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    SubjectRe: [PATCH 02/11] mm,migration: Do not try to migrate unmapped anonymous pages
    On Thu, 25 Mar 2010 08:32:35 +0000
    Mel Gorman <> wrote:

    > On Thu, Mar 25, 2010 at 11:49:23AM +0900, KOSAKI Motohiro wrote:
    > > > On Fri, Mar 19, 2010 at 03:21:41PM +0900, KOSAKI Motohiro wrote:
    > > Hmmm...
    > > I haven't understand your mention because I guess I was wrong.
    > >
    > > probably my last question was unclear. I mean,
    > >
    > > 1) If we still need SLAB_DESTROY_BY_RCU, why do we need to add refcount?
    > > Which difference is exist between normal page migration and compaction?
    > The processes typically calling migration today own the page they are moving
    > and is not going to exit unexpectedly during migration.
    > > 2) If we added refcount, which race will solve?
    > >
    > The process exiting and the last anon_vma being dropped while compaction
    > is running. This can be reliably triggered with compaction.
    > > IOW, Is this patch fix old issue or compaction specific issue?
    > >
    > Strictly speaking, it's an old issue but in practice it's impossible to
    > trigger because the process migrating always owns the page. Compaction
    > moves pages belonging to arbitrary processes.

    IIUC, the race in memory-hotunplug was fixed by this patch [2/11].

    But, this behavior of unmap_and_move() requires access to _freed_
    objects (spinlock). Even if it's safe because of SLAB_DESTROY_BY_RCU,
    it't not good habit in general.

    After direct compaction, page-migration will be one of "core" code of
    memory management. Then, I agree to patch [1/11] as our direction for
    keeping sanity and showing direction to more updates. Maybe adding
    refcnt and removing RCU in futuer is good.

    IMHO, pushing this patch [2/11] as "BUGFIX" independent of this set and
    adding anon_vma->refcnt [1/11] and [3/11] in 1st Direct-compaction patch
    series to show the direction will makse sense.
    (I think merging 1/11 and 3/11 will be okay...)


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