Re: [syzbot] [ntfs3?] INFO: task hung in do_user_addr_fault (3)

[Tetsuo Handa]

On 2023/01/02 10:40, Linus Torvalds wrote:
> So I think that we have:
> 
>  - ntfs_truncate()  gets the ni_lock (fs/ntfs3/file.c:393)
> 
>  - it then - while holding that lock - calls (on line 395):
> 
>       truncate_setsize ->
>          truncate_pagecache ->
>             truncate_inode_pages ->
>                truncate_inode_pages_range ->
>                   folio_lock
> 
> but that deadlocks on another process that wants to read that page,
> and that needs ni_lock to do so.
> 
> So yes, it does look like a ntfs3 deadlock involving ni_lock.

Yes, I think you are right. My patch confirmed that other threads are not
holding ni_lock lock, which means that this is a deadlock between
PG_locked bit and ni_lock lock.

filemap_update_page() calls filemap_read_folio() after calling
folio_trylock(). Since folio_trylock() sets PG_locked bit,

 mutex_lock_nested+0x17/0x20 kernel/locking/mutex.c:799
 ni_lock fs/ntfs3/ntfs_fs.h:1122 [inline]
 attr_data_get_block+0x4a6/0x2e40 fs/ntfs3/attrib.c:919
 ntfs_get_block_vbo+0x374/0xd20 fs/ntfs3/inode.c:573
 do_mpage_readpage+0x98b/0x1bb0 fs/mpage.c:208
 mpage_read_folio+0x103/0x1d0 fs/mpage.c:379
 filemap_read_folio+0x1ba/0x7f0 mm/filemap.c:2426
 filemap_update_page+0x3ca/0x550 mm/filemap.c:2511
 filemap_get_pages+0x8d8/0x1110 mm/filemap.c:2624
 filemap_read+0x3e7/0xee0 mm/filemap.c:2694

is trying to take ni_lock after setting PG_locked bit.

On the other hand, folio_lock() waits until PG_locked bit is cleared,
but unfortunately ntfs3_setattr() already took ni_lock before calling
folio_lock().

 io_schedule+0x83/0x100 kernel/sched/core.c:8811
 folio_wait_bit_common+0x8ca/0x1390 mm/filemap.c:1297
 folio_lock include/linux/pagemap.h:938 [inline]
 truncate_inode_pages_range+0xc8d/0x1650 mm/truncate.c:421
 truncate_inode_pages mm/truncate.c:448 [inline]
 truncate_pagecache mm/truncate.c:743 [inline]
 truncate_setsize+0xcb/0xf0 mm/truncate.c:768
 ntfs_truncate fs/ntfs3/file.c:395 [inline]
 ntfs3_setattr+0x5a5/0xca0 fs/ntfs3/file.c:696

Since no lockdep annotation is used for e.g. PG_locked bit, this deadlock
cannot be detected by lockdep...