On Thu, Dec 26, 2013 at 03:18:58PM +0900, Konstantin Khlebnikov wrote: >Bob Liu <bob.liu@xxxxxxxxxx> wrote: >>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. > >Hmm. This kind of race looks impossible: dup_mmap() always places child's >vma in into rmap tree after parent's one. For file-vma it's done explicitly >(vma_interval_tree_insert_after), for anon vma it's true because rb-tree >insert function goes to right branch if elements are equal. > >Thus remove_migration_ptes() sees parent's pte first: >If child has the copy this function will check it after that. >And they are already synchronized with parent's and child's pte locks. > Agreed. >Sorry for double posting, gmail cannot into plain text =) > >-- >Sent from my Android device with K-9 Mail. Please excuse my brevity. > >-- >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>