On Tue, Mar 01, 2016 at 06:17:58PM +0100, Michal Hocko wrote: > On Tue 01-03-16 18:46:38, Michael S. Tsirkin wrote: > > On Tue, Mar 01, 2016 at 05:35:37PM +0100, Michal Hocko wrote: > > > On Tue 01-03-16 18:22:32, Michael S. Tsirkin wrote: > > > > On Tue, Mar 01, 2016 at 05:08:13PM +0100, Michal Hocko wrote: > > > > > On Tue 01-03-16 17:57:04, Michael S. Tsirkin wrote: > > > > > > On Tue, Mar 01, 2016 at 04:52:12PM +0100, Michal Hocko wrote: > > > > > > > [CCing vhost-net maintainer] > > > > > > > > > > > > > > On Mon 29-02-16 20:02:09, Vladimir Davydov wrote: > > > > > > > > An mm_struct may be pinned by a file. An example is vhost-net device > > > > > > > > created by a qemu/kvm (see vhost_net_ioctl -> vhost_net_set_owner -> > > > > > > > > vhost_dev_set_owner). > > > > > > > > > > > > > > The more I think about that the more I am wondering whether this is > > > > > > > actually OK and correct. Why does the driver have to pin the address > > > > > > > space? Nothing really prevents from parallel tearing down of the address > > > > > > > space anyway so the code cannot expect all the vmas to stay. Would it be > > > > > > > enough to pin the mm_struct only? > > > > > > > > > > > > I'll need to research this. It's a fact that as long as the > > > > > > device is not stopped, vhost can attempt to access > > > > > > the address space. > > > > > > > > > > But does it expect any specific parts of the address space to be mapped? > > > > > E.g. proc needs to keep the mm allocated as well for some files but it > > > > > doesn't pin the address space (mm_users) but rather mm_count (see > > > > > proc_mem_open). > > > > > > > > At a quick glance, it seems that it's needed: it calls > > > > get_user_pages(mm) and that looks like it will not DTRT (or even fail > > > > gracefully) if mm->mm_users == 0 and exit_mmap/etc was already called > > > > (or is in progress). > > > > > > yes it will fail gracefully > > > > > > What makes get_user_pages fail gracefully in this case, > > if it races with task exiting? > > Sorry, I could have been more verbose... The code would have to make sure > that the mm is still alive before calling g-u-p by > atomic_inc_not_zero(&mm->mm_users) and fail if the user count dropped to > 0 in the mean time. See how fs/proc/task_mmu.c does that (proc_mem_open > + m_start + m_stop. > > The biggest advanatage would be that the mm address space pin would be > only for the particular operation. Not sure whether that is possible in > the driver though. Anyway pinning the mm for a potentially unbounded > amount of time doesn't sound too nice. > -- > Michal Hocko > SUSE Labs Hmm that would be another atomic on data path ... I'd have to explore that. -- MST -- 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>