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    SubjectRe: [PATCH 02/11] mm,migration: Do not try to migrate unmapped anonymous pages
    Date
    > On Thu, 25 Mar 2010 08:32:35 +0000
    > Mel Gorman <mel@csn.ul.ie> 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.
    > >
    > Kosaki-san,
    >
    > 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.

    But Christoph seems oppose to remove SLAB_DESTROY_BY_RCU. then refcount
    is meaningless now. I agree you if we will remove SLAB_DESTROY_BY_RCU
    in the future.

    refcount is easy understanding than rcu trick.


    > 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...)

    agreed.

    >
    > Thanks,
    > -Kame
    >
    >





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