On Mon, Apr 3, 2017 at 5:40 PM, Alex Williamson <alex.william...@redhat.com> wrote: > > On Mon, 3 Apr 2017 10:58:22 +0200 > Sergio Lopez <s...@redhat.com> wrote: > > > When quickly unmapping and mapping memory regions (as may happen in > > address_space_update_topology), if running with a non-unlimited > > RLIMIT_MEMLOCK, the kernel may return ENOMEM for a map request > > because the previous unmap has been processed, but accounted yet. > > > > Probably this should be fixed in the kernel ensuring a deterministic > > behavior for VFIO map and unmap operations. Until then, this works > > around the issue, waiting 10ms and trying again. > > I think we need to know what that kernel fix is before adding arbitrary > delays and retries in userspace code (Do we know why 10ms works? Is > it too long/short?).
AFAIK from userspace we can't know when a certain work scheduled in a kernel workqueue has been completed. Calling usleep ensures the process will yield, and 10ms looks enough time for a full world of context switches, but I agree with you that's pretty arbitrary. On the other hand, this code is only reached in a pretty exceptional situation, which is not relevant from a performance point of view, and there's already a workaround for a non-deterministic EBUSY while mapping VGA ROM space. There's the option of leaving this as is, and waiting for a fix in the kernel, but I think I'd a good idea to work around the issue for older kernels too. Sergio.
