> Am 20.11.2020 um 22:58 schrieb Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>: > > On Fri, Nov 20, 2020 at 3:59 PM David Hildenbrand <david@xxxxxxxxxx> wrote: >> >> >>>> Am 20.11.2020 um 21:28 schrieb Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>: >>> >>> 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. >> >> What happens when we can‘t migrate (OOM on !MOVABLE memory, short-term pinning)? TBD. >> >>> 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. >>> >>> 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. >>> >>> 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. >>> >>> Sample function stacks through which this info needs to be passed is this: >>> >>> pin_user_pages_remote(gup_flags) >>> __get_user_pages_remote(gup_flags) >>> __gup_longterm_locked(gup_flags) >>> __get_user_pages_locked(gup_flags) >>> __get_user_pages(gup_flags) >>> faultin_page(gup_flags) >>> Convert gup_flags into fault_flags >>> handle_mm_fault(fault_flags) >>> >>> From handle_mm_fault(), the stack diverges into various faults, >>> examples include: >>> >>> Transparent Huge Page >>> handle_mm_fault(fault_flags) >>> __handle_mm_fault(fault_flags) >>> Create: struct vm_fault vmf, use fault_flags to specify correct gfp_mask >>> create_huge_pmd(vmf); >>> do_huge_pmd_anonymous_page(vmf); >>> mm_get_huge_zero_page(vma->vm_mm); -> flag is lost, so flag from >>> vmf.gfp_mask should be passed as well. >>> >>> There are several other similar paths in a transparent huge page, also >>> there is a named path where allocation is based on filesystems, and >>> the flag should be honored there as well, but it does not have to be >>> added at the same time. >>> >>> Regular Pages >>> handle_mm_fault(fault_flags) >>> __handle_mm_fault(fault_flags) >>> Create: struct vm_fault vmf, use fault_flags to specify correct gfp_mask >>> handle_pte_fault(vmf) >>> do_anonymous_page(vmf); >>> page = alloc_zeroed_user_highpage_movable(vma, vmf->address); -> >>> replace change this call according to gfp_mask. >>> >>> The above only take care of the case if user application faults on the >>> page during pinning time, but there are also cases where pages already >>> exist. >>> >>> 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. >>> >>> 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. >>> >> >> Hm, I am not sure we want to expose these details. What would be the semantics? „Might pin“? Hm, not sure. > > The semantic would be PA must not change, something that DPDK > currently excpects from huge pages, which by the way is not true, as > huge pages are migratable. > >> >> Assume you start a fresh VM via QEMU with vfio. When we start mapping guest memory via vfio, that‘s usually the time memory will get populated. Not really much has to be migrated. I think this is even true during live migration. >> >> I think selective DMA pinning (e.g., vIOMMU in QEMU) is different, where we keep pinning/unpinning on demand. But I guess even here, we will often reuse some pages over and over again. >> >> >>> Before I start working on the above approaches, I would like to get an >>> opinion from the community on an appropriate path forward for this >>> problem. If what I described sounds reasonable, or if there are other >>> ideas on how to address the problem that I am seeing. >> >> At least 1 and 2 sound sane. 3 is TBD - but it‘s a pure optimization, so it can wait. > > Makes sense, I am also worried about 3, but most of madvise() flags > are for pure optimization purposes: MADV_HUGEPAGE, MADV_SEQUENTIAL, > MADV_WILLNEED etc. BTW, I assume we should also directly tackle migrating pages off CMA regions when pinning, I guess quite some people will be interested in that as well. Have a nice weekend and thanks for looking into this issue :)
