Hi,

On 03/04/18 13:05, Jan Kara wrote:
Hello,

On Sun 01-04-18 10:01:02, syzbot wrote:
syzbot hit the following crash on upstream commit
10b84daddbec72c6b440216a69de9a9605127f7a (Sat Mar 31 17:59:00 2018 +0000)
Merge branch 'perf-urgent-for-linus' of
git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
syzbot dashboard link:
https://syzkaller.appspot.com/bug?extid=b7772c65a1d88bfd8fca

C reproducer: https://syzkaller.appspot.com/x/repro.c?id=5705587757154304
syzkaller reproducer:
https://syzkaller.appspot.com/x/repro.syz?id=5644332530925568
Raw console output:
https://syzkaller.appspot.com/x/log.txt?id=5472755969425408
Kernel config:
https://syzkaller.appspot.com/x/.config?id=-2760467897697295172
compiler: gcc (GCC) 7.1.1 20170620

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+b7772c65a1d88bfd8...@syzkaller.appspotmail.com
It will help syzbot understand when the bug is fixed. See footer for
details.
If you forward the report, please keep this part and the footer.

gfs2: fsid=loop0.0: jid=0, already locked for use
gfs2: fsid=loop0.0: jid=0: Looking at journal...
gfs2: fsid=loop0.0: jid=0: Done
gfs2: fsid=loop0.0: first mount done, others may mount
gfs2: fsid=loop0.0: found 1 quota changes
WARNING: CPU: 0 PID: 4469 at ./include/linux/backing-dev.h:341 inode_to_wb
include/linux/backing-dev.h:338 [inline]
WARNING: CPU: 0 PID: 4469 at ./include/linux/backing-dev.h:341
account_page_dirtied+0x8f9/0xcb0 mm/page-writeback.c:2416
Kernel panic - not syncing: panic_on_warn set ...

CPU: 0 PID: 4469 Comm: syzkaller368843 Not tainted 4.16.0-rc7+ #9
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
Call Trace:
  __dump_stack lib/dump_stack.c:17 [inline]
  dump_stack+0x194/0x24d lib/dump_stack.c:53
  panic+0x1e4/0x41c kernel/panic.c:183
  __warn+0x1dc/0x200 kernel/panic.c:547
  report_bug+0x1f4/0x2b0 lib/bug.c:186
  fixup_bug.part.10+0x37/0x80 arch/x86/kernel/traps.c:178
  fixup_bug arch/x86/kernel/traps.c:247 [inline]
  do_error_trap+0x2d7/0x3e0 arch/x86/kernel/traps.c:296
  do_invalid_op+0x1b/0x20 arch/x86/kernel/traps.c:315
  invalid_op+0x1b/0x40 arch/x86/entry/entry_64.S:986
RIP: 0010:inode_to_wb include/linux/backing-dev.h:338 [inline]
RIP: 0010:account_page_dirtied+0x8f9/0xcb0 mm/page-writeback.c:2416
RSP: 0018:ffff8801d966e5c0 EFLAGS: 00010093
RAX: ffff8801acb7e600 RBX: 1ffff1003b2cdcba RCX: ffffffff818f47a9
RDX: 0000000000000000 RSI: ffff8801d3338148 RDI: 0000000000000082
RBP: ffff8801d966e698 R08: 1ffff1003b2cdc13 R09: 000000000000000c
R10: ffff8801d966e558 R11: 0000000000000002 R12: ffff8801c96f0368
R13: ffffea0006b12780 R14: ffff8801c96f01d8 R15: ffff8801c96f01d8
  __set_page_dirty+0x100/0x4b0 fs/buffer.c:605
  mark_buffer_dirty+0x454/0x5d0 fs/buffer.c:1126
Huh, I don't see how this could possibly happen. The warning is:

         WARN_ON_ONCE(debug_locks &&
                      (!lockdep_is_held(&inode->i_lock) &&
                       !lockdep_is_held(&inode->i_mapping->tree_lock) &&
                       !lockdep_is_held(&inode->i_wb->list_lock)));

