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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 <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.

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...)

Thanks,
-Kame




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