On 12/24/2013 03:45 PM, Joonsoo Kim wrote: > 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> > move_to_new_page() > > 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. > Yes, Looks like this is a potential race condition. > 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? > I think we can down_read(&mm->mmap_sem) before remove_migration_ptes() to fix this issue, but I don't have time to verify it currently. -- Regards, -Bob -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>