NTFS: Fix a mount time deadlock.

Big thanks go to Mathias Kolehmainen for reporting the bug, providing
debug output and testing the patches I sent him to get it working.

The fix was to stop calling ntfs_attr_set() at mount time as that causes
balance_dirty_pages_ratelimited() to be called which on systems with
little memory actually tries to go and balance the dirty pages which tries
to take the s_umount semaphore but because we are still in fill_super()
across which the VFS holds s_umount for writing this results in a
deadlock.

We now do the dirty work by hand by submitting individual buffers.  This
has the annoying "feature" that mounting can take a few seconds if the
journal is large as we have clear it all.  One day someone should improve
on this by deferring the journal clearing to a helper kernel thread so it
can be done in the background but I don't have time for this at the moment
and the current solution works fine so I am leaving it like this for now.

Signed-off-by: Anton Altaparmakov <aia21@cantab.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 12 insertions, 0 deletions

diff --git a/fs/ntfs/ChangeLog b/fs/ntfs/ChangeLog
index af4ef808fa9..345798ebd36 100644
--- a/fs/ntfs/ChangeLog
+++ b/fs/ntfs/ChangeLog
@@ -17,6 +17,18 @@ ToDo/Notes:
           happen is unclear however so it is worth waiting until someone hits
           the problem.
 
+2.1.29 - Fix a deadlock at mount time.
+
+        - During mount the VFS holds s_umount lock on the superblock.  So when
+          we try to empty the journal $LogFile contents by calling
+          ntfs_attr_set() when the machine does not have much memory and the
+          journal is large ntfs_attr_set() results in the VM trying to balance
+          dirty pages which in turn tries to that the s_umount lock and thus we
+          get a deadlock.  The solution is to not use ntfs_attr_set() and
+          instead do the zeroing by hand at the block level rather than page
+          cache level.
+        - Fix sparse warnings.
+
 2.1.28 - Fix a deadlock.
 
         - Fix deadlock in fs/ntfs/inode.c::ntfs_put_inode().  Thanks to Sergey