On (01/18/16 13:14), Sergey Senozhatsky wrote: > Cc Vlastimil, > > Hello, > > On (01/18/16 10:15), Junil Lee wrote: > > To prevent unlock at the not correct situation, tagging the new obj to > > assure lock in migrate_zspage() before right unlock path. > > > > Two functions are in race condition by tag which set 1 on last bit of > > obj, however unlock succrently when update new obj to handle before call > > unpin_tag() which is right unlock path. > > > > summarize this problem by call flow as below: > > > > CPU0 CPU1 > > migrate_zspage > > find_alloced_obj() > > trypin_tag() -- obj |= HANDLE_PIN_BIT > > obj_malloc() -- new obj is not set zs_free > > record_obj() -- unlock and break sync pin_tag() -- get lock > > unpin_tag() > > Junil, can something like this be a bit simpler problem description? > > --- > > record_obj() in migrate_zspage() does not preserve handle's > HANDLE_PIN_BIT, set by find_alloced_obj()->trypin_tag(), and > implicitly (accidentally) un-pins the handle, while migrate_zspage() > still performs an explicit unpin_tag() on the that handle. > This additional explicit unpin_tag() introduces a race condition > with zs_free(), which can pin that handle by this time, so the handle > becomes un-pinned. Schematically, it goes like this: > > CPU0 CPU1 > migrate_zspage > find_alloced_obj > trypin_tag > set HANDLE_PIN_BIT zs_free() > pin_tag() > obj_malloc() -- new object, no tag > record_obj() -- remove HANDLE_PIN_BIT set HANDLE_PIN_BIT > unpin_tag() -- remove zs_free's HANDLE_PIN_BIT > > The race condition may result in a NULL pointer dereference: > Unable to handle kernel NULL pointer dereference at virtual address 00000000 > CPU: 0 PID: 19001 Comm: CookieMonsterCl Tainted: > PC is at get_zspage_mapping+0x0/0x24 > LR is at obj_free.isra.22+0x64/0x128 > Call trace: > [<ffffffc0001a3aa8>] get_zspage_mapping+0x0/0x24 > [<ffffffc0001a4918>] zs_free+0x88/0x114 > [<ffffffc00053ae54>] zram_free_page+0x64/0xcc > [<ffffffc00053af4c>] zram_slot_free_notify+0x90/0x108 > [<ffffffc000196638>] swap_entry_free+0x278/0x294 > [<ffffffc000199008>] free_swap_and_cache+0x38/0x11c > [<ffffffc0001837ac>] unmap_single_vma+0x480/0x5c8 > [<ffffffc000184350>] unmap_vmas+0x44/0x60 > [<ffffffc00018a53c>] exit_mmap+0x50/0x110 > [<ffffffc00009e408>] mmput+0x58/0xe0 > [<ffffffc0000a2854>] do_exit+0x320/0x8dc > [<ffffffc0000a3cb4>] do_group_exit+0x44/0xa8 > [<ffffffc0000ae1bc>] get_signal+0x538/0x580 > [<ffffffc000087e44>] do_signal+0x98/0x4b8 > [<ffffffc00008843c>] do_notify_resume+0x14/0x5c > > Fix the race by removing explicit unpin_tag() from migrate_zspage(). > > --- > > > > and for test, print obj value after pin_tag() in zs_free(). > > Sometimes obj is even number means break synchronization. > > > > After patched, crash is not occurred and obj is only odd number in same > > situation. > > > > Signed-off-by: Junil Lee <junil0814.lee@xxxxxxx> > > I believe Vlastimil deserves a credit here (at least Suggested-by) > Suggested-by: Vlastimil Babka <vbabka@xxxxxxx> > > > now, can the compiler re-order > > record_obj(handle, free_obj); > obj_free(pool, class, used_obj); oh, disregard the last "re-ordering" commentary, sorry. -ss -- 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>