Re: Pinning ZONE_MOVABLE pages

[David Hildenbrand]

> 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 :)