lists  /  announce  owl-users  owl-dev  john-users  john-dev  passwdqc-users  yescrypt  popa3d-users  /  oss-security  kernel-hardening  musl  sabotage  tlsify  passwords  /  crypt-dev  xvendor  /  Bugtraq  Full-Disclosure  linux-kernel  linux-netdev  linux-ext4  PHC 
Open Source and information security mailing list archives
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Date:	Wed, 11 Jan 2012 13:10:22 +0100
From:	Jan Kara <>
To:	Surbhi Palande <>
Cc:	Jan Kara <>, Eric Sandeen <>,
	Kamal Mostafa <>,
	Andreas Dilger <>,
	Randy Dunlap <>,
	Theodore Tso <>,,
	Valerie Aurora <>,
	Christopher Chaltain <>,
	"Peter M. Petrakis" <>,
	Mikulas Patocka <>
Subject: Re: [PATCH v2 1/7] Adding support to freeze and unfreeze a journal

On Tue 10-01-12 21:38:29, Surbhi Palande wrote:
> On second thoughts, I fail to see why there is still a race window
> after this patch.
> Here are the reasons why i fail to see how the data can be dirtied
> when all the operations involve a journal:
> ----------
> So here is the problem that we see
> 	CPU1						         CPU2
>        Task1 (write operation)				          Task2
> ---------------------------------------------------------------------------------------
> t1 	ext4_journal_start()
> t2	  ext4_journal_start_sb()
> t3	    vfs_check_frozen				sb->frozen=SB_FREEZE_WRITE
> t4		jbd2_journal_start()			/* hence forth all processes calling
> vfs_check_frozen will wait */
  Note that we call vfs_check_frozen(sb, SB_FREEZE_TRANS) in
ext4_journal_start_sb(). Thus we start blocking only when s_frozen ==
SB_FREEZE_TRANS and we just ignore s_frozen == SB_FREEZE_WRITE.

> Now, our aim is to stop Task1 from dirtying the page cache ie in
> starting this transaction. However if it is successful in starting
> this transaction, then we want to make sure that this transaction is
> flushed out.
> Correct?
  Not quite. Flushing a journal will flush dirty metadata but we will still
have dirty pages (dirty data is not part of any transaction). So in the
scenarion I describe in
all metadata changes will be flushed inside ->freeze_fs (at least for
journalling filesystems) but pages will be left dirty. Is it clearer now?

But your comment makes me realize that the situation is simpler than I
thought by the fact that we only have to protect paths that create dirty
data as dirty metadata can be handled by flushing a journal. And there are
only a few places creating dirty data. So a reasonably clean solution
shouldn't be that complicated after all. I'll tweak my patch and try it in
a moment.

Jan Kara <>
To unsubscribe from this list: send the line "unsubscribe linux-ext4" in
the body of a message to
More majordomo info at

Powered by blists - more mailing lists