On Fri, Jan 15, 2016 at 04:39:11PM +0900, 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() > > Before code make crash as below: > 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 > > 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. If you verified it solved your problem, we should mark this patch as stable. > > Signed-off-by: Junil Lee <junil0814.lee@xxxxxxx> Acked-by: Minchan Kim <minchan@xxxxxxxxxx> Below comment. > --- > mm/zsmalloc.c | 2 ++ > 1 file changed, 2 insertions(+) > > diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c > index e7414ce..a24ccb1 100644 > --- a/mm/zsmalloc.c > +++ b/mm/zsmalloc.c > @@ -1635,6 +1635,8 @@ static int migrate_zspage(struct zs_pool *pool, struct size_class *class, > free_obj = obj_malloc(d_page, class, handle); > zs_object_copy(free_obj, used_obj, class); > index++; > + /* Must not unlock before unpin_tag() */ I want to make comment more clear. /* * record_obj updates handle's value to free_obj and it will invalidate * lock bit(ie, HANDLE_PIN_BIT) of handle, which breaks synchronization * using pin_tag(e,g, zs_free) so let's keep the lock bit. */ Thanks. > + free_obj |= BIT(HANDLE_PIN_BIT); > record_obj(handle, free_obj); > unpin_tag(handle); > obj_free(pool, class, used_obj); > -- > 2.6.2 > -- Kind regards, Minchan Kim -- 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>