Re: Pinning ZONE_MOVABLE pages

[David Hildenbrand]

> 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.

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.

Thanks!

> 
> Thank you,
> Pasha
>