On Sat, Mar 03, 2012 at 10:27:45PM +0800, Fengguang Wu wrote:

[correct email addresses for Artem and Adrian]

On Sat, Mar 03, 2012 at 09:55:58PM +0800, Fengguang Wu wrote:

quoted

On Fri, Mar 02, 2012 at 11:57:00AM -0800, Andrew Morton wrote:

quoted

On Fri, 2 Mar 2012 18:39:51 +0800
Fengguang Wu [off-list ref] wrote:

quoted

quoted

And I agree it's unlikely but given enough time and people, I
believe someone finds a way to (inadvertedly) trigger this.

Right. The pageout works could add lots more iput() to the flusher
and turn some hidden statistical impossible bugs into real ones.

Fortunately the "flusher deadlocks itself" case is easy to detect and
prevent as illustrated in another email.

It would be a heck of a lot safer and saner to avoid the iput().  We
know how to do this, so why not do it?

My concern about the page lock is, it costs more code and sounds like
hacking around something. It seems we (including me) have been trying
to shun away from the iput() problem. Since it's unlikely we are to
get rid of the already existing iput() calls from the flusher context,
why not face the problem, sort it out and use it with confident in new
code?

Let me try it now. The only scheme iput() can deadlock the flusher is
for the iput() path to come back to queue some work and wait for it.
Here are the exhaust list of the queue+wait paths:

quoted

writeback_inodes_sb_nr_if_idle

Sorry the above function is actually called from all over btrfs. ext4
uses the much more heavy weight writeback_inodes_sb_if_idle(). I'm not
sure if ext4/ubifs developers are fully aware that these functions may
take seconds or even dozens of seconds to complete and show up as long
delays to the users, because they write and wait ALL dirty pages on
the superblock. Even w/o the iput() deadlock problem, it's still
questionable to start and wait for such big writeback works from fs
code.  If these waits could be turned into congestion_wait() style
throttling, it should not only completely remove the possibility of
iput() deadlock, but also make the delays much smaller.
congestion_wait() is just an example and may not be a good fit, but
the *_if_idle thing does indicate that the calling ext4/btrfs sites
are pretty flexible (or careless) about the exact policy used.

Thanks,
Fengguang

quoted

  ext4_nonda_switch
    ext4_page_mkwrite                   # from page fault
    ext4_da_write_begin                 # from user writes

writeback_inodes_sb_nr
  quotactl syscall                      # from syscall
  __sync_filesystem                     # from sync/umount
  shrink_liability                      # ubifs
    make_free_space
      ubifs_budget_space                # from all over ubifs:

   2    274  /c/linux/fs/ubifs/dir.c <<ubifs_create>>
   3    531  /c/linux/fs/ubifs/dir.c <<ubifs_link>>
   4    586  /c/linux/fs/ubifs/dir.c <<ubifs_unlink>>
   5    675  /c/linux/fs/ubifs/dir.c <<ubifs_rmdir>>
   6    731  /c/linux/fs/ubifs/dir.c <<ubifs_mkdir>>
   7    803  /c/linux/fs/ubifs/dir.c <<ubifs_mknod>>
   8    871  /c/linux/fs/ubifs/dir.c <<ubifs_symlink>>
   9   1006  /c/linux/fs/ubifs/dir.c <<ubifs_rename>>
  10   1009  /c/linux/fs/ubifs/dir.c <<ubifs_rename>>
  11    246  /c/linux/fs/ubifs/file.c <<write_begin_slow>>
  12    388  /c/linux/fs/ubifs/file.c <<allocate_budget>>
  13   1125  /c/linux/fs/ubifs/file.c <<do_truncation>>   <===== deadlockable
  14   1217  /c/linux/fs/ubifs/file.c <<do_setattr>>
  15   1381  /c/linux/fs/ubifs/file.c <<update_mctime>>
  16   1486  /c/linux/fs/ubifs/file.c <<ubifs_vm_page_mkwrite>>
  17    110  /c/linux/fs/ubifs/ioctl.c <<setflags>>
  19    122  /c/linux/fs/ubifs/xattr.c <<create_xattr>>
  20    201  /c/linux/fs/ubifs/xattr.c <<change_xattr>>
  21    494  /c/linux/fs/ubifs/xattr.c <<remove_xattr>>

It seems they are all safe except for ubifs. ubifs may actually
deadlock from the above do_truncation() caller. However it should be

Sorry that do_truncation() is actually called from ubifs_setattr()
which is not related to iput().

Are there other possibilities for iput() to call into the above list
of ubifs functions, then start writeback work and wait for it which
will deadlock the flusher? ubifs_unlink() and perhaps remove_xattr()?

quoted

fixable because the ubifs call for writeback_inodes_sb_nr() sounds
very brute force writeback and wait and there may well be better way
out.

CCing ubifs developers for possible thoughts..

Thanks,
Fengguang

PS. I'll be on travel in the following week and won't have much time
for replying emails. Sorry about that.

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