Re: Pinning ZONE_MOVABLE pages

[Michal Hocko]

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