Now __set_page_dirty() which called account_page_dirtied() just did:

spin_lock_irqsave(&mapping->tree_lock, flags);

Now the fact is that account_page_dirtied() actually checks
mapping->host->i_mapping->tree_lock so if mapping->host->i_mapping doesn't
get us back to 'mapping', that would explain the warning. But then
something would have to be very wrong in the GFS2 land... Adding some GFS2
related CCs just in case they have some idea.
So I looked at this for some time trying to work out what is going on. I'm sill not 100% sure now, but lets see if we can figure it out....

The stack trace shows a call path to the end of the journal flush code where we are unpinning pages that have been through the journal. Assuming that jdata is not in use (it is used for some internal files, even if it is not selected by the user) then it is most likely that this applies to a metadata page.

For recent gfs2, all the metadata pages are kept in an address space which for inodes is in the relevant glock, and for resource groups is a single address space kept for only that purpose in the super block. In both of those cases the mapping->host points to the block device inode. Since the inode's mapping->host reflects only the block device address space (unused by gfs2) we would not expect it to point back to the relevant address space.

As far as I can tell this usage is ok, since it doesn't make much sense to require lots of inodes to be hanging around uselessly just to keep metadata pages in. That after all, is why the address space and inode are separate structures in the first place since it is not a one to one relationship. So I think that probably explains why this triggers, since the test is not really a valid one in all cases,

Steve.

  gfs2_unpin+0x143/0x12c0 fs/gfs2/lops.c:108
  buf_lo_after_commit+0x273/0x430 fs/gfs2/lops.c:512
  lops_after_commit fs/gfs2/lops.h:67 [inline]
  gfs2_log_flush+0xe2a/0x2750 fs/gfs2/log.c:809
  do_sync+0x666/0xe40 fs/gfs2/quota.c:958
  gfs2_quota_sync+0x2cc/0x570 fs/gfs2/quota.c:1301
  gfs2_sync_fs+0x46/0xb0 fs/gfs2/super.c:956
  __sync_filesystem fs/sync.c:39 [inline]
  sync_filesystem+0x188/0x2e0 fs/sync.c:64
  generic_shutdown_super+0xd5/0x540 fs/super.c:425
  kill_block_super+0x9b/0xf0 fs/super.c:1146
  gfs2_kill_sb+0x133/0x1b0 fs/gfs2/ops_fstype.c:1392
  deactivate_locked_super+0x88/0xd0 fs/super.c:312
  deactivate_super+0x141/0x1b0 fs/super.c:343
  cleanup_mnt+0xb2/0x150 fs/namespace.c:1173
  __cleanup_mnt+0x16/0x20 fs/namespace.c:1180
  task_work_run+0x199/0x270 kernel/task_work.c:113
  exit_task_work include/linux/task_work.h:22 [inline]
  do_exit+0x9bb/0x1ad0 kernel/exit.c:865
  do_group_exit+0x149/0x400 kernel/exit.c:968
  SYSC_exit_group kernel/exit.c:979 [inline]
  SyS_exit_group+0x1d/0x20 kernel/exit.c:977
  do_syscall_64+0x281/0x940 arch/x86/entry/common.c:287
  entry_SYSCALL_64_after_hwframe+0x42/0xb7
RIP: 0033:0x456c29
RSP: 002b:00007fff74938dc8 EFLAGS: 00000202 ORIG_RAX: 00000000000000e7
RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 0000000000456c29
RDX: 00000000004170e0 RSI: 0000000000000000 RDI: 0000000000000001
RBP: 0000000000000003 R08: 000000000000000a R09: 0000000000418100
R10: 00000000200a9300 R11: 0000000000000202 R12: 0000000000000004
R13: 0000000000418100 R14: 0000000000000000 R15: 0000000000000000
Dumping ftrace buffer:
    (ftrace buffer empty)
Kernel Offset: disabled
Rebooting in 86400 seconds..
                                                                Honza

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