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Date:	Mon, 4 Apr 2011 16:09:45 +0200
From:	Jan Kara <jack@...e.cz>
To:	Yongqiang Yang <xiaoqiangnk@...il.com>
Cc:	Jan Kara <jack@...e.cz>, linux-ext4@...r.kernel.org
Subject: Re: [PATCH v0 RFC] ext4: Fix a bug in ext4_journal_start_sb().

On Mon 04-04-11 20:31:41, Yongqiang Yang wrote:
> >> Reported-by: Amir Goldstein <amir73il@...rs.sf.net>
> >> Signed-off-by: Yongqiang Yang <xiaoqiangnk@...il.com>
> >> ---
> >>  fs/ext4/super.c |   49 ++++++++++++++++++++++++++++++++++++++-----------
> >>  1 files changed, 38 insertions(+), 11 deletions(-)
> >>
> >> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> >> index ccfa686..f35b53e 100644
> >> --- a/fs/ext4/super.c
> >> +++ b/fs/ext4/super.c
> >> @@ -242,27 +242,49 @@ static void ext4_put_nojournal(handle_t *handle)
> >>   * journal_end calls result in the superblock being marked dirty, so
> >>   * that sync() will call the filesystem's write_super callback if
> >>   * appropriate.
> >> + *
> >> + * To avoid j_barrier hold in userspace when a user calls freeze(),
> >> + * ext4 prevents a new handle from being started by s_frozen, which
> >> + * is in an upper layer.
> >>   */
> >>  handle_t *ext4_journal_start_sb(struct super_block *sb, int nblocks)
> >>  {
> >>       journal_t *journal;
> >> +     handle_t  *handle;
> >>
> >>       if (sb->s_flags & MS_RDONLY)
> >>               return ERR_PTR(-EROFS);
> >>
> >> -     vfs_check_frozen(sb, SB_FREEZE_TRANS);
> >> -     /* Special case here: if the journal has aborted behind our
> >> -      * backs (eg. EIO in the commit thread), then we still need to
> >> -      * take the FS itself readonly cleanly. */
> >>       journal = EXT4_SB(sb)->s_journal;
> >> -     if (journal) {
> >> -             if (is_journal_aborted(journal)) {
> >> -                     ext4_abort(sb, "Detected aborted journal");
> >> -                     return ERR_PTR(-EROFS);
> >> -             }
> >> -             return jbd2_journal_start(journal, nblocks);
> >> +     if (!journal)
> >> +             /*
> >> +              * Under no-journal mode, vfs_check_frozen() is not neeed.
> >> +              */
> >  Why is this? Previously we waited also in the nojournal case and I don't
> > see anything that would stop modifications in the nojournal case after your
> > change...
> 
> I think that ext4 in the nojournal case should do as filesystems
> without journal, such as ext2.   ext4_ext_truncate() upwrite
  But ext2 does not support filesystem freezing...

> i_data_sem only if ext4_journal_extend() fails before
> ext4_journal_restart() is called, ext4_journal_extend() however always
> succeeds in nojournal case.
  OK, but again, I'm failing to see how i_data_sem behavior is relevant
for waiting if a filesystem is frozen... So to be clear I believe we must
do vfs_check_frozen() even in nojournal case if and only if the handle
reference count is 0 (it's stored directly in current->journal_info in
nojournal mode).

									Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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