Re: [PATCH] fs / ext3: Always unlock updates in ext3_freeze()

Hi,

On Monday, August 15, 2011, Jan Kara wrote:

  Hello,

On Mon 15-08-11 20:09:13, Rafael J. Wysocki wrote:

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On Monday, August 15, 2011, Jan Kara wrote:

...

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  It's not so simple as this. Ext3 relies on the mutex (the one hidden in
journal_lock_updates()) to make sure that new transaction cannot be started
while the filesystem is frozen - that's essentially what makes the
filesystem frozen. So if we want to get rid of the mutex we have to achieve
blocking by something else - ext4 uses vfs_check_frozen() in
ext4_journal_start().

I see.  Still, freeze_bdev() may be called by user space through a syscall,
as far as I can say, so it shouldn't leave the mutex locked.

  Yes, I agree with you. That's an ugliness left over from a long time ago.
I'll have a look at fixing this...

Thanks!

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  BTW,  filesystem freezing never really worked for mmaped writes under
ext3 - ext3 would have to implement page_mkwrite() callback for that - so
if you want to rely on it for suspending, this will be non-trivial.

At this point the purpose of freezing filesystems is basically to
prevent XFS from deadlocking with hibernation's memory preallocation.
For other filesystems it may or may not make a difference depending on
their implementation of freeze/unfreeze_super().

  What's exactly the problem? Memory preallocation enters direct reclaim
and that deadlocks in the filesystem?

Yes, that seems to be the case.

Thanks,
Rafael