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    SubjectRe: [BUG REPORT] [mm-hotplug, aio] aio ring_pages can't be offlined
    On Thu, Nov 29, 2012 at 11:55:02PM -0800, Andrew Morton wrote:
    > On Fri, 30 Nov 2012 15:01:26 +0800 Lin Feng <linfeng@cn.fujitsu.com> wrote:
    >
    > >
    > >
    > > On 11/30/2012 01:57 PM, Andrew Morton wrote:
    > > > On Fri, 30 Nov 2012 11:42:05 +0800 Lin Feng <linfeng@cn.fujitsu.com> wrote:
    > > >
    > > >> hi Andrew,
    > > >>
    > > >> On 11/30/2012 07:39 AM, Andrew Morton wrote:
    > > >>> Tricky.
    > > >>>
    > > >>> I expect the same problem would occur with pages which are under
    > > >>> O_DIRECT I/O. Obviously O_DIRECT pages won't be pinned for such long
    > > >>> periods, but the durations could still be lengthy (seconds).
    > > >> the offline retry timeout duration is 2 minutes, so to O_DIRECT pages
    > > >> seem maybe not a problem for the moment.
    > > >>>
    > > >>> Worse is a futex page, which could easily remain pinned indefinitely.
    > > >>>
    > > >>> The best I can think of is to make changes in or around
    > > >>> get_user_pages(), to steal the pages from userspace and replace them
    > > >>> with non-movable ones before pinning them. The performance cost of
    > > >>> something like this would surely be unacceptable for direct-io, but
    > > >>> maybe OK for the aio ring and futexes.
    > > >> thanks for your advice.
    > > >> I want to limit the impact as little as possible, as mentioned above,
    > > >> direct-io seems not a problem, we needn't touch them. Maybe we can
    > > >> just change the use of get_user_pages()(in or around) such as aio
    > > >> ring pages. I will try to find a way to do this.
    > > >
    > > > What about futexes?
    > > hi Andrew,
    > >
    > > Yes, better to find an approach to solve them all.
    > >
    > > But I'm worried about that if we just confine get_user_pages() to use
    > > none-movable pages, it will drain the none-movable pages soon. Because
    > > there are many places using get_user_pages() such as some drivers.
    >
    > Obviously we shouldn't change get_user_pages() for all callers.
    >
    > > IMHO in most cases get_user_pages() callers should release the pages soon,
    > > so pages allocated from movable zone should be OK. But I'm not sure if
    > > we get such rule upon get_user_pages().
    > > And in other cases we specify get_user_pages() to allocate pages from
    > > none-movable zone.
    > >
    > > So could we add a zone-alloc flags when we call get_user_pages()?
    >
    > Well, that's a fairly low-level implementation detail. A more typical
    > approach would be to add a new get_user_pages_non_movable() or such.
    > That would probably have the same signature as get_user_pages(), with
    > one additional argument. Then get_user_pages() becomes a one-line
    > wrapper which passes in a particular value of that argument.
    >

    That is going in the direction that all pinned pages become MIGRATE_UNMOVABLE
    allocations. That will impact THP availability by increasing the number
    of MIGRATE_UNMOVABLE blocks that exist and it would hit every user --
    not just those that care about ZONE_MOVABLE.

    I'm likely to NAK such a patch if it's only about node hot-remove because
    it's much more of a corner case than wanting to use THP.

    I would prefer if get_user_pages() checked if the page it was about to
    pin was in ZONE_MOVABLE and if so, migrate it at that point before it's
    pinned. It'll be expensive but will guarantee ZONE_MOVABLE availability
    if that's what they want. The CMA people might also want to take
    advantage of this if the page happened to be in the MIGRATE_CMA
    pageblock.

    --
    Mel Gorman
    SUSE Labs


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