On Mon, Mar 02, 2015 at 01:35:05PM +0100, Cornelia Huck wrote: > On Mon, 2 Mar 2015 13:19:43 +0100 > "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote: > > > On Mon, Mar 02, 2015 at 01:11:02PM +0100, Cornelia Huck wrote: > > > On Mon, 2 Mar 2015 12:46:57 +0100 > > > "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote: > > > > > > > On Mon, Mar 02, 2015 at 12:31:06PM +0100, Cornelia Huck wrote: > > > > > On Mon, 2 Mar 2015 12:13:58 +0100 > > > > > "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote: > > > > > > > > > > > On Mon, Mar 02, 2015 at 10:37:26AM +1030, Rusty Russell wrote: > > > > > > > Thomas Huth <thuth@xxxxxxxxxxxxxxxxxx> writes: > > > > > > > > On Thu, 26 Feb 2015 11:50:42 +1030 > > > > > > > > Rusty Russell <rusty@xxxxxxxxxxxxxxx> wrote: > > > > > > > > > > > > > > > >> Thomas Huth <thuth@xxxxxxxxxxxxxxxxxx> writes: > > > > > > > >> > Hi all, > > > > > > > >> > > > > > > > > >> > with the recent kernel 3.19, I get a kernel warning when I start my > > > > > > > >> > KVM guest on s390 with virtio balloon enabled: > > > > > > > >> > > > > > > > >> The deeper problem is that virtio_ccw_get_config just silently fails on > > > > > > > >> OOM. > > > > > > > >> > > > > > > > >> Neither get_config nor set_config are expected to fail. > > > > > > > > > > > > > > > > AFAIK this is currently not a problem. According to > > > > > > > > http://lwn.net/Articles/627419/ these kmalloc calls never > > > > > > > > fail because they allocate less than a page. > > > > > > > > > > > > > > I strongly suggest you unlearn that fact. > > > > > > > The fix for this is in two parts: > > > > > > > > > > > > > > 1) Annotate using sched_annotate_sleep() and add a comment: we may spin > > > > > > > a few times in low memory situations, but this isn't a high > > > > > > > performance path. > > > > > > > > > > > > > > 2) Handle get_config (and other) failure in some more elegant way. > > > > > > > > > > Do you mean we need to enable the caller to deal with get_config > > > > > failures (and the transport to relay those failures)? I agree with that. > > > > > > > > We can certainly tweak code to bypass need to kmalloc > > > > on get_config. > > > > > > > > Why is it doing these allocs? What's wrong with using > > > > vcdev->config directly? > > > > > > We'd need to make sure that vcdev->config is allocated with GFP_DMA, as > > > we need it to be under 2G. And we need to be more careful wrt > > > serialization, especially if we want to reuse the ccw structure as > > > well, for example. Nothing complicated, I'd just need some free time to > > > do it :) > > > > > > The more likely reason for get_config to fail is a device hotunplug, > > > however. We'll get a seperate notification about that (via machine > > > check + channel report), but it would be nice if we could stop poking > > > the device immediately, as there's no use trying to do something with > > > it anymore. > > > > Normally, hotunplug requires guest cooperation. > > IOW unplug request should send guest interrupt, > > then block until guest confirms it's not using the > > device anymore. > > virtio pci already handles that fine, can't ccw > > do something similar? > > Hotunplug for channel devices does not require guest feedback. (In > fact, I was surprised to hear that there is somthing like guest > cooperation on other platforms.) Consider a storage device. If you don't flush out caches before removing the disk, you might lose a bunch of data. > Basically, the guest is simply > presented with the fact that the device is gone and has to deal with > it. It does not matter whether the device was removed by operator > request or due to a hardware failure. > > (We do have support in the s390 channel device core to be able to deal > with devices going away and coming back gracefully. ccw devices can be > put into a special state where they retain their configuration so that > they can be reactivated if they become available again. For example, > dasd (disk) devices survive being detached and reattached just fine, > even under I/O load. > See the ->notify() callback of the ccw driver for > details.) How does guest distinguish between this and intentional permanent removal? -- MST -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
