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    SubjectRe: Pinning ZONE_MOVABLE pages
    On Mon 23-11-20 11:06:21, Pavel Tatashin wrote:
    > On Mon, Nov 23, 2020 at 4:01 AM Michal Hocko <mhocko@suse.com> wrote:
    > >
    > > 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.
    >
    > Exactly, this is what I am trying to solve, and started this thread to
    > figure out what is the best approach to address this problem.
    >
    > >
    > > > 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.
    >
    > You are right, the hot-remove is killable operation. In our case,
    > however, systemd stops petting watchdog during kexec reboot to ensure
    > that reboot finishes, however, because we hot-remove memory during
    > shutdown, and kernel is unable to hot-remove memory within 60s we get
    > a watchdog reset.

    Well, this should be worked around quite trivially. You can kill your
    attempt before the timeout fires.

    [...]
    > > > 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?
    >
    > Here I am talking about internal implementation not user API. We do
    > not have a function that migrates pages in a user address space from
    > one zone to another zone. We only have a function that is exposed as a
    > syscall that migrates pages from one node to another node.

    We do have migrate_pages and its interface should make it trivial enough
    that a new general purpose helper shouldn't be really needed.

    struct migration_target_control mtc = {
    .gfp_mask = GFP_USER | __GFP_RETRY_MAYFAIL,
    };
    migrate_pages(&list_of_pages, alloc_migration_target, NULL,
    (unsigned long)&mtc, MIGRATE_SYNC, MR_PINNING);

    note that NR_PINNING would have to added.

    > > > 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.
    >
    > What I mean here is allowing users to guarantee that the page's PA is
    > going to stay the same. Sort of a stronger mlock. Mlock only
    > guarantees that the page is not swapped, but something like
    > MADV_PINNED would guarantee that page is not going to be swapped and
    > also not migrated.

    There were some discussions around vmpin/unpin syscalls. This didn't
    really lead anywhere. One of the roadblock was a proper accounting IIRC.
    You might want to look for those discussions in email archives.

    > If a user determines the PA of that page, that PA
    > is going to stay the same throughout the life of the page. This is not
    > exposing internal implementation in any way, this guarantee could be
    > honored in various ways: i.e. pinned or allocating from ZONE_NORMAL.
    > The fact that we would honor it by allocating memory from ZONE_NORMAL
    > is implementation detail that would not be exposed to the user.

    Jason has already replied to this and I do not have much to add.

    [...]

    > I just think it is inefficient to first allocate memory from
    > ZONE_MOVABLE, and later migrate it to ZONE_NORMAL.

    Yes it is inefficient. Is it usual that the memory is already faulted in
    when it is pinned?

    --
    Michal Hocko
    SUSE Labs

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