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. Signed-off-by: Junil Lee <junil0814.lee@xxxxxxx> --- 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() */ + free_obj |= BIT(HANDLE_PIN_BIT); record_obj(handle, free_obj); unpin_tag(handle); obj_free(pool, class, used_obj); -- 2.6.2 -- 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>