On Fri 20-11-20 15:27:46, Pavel Tatashin wrote: > 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. Long term or effectively time unbound pinning on zone movable is fundamentaly broken. The sole reason of ZONE_MOVABLE existence is to guarantee migrateability. If the cosumer of this memory cannot guarantee that then it shouldn't use __GFP_MOVABLE in the first place. > Kernel keeps trying to > hot-remove them, but refcnt never gets to zero, so we are looping > until the hardware watchdog kicks in. Yeah, the existing offlining behavior doesn't stop trying because the current implementation of the migration cannot tell a diffence between short and long term failures. Maybe the recent ref count for long term pinning can be used to help out there. Anyway, I am wondering what do you mean by watchdog firing. The operation should trigger neither of soft, hard or hung detectors. > 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. > > The solution is for dpdk to allocate pages from a zone below > ZONE_MOVAVLE, i.e. ZONE_NORMAL/ZONE_HIGHMEM, but this is not possible. > There is no user interface that we have that allows applications to > select what zone the memory should come from. Our existing interface is __GFP_MOVABLE. It is a responsibility of the driver to know whether the resulting memory is migratable. Users shouldn't even have to think about that. > I've spoken with Stephen Hemminger, and he said that DPDK is moving in > the direction of using transparent huge pages instead of HugeTLBs, > which means that we need to allow at least anonymous, and anonymous > transparent huge pages to come from non-movable zones on demand. You can migrate before pinning. > Here is what I am proposing: > 1. Add a new flag that is passed through pin_user_pages_* down to > fault handlers, and allow the fault handler to allocate from a > non-movable zone. gup already tries to deal with long term pins on CMA regions and migrate to a non CMA region. Have a look at __gup_longterm_locked. Migrating of the movable zone sounds like a reasonable solution to me. > 2. Add an internal move_pages_zone() similar to move_pages() syscall > but instead of migrating to a different NUMA node, migrate pages from > ZONE_MOVABLE to another zone. > Call move_pages_zone() on demand prior to pinning pages from > vfio_pin_map_dma() for instance. Why is the existing migration API insufficient? > 3. Perhaps, it also makes sense to add madvise() flag, to allocate > pages from non-movable zone. When a user application knows that it > will do DMA mapping, and pin pages for a long time, the memory that it > allocates should never be migrated or hot-removed, so make sure that > it comes from the appropriate place. > The benefit of adding madvise() flag is that we won't have to deal > with slow page migration during pin time, but the disadvantage is that > we would need to change the user interface. No, the MOVABLE_ZONE like other zone types are internal implementation detail of the MM. I do not think we want to expose that to the userspace and carve this into stone. -- Michal Hocko SUSE Labs
