: > On Thu, Jun 18, 2020 at 12:31 AM Jan Beulich <[email protected]> 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.
But that's still a lock order issue then, isn't it? Just one that the machinery can't detect / assert upon. Jan
