Hi Joonsoo, On Tue, Dec 24, 2013 at 04:45:46PM +0900, 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 >... >From Sasha's report: | [ 3800.520039] page:ffffea0000245800 count:12 mapcount:4 mapping:ffff88001d0c3668 index:0x7de | [ 3800.521404] page flags: 0x1fffff8038003c(referenced|uptodate|dirty|lru|swapbacked|unevictable|mlocked) | [ 3800.522585] pc:ffff88001ed91600 pc->flags:2 pc->mem_cgroup:ffffc90000c0a000 IIUC, C's mapcount should be 0 as B's in the race condition you mentioned. Regards, Wanpeng Li >... >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? > >Thanks. > >-- >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> -- 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>