On Thu 11-02-21 12:57:17, Matthew Wilcox wrote: > On Thu, Feb 11, 2021 at 01:34:48PM +0100, Michal Hocko wrote: > > On Thu 11-02-21 13:10:20, Jan Kara wrote: > > > On Thu 11-02-21 12:28:48, Dmitry Vyukov wrote: > > > > On Thu, Feb 11, 2021 at 12:22 PM Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote: > > > > > > > > > > On Thu, Feb 11, 2021 at 11:49 AM Jan Kara <jack@xxxxxxx> wrote: > > > > > > > > > > > > Hello, > > > > > > > > > > > > added mm guys to CC. > > > > > > > > > > > > On Wed 10-02-21 05:35:18, syzbot wrote: > > > > > > > HEAD commit: 1e0d27fc Merge branch 'akpm' (patches from Andrew) > > > > > > > git tree: upstream > > > > > > > console output: https://syzkaller.appspot.com/x/log.txt?x=15cbce90d00000 > > > > > > > kernel config: https://syzkaller.appspot.com/x/.config?x=bd1f72220b2e57eb > > > > > > > dashboard link: https://syzkaller.appspot.com/bug?extid=bfdded10ab7dcd7507ae > > > > > > > userspace arch: i386 > > > > > > > > > > > > > > Unfortunately, I don't have any reproducer for this issue yet. > > > > > > > > > > > > > > IMPORTANT: if you fix the issue, please add the following tag to the commit: > > > > > > > Reported-by: syzbot+bfdded10ab7dcd7507ae@xxxxxxxxxxxxxxxxxxxxxxxxx > > > > > > > > > > > > > > ====================================================== > > > > > > > WARNING: possible circular locking dependency detected > > > > > > > 5.11.0-rc6-syzkaller #0 Not tainted > > > > > > > ------------------------------------------------------ > > > > > > > kswapd0/2246 is trying to acquire lock: > > > > > > > ffff888041a988e0 (jbd2_handle){++++}-{0:0}, at: start_this_handle+0xf81/0x1380 fs/jbd2/transaction.c:444 > > > > > > > > > > > > > > but task is already holding lock: > > > > > > > ffffffff8be892c0 (fs_reclaim){+.+.}-{0:0}, at: __fs_reclaim_acquire+0x0/0x30 mm/page_alloc.c:5195 > > > > > > > > > > > > > > which lock already depends on the new lock. > > > > > > > > > > > > > > the existing dependency chain (in reverse order) is: > > > > > > > > > > > > > > -> #2 (fs_reclaim){+.+.}-{0:0}: > > > > > > > __fs_reclaim_acquire mm/page_alloc.c:4326 [inline] > > > > > > > fs_reclaim_acquire+0x117/0x150 mm/page_alloc.c:4340 > > > > > > > might_alloc include/linux/sched/mm.h:193 [inline] > > > > > > > slab_pre_alloc_hook mm/slab.h:493 [inline] > > > > > > > slab_alloc_node mm/slub.c:2817 [inline] > > > > > > > __kmalloc_node+0x5f/0x430 mm/slub.c:4015 > > > > > > > kmalloc_node include/linux/slab.h:575 [inline] > > > > > > > kvmalloc_node+0x61/0xf0 mm/util.c:587 > > > > > > > kvmalloc include/linux/mm.h:781 [inline] > > > > > > > ext4_xattr_inode_cache_find fs/ext4/xattr.c:1465 [inline] > > > > > > > ext4_xattr_inode_lookup_create fs/ext4/xattr.c:1508 [inline] > > > > > > > ext4_xattr_set_entry+0x1ce6/0x3780 fs/ext4/xattr.c:1649 > > > > > > > ext4_xattr_ibody_set+0x78/0x2b0 fs/ext4/xattr.c:2224 > > > > > > > ext4_xattr_set_handle+0x8f4/0x13e0 fs/ext4/xattr.c:2380 > > > > > > > ext4_xattr_set+0x13a/0x340 fs/ext4/xattr.c:2493 > > > > > > > ext4_xattr_user_set+0xbc/0x100 fs/ext4/xattr_user.c:40 > > > > > > > __vfs_setxattr+0x10e/0x170 fs/xattr.c:177 > > > > > > > __vfs_setxattr_noperm+0x11a/0x4c0 fs/xattr.c:208 > > > > > > > __vfs_setxattr_locked+0x1bf/0x250 fs/xattr.c:266 > > > > > > > vfs_setxattr+0x135/0x320 fs/xattr.c:291 > > > > > > > setxattr+0x1ff/0x290 fs/xattr.c:553 > > > > > > > path_setxattr+0x170/0x190 fs/xattr.c:572 > > > > > > > __do_sys_setxattr fs/xattr.c:587 [inline] > > > > > > > __se_sys_setxattr fs/xattr.c:583 [inline] > > > > > > > __ia32_sys_setxattr+0xbc/0x150 fs/xattr.c:583 > > > > > > > do_syscall_32_irqs_on arch/x86/entry/common.c:77 [inline] > > > > > > > __do_fast_syscall_32+0x56/0x80 arch/x86/entry/common.c:139 > > > > > > > do_fast_syscall_32+0x2f/0x70 arch/x86/entry/common.c:164 > > > > > > > entry_SYSENTER_compat_after_hwframe+0x4d/0x5c > > > > > > > > > > > > This stacktrace should never happen. ext4_xattr_set() starts a transaction. > > > > > > That internally goes through start_this_handle() which calls: > > > > > > > > > > > > handle->saved_alloc_context = memalloc_nofs_save(); > > > > > > > > > > > > and we restore the allocation context only in stop_this_handle() when > > > > > > stopping the handle. And with this fs_reclaim_acquire() should remove > > > > > > __GFP_FS from the mask and not call __fs_reclaim_acquire(). > > > > > > > > > > > > Now I have no idea why something here didn't work out. Given we don't have > > > > > > a reproducer it will be probably difficult to debug this. I'd note that > > > > > > about year and half ago similar report happened (got autoclosed) so it may > > > > > > be something real somewhere but it may also be just some HW glitch or > > > > > > something like that. > > > > > > > > > > HW glitch is theoretically possible. But if we are considering such > > > > > causes, I would say a kernel memory corruption is way more likely, we > > > > > have hundreds of known memory-corruption-capable bugs open. In most > > > > > cases they are caught by KASAN before doing silent damage. But KASAN > > > > > can miss some cases. > > > > > > > > > > I see at least 4 existing bugs with similar stack: > > > > > https://syzkaller.appspot.com/bug?extid=bfdded10ab7dcd7507ae > > > > > https://syzkaller.appspot.com/bug?extid=a7ab8df042baaf42ae3c > > > > > https://syzkaller.appspot.com/bug?id=c814a55a728493959328551c769ede4c8ff72aab > > > > > https://syzkaller.appspot.com/bug?id=426ad9adca053dafcd698f3a48ad5406dccc972b > > > > > > > > > > All in all, I would not assume it's a memory corruption. When we had > > > > > bugs that actually caused silent memory corruption, that caused a > > > > > spike of random one-time crashes all over the kernel. This does not > > > > > look like it. > > > > > > > > I wonder if memalloc_nofs_save (or any other manipulation of > > > > current->flags) could have been invoked from interrupt context? I > > > > think it could cause the failure mode we observe (extremely rare > > > > disappearing flags). It may be useful to add a check for task context > > > > there. > > > > > > That's an interesting idea. I'm not sure if anything does manipulate > > > current->flags from inside an interrupt (definitely memalloc_nofs_save() > > > doesn't seem to be) but I'd think that in fully preemtible kernel, > > > scheduler could preempt the task inside memalloc_nofs_save() and the > > > current->flags manipulation could also clash with a manipulation of these > > > flags by the scheduler if there's any? > > > > current->flags should be always manipulated from the user context. But > > who knows maybe there is a bug and some interrupt handler is calling it. > > This should be easy to catch no? > > Why would it matter if it were? I was thinking about a clobbered state because updates to ->flags are not atomic because this shouldn't ever be updated concurrently. So maybe a racing interrupt could corrupt the flags state? -- Michal Hocko SUSE Labs
