Re: mm: kernel BUG at include/linux/swapops.h:131!

From: Joonsoo Kim
Date: Tue Dec 24 2013 - 02:46:14 EST

On Tue, Dec 24, 2013 at 03:07:05PM +0900, Joonsoo Kim wrote:
> On Mon, Dec 23, 2013 at 10:01:10PM -0500, Sasha Levin wrote:
> > On 12/23/2013 09:51 PM, Joonsoo Kim wrote:
> > >On Mon, Dec 23, 2013 at 12:24:02PM -0500, Sasha Levin wrote:
> > >>>Ping?
> > >>>
> > >>>I've also Cc'ed the "this page shouldn't be locked at all" team.
> > >Hello,
> > >
> > >I can't find the reason of this problem.
> > >If it is reproducible, how about bisecting?
> >
> > While it reproduces under fuzzing it's pretty hard to bisect it with
> > the amount of issues uncovered by trinity recently.
> >
> > I can add any debug code to the site of the BUG if that helps.
> Good!
> It will be helpful to add dump_page() in migration_entry_to_page().
> Thanks.

Minchan teaches me that there is possible race condition between
fork and migration.

Please consider following situation.

Process A (do migration) Process B (parents) Process C (child)

try_to_unmap() for migration <begin> fork
setup migration entry to B's vma
try_to_unmap() for migration <end>

link new vma
into interval tree
remove_migration_ptes() <begin>
check and clear migration entry on C's vma
... copy_one_pte:
... now, B and C have migration entry
check and clear migration entry on B's vma
remove_migration_ptes() <end>

Eventually, migration entry on C's vma is left.
And then, when C exits, above BUG_ON() can be triggered.

I'm not sure the I am right, so please think of it together. :)
And I'm not sure again that above assumption is related to this trigger report,
since this may exist for a long time.

So my question to mm folks is is above assumption possible and do we have
any protection mechanism on this race?

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