Re: Possible race between direct IO and JBD?

[Jan Kara]

On Mon 28-04-08 12:09:23, Mingming Cao wrote:
> On Mon, 2008-04-28 at 20:09 +0200, Jan Kara wrote: 
> > On Mon 28-04-08 10:11:34, Badari Pulavarty wrote:
> > > 
> > > On Mon, 2008-04-28 at 14:26 +0200, Jan Kara wrote:
> > > > Hi,
> > > > 
> > > > On Fri 25-04-08 16:38:23, Mingming Cao wrote:
> > > > > While looking at a bug related to direct IO returns to EIO, after
> > > > > looking at the code, I found there is a window that
> > > > > try_to_free_buffers() from direct IO could race with JBD, which holds
> > > > > the reference to the data buffers before journal_commit_transaction()
> > > > > ensures the data buffers has reached to the disk.
> > > > > 
> > > > > A little more detail: to prepare for direct IO, generic_file_direct_IO()
> > > > > calls invalidate_inode_pages2_range() to invalidate the pages in the
> > > > > cache before performaning direct IO.  invalidate_inode_pages2_range()
> > > > > tries to free the buffers via try_to free_buffers(), but sometimes it
> > > > > can't, due to the buffers is possible still on some transaction's
> > > > > t_sync_datalist or t_locked_list waiting for
> > > > > journal_commit_transaction() to process it. 
> > > > > 
> > > > > Currently Direct IO simply returns EIO if try_to_free_buffers() finds
> > > > > the buffer is busy, as it has no clue that JBD is referencing it.
> > > > > 
> > > > > Is this a known issue and expected behavior? Any thoughts?
> > > >   Are you seeing this in data=ordered mode? As Andrew pointed out we do
> > > > filemap_write_and_wait() so all the relevant data buffers of the inode
> > > > should be already on disk. In __journal_try_to_free_buffer() we check
> > > > whether the buffer is already-written-out data buffer and unfile and free
> > > > it in that case. It shouldn't happen that a data buffer has
> > > > b_next_transaction set so really the only idea why try_to_free_buffers()
> > > > could fail is that somebody manages to write to a page via mmap before
> > > > invalidate_inode_pages2_range() gets to it. Under which kind of load do you
> > > > observe the problem? Do you know exactly because of which condition does
> > > > journal_try_to_free_buffers() fail?
> > > > 
> > > 
> > > Thank you for your reply.
> > > 
> > > What we are noticing is invalidate_inode_pages2_range() fails with -EIO
> > > (from try_to_free_buffers() since b_count > 0).
> > > 
> > > I don't think the file is being updated through mmap(). Previous
> > > writepage() added these buffers to t_sync_data list (data=ordered).
> > > filemap_write_and_wait() waits for pagewrite back to be cleared.
> > > So, buffers are no longer dirty, but still on the t_sync_data and
> > > kjournald didn't get chance to process them yet :(
> > > 
> > > Since we have elevated b_count on these buffers, try_to_free_buffers()
> > > fails. How can we make filemap_write_and_wait() to wait for kjournald
> > > to unfile these buffers ?
> >   Hmm, I don't get one thing:
> >   The call chain is invalidate_inode_pages2_range() ->
> > invalidate_complete_page2() -> try_to_release_page() -> ext3_releasepage()
> > -> journal_try_to_free_buffers() -> __journal_try_to_free_buffer() and this
> > function should remove the buffer from the committing transaction. 
> 
> Thanks, yes I noticed that after you pointing this out.
> 
> But __journal_try_to_free_buffer() only unfile the buffer from
> t_sync_datalist or t_locked list, the journal head is not removed in
> journal_remove_journal_head() there, at that time,
> journal_remove_journal_head() just check if counter b_jcount is 0. But
> before calling __journal_try_to_free_buffer(), since
> journal_try_to_free_buffers() already increase the b_jcount in
> journal_grab_journal_head(), so the journal head is not removed in
> __journal_try_to_free_buffer-> journal_remove_journal_head()
> 
> > So who's
> > holding the reference to those buffers? 
> 
> Looking at the code, it seems the it's the journal_put_journal_head(jh)
> who remove the journal head and decrease the bh
> 
> journal_try_to_free_buffers() 
> {
> ...
> 
> jh = journal_grab_journal_head(bh);
>                 if (!jh)
>                         continue;
> 
>                 jbd_lock_bh_state(bh);
>                 __journal_try_to_free_buffer(journal, bh);
>                 journal_put_journal_head(jh);
>                 jbd_unlock_bh_state(bh);
> 
> ...
> 
> }
>  so when journal_put_journal_head()-> __journal_remove_journal_head(),
> now the b_jcount is zero, but is  
> jh->b_transaction is NULL? So it seems possible that bh ref count is non
> zero when exit from journal_put_journal_head() if jh_b_transaction is
> not cleared.
> 
> I miss where jh->b_transaction is clear to NULL? 
  __journal_unfile_buffer() called from __journal_try_to_free_buffer() sets
jh->b_transaction to NULL. So as soon as journal_put_journal_head() is
called, it results in freeing of journal head and releasing buffer
reference. So really the only possible race I see is what I describe
below...

> >   Hmm, maybe I have one idea - in theory we could call
> > __journal_try_to_free_buffer() exactly at the moment commit code inspects
> > the buffer. Then we'd release the buffer from the transaction but
> > try_to_free_buffers() would fail because of elevated b_count exactly as you
> > described. Could you maybe verify this? Not that I'd know how to easily fix
> > this ;)...

									Honza
-- 
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
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