On Fri, Nov 20, 2020 at 4:34 PM David Hildenbrand <[email protected]> wrote: > > > > Am 20.11.2020 um 22:17 schrieb Matthew Wilcox <[email protected]>: > > > > On Fri, Nov 20, 2020 at 09:59:24PM +0100, David Hildenbrand wrote: > >> > >>>> Am 20.11.2020 um 21:28 schrieb Pavel Tatashin > >>>> <[email protected]>: > >>> > >>> Recently, I encountered a hang that is happening during memory hot > >>> remove operation. It turns out that the hang is caused by pinned user > >>> pages in ZONE_MOVABLE. > >>> > >>> Kernel expects that all pages in ZONE_MOVABLE can be migrated, but > >>> this is not the case if a user applications such as through dpdk > >>> libraries pinned them via vfio dma map. Kernel keeps trying to > >>> hot-remove them, but refcnt never gets to zero, so we are looping > >>> until the hardware watchdog kicks in. > >>> > >>> We cannot do dma unmaps before hot-remove, because hot-remove is a > >>> slow operation, and we have thousands for network flows handled by > >>> dpdk that we just cannot suspend for the duration of hot-remove > >>> operation. > >>> > >> > >> Hi! > >> > >> It‘s a known problem also for VMs using vfio. I thought about this some > >> while ago an came to the same conclusion: before performing long-term > >> pinnings, we have to migrate pages off the movable zone. After that, it‘s > >> too late. > > > > We can't, though. VMs using vfio pin their entire address space (right?) > > so we end up with basically all of the !MOVABLE memory used for VMs and > > the MOVABLE memory goes unused (I'm thinking about the case of a machine > > which only hosts VMs and has nothing else to do with its memory). In > > that case, the sysadmin is going to reconfigure ZONE_MOVABLE away, and > > now we just don't have any ZONE_MOVABLE. So what's the point? > > When the guest is using an vIOMMU, it will only pin what‘s currently mapped > by the guest into the vIOMMU. Otherwise: yes.
Right, not all guest memory needs to be pinned, so ZONE_MOVABLE can still be used for a vast amount of allocations. > > If you assume all memory will be used for VMs with vfio, then yes: no > ZONE_MOVABLE, no memory hotunplug. If its‘s only some VMs, it‘s a different > story. Sounds like in such an extreme case it is reasonable to assume no hot-plug. But, when you have 8G, and need to remove 2G movable zone, but can't guarantee it even if you have 6G of free mem, this is unreasonable. > > > > > ZONE_MOVABLE can also be pinned by mlock() and other such system calls. > > Mlocked pages can be migrated, no? They are simply not swappable iirc. Yes, mlocked they are simply in memory, but the content of the pages can be migrated to a different place in RAM. > > > The kernel needs to understand that ZONE_MOVABLE memory may not actually > > be movable, and skip the unmovable stuff. > > > > Then you don‘t have unplug guarantees. Memory unplug broken by design. Then > there is no point in optimizing that case at all and tell customers „vfio and > memory hotunplug is incompatible“. The only ugly thing is the endless loop. Right, if memory in ZONE_MOVABLE is not guaranteed to be movable, we can never guarantee memory hot-remove even when we have a lot of free memory to migrate to. >
