On 05/02/2011 04:16 PM, Jan Kara wrote:
On Mon 02-05-11 15:30:23, Surbhi Palande wrote:
On 05/02/2011 03:20 PM, Jan Kara wrote:
On Mon 02-05-11 14:27:51, Surbhi Palande wrote:
On 05/02/2011 01:56 PM, Jan Kara wrote:
On Mon 02-05-11 12:07:59, Surbhi Palande wrote:
On 04/06/2011 02:21 PM, Dave Chinner wrote:
On Wed, Apr 06, 2011 at 08:18:56AM +0200, Jan Kara wrote:
On Wed 06-04-11 15:40:05, Dave Chinner wrote:
On Fri, Apr 01, 2011 at 04:08:56PM +0200, Jan Kara wrote:
On Fri 01-04-11 10:40:50, Dave Chinner wrote:
If you don't allow the page to be dirtied in the fist place, then
nothing needs to be done to the writeback path because there is
nothing dirty for it to write back.
Sure but that's only the problem he was able to hit. But generally,
there's a problem with needing s_umount for unfreezing because it isn't
clear there aren't other code paths which can block with s_umount held
waiting for fs to get unfrozen. And these code paths would cause the same
deadlock. That's why I chose to get rid of s_umount during thawing.
Holding the s_umount lock while checking if frozen and sleeping
is essentially an ABBA lock inversion bug that can bite in many more
places that just thawing the filesystem. Any where this is done should
be fixed, so I don't think just removing the s_umount lock from the thaw
path is sufficient to avoid problems.
That's easily said but hard to do - any transaction start in ext3/4 may
block on filesystem being frozen (this seems to be similar for XFS as I'm
looking into the code) and transaction start traditionally nests inside
s_umount (and basically there's no way around that since sync() calls your
fs code with s_umount held).
Sure, but the question must be asked - why is ext3/4 even starting a
transaction on a clean filesystem during sync? A frozen filesystem,
by definition, is a clean filesytem, and therefore sync calls of any
kind should not be trying to write to the FS or start transactions.
XFS does this just fine, so I'd consider such behaviour on a frozen
filesystem a bug in ext3/4...
I had a look at the xfs code for seeing how this is done.
xfs_file_aio_write()
xfs_wait_for_freeze()
vfs_check_frozen()
So xfs_file_aio_write() writes to buffers when the FS is not frozen.
Now, I want to know what stops the following scenario from happening:
--------------------
xfs_file_aio_write()
xfs_wait_for_freeze()
vfs_check_frozen()
At this point F.S was not frozen, so the next instruction in the
xfs_file_aio_write() will be executed next.
However at this point (i.e after checking if F.S is frozen) the
write process gets pre-empted and say the _freeze_ process gets
control.
Now the F.S freezes and the write process gets the control back. And
so we end up writing to the page cache when the F.S is frozen.
--------------------
Can anyone please enlighten me on how& why this premption is _not_
possible?
Thanks for your reply.
XFS works similarly as ext4 in this regard I believe. They have the log
frozen in xfs_freeze() so if the race you describe above happens, either
the writing process gets caught waiting for log to unfreeze
Agreed.
or it manages
to start a transaction and then freezing process waits for transaction to
finish before it can proceed with freezing. I'm not sure why is there the
check in xfs_file_aio_write()...
I am sorry, but I don't understand how this will happen - i.e I
can't understand what stops freeze_super() (or ext4_freeze) from
freezing a superblock (as the write process stopped just before
writing anything for this transaction and has not taken any locks?)
So ext4_freeze() does
jbd2_journal_lock_updates(journal)
which waits for all running transactions to finish and updates
j_barrier_count which stops any news ones from proceeding (check
function start_this_handle()).
Yes, but ext4_freeze() also calls
jbd2_journal_unlock_updates(journal) which decrements the
j_barrier_count (which was previously updated/incremented in
jbd2_journal_lock_updates) ? before it returns. So after this call a
new transaction/handle can be accepted/started.
A comment in ext4_freeze() says:
/* we rely on s_frozen to stop further updates */
(before calling jbd2_journal_unlock_updates())
Ah, drat, you're right. I've missed this other part. It's the problem
that if you expect to see something, you'll see it regardless of the real
code ;).
The fact is we do vfs_check_frozen() in ext4_journal_start_sb() but indeed
it's still racy (although the race window is relatively small) because the
filesystem can become frozen the instant after we check vfs_check_frozen().
Commit 6b0310fb broke it for ext4.
I guess the code was mostly copied from XFS which seems to have the same
problem in xfs_trans_alloc() since the git history beginning. I see two
ways to fix this - either fix ext4/xfs to check s_frozen after starting
a transaction and if the filesystem is being frozen, we stop the
transaction, wait for fs to get unfrozen, and restart. Another option is
to create an analogous logic using a atomic counter of write ops in vfs
that could be used by all filesystems. We'd just have to replace
vfs_check_frozen() with vfs_start_write() and add vfs_stop_write() at
appropriate places...
How about calling jbd2_journal_unlock_updates(EXT4_SB(sb)->s_journal);
from ext4_unfreeze()?
So that indeed no transactions can be started before unfreeze is called.
This has another advantage, that it rightfully does not let you update
the access time when the F.S is frozen (touch_atime called from a read
path when the F.S is frozen) Otherwise we also need to fix this path.
Warm Regards,
Surbhi.
Dave, Christoph, any opinions on this?
Honza
--
To unsubscribe from this list: send the line "unsubscribe linux-ext4" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
