Re: [PATCH] Fix commit of ordered data buffers

[Zhang, Yanmin]

On Fri, 2006-09-29 at 05:35, Jan Kara wrote:
>   Hello,
> 
> > On Tue, 2006-09-12 at 05:05, Jan Kara wrote:
> > >   Hi Andrew,
> > > 
> > >   here is the patch that came out of the thread "set_page_buffer_dirty
> > > should skip unmapped buffers". It fixes several flaws in the code
> > > writing out ordered data buffers during commit. It definitely fixed the
> > > problem Badari was seeing with fsx-linux test.  Could you include it
> > > into -mm? Since there are quite complex interactions with other JBD code
> > > and the locking is kind of ugly, I'd leave it in -mm for a while whether
> > > some bug does not emerge ;). Thanks.
> > > 
> > > 								Honza
> > The fsx-linux test issue is a race between journal_commit_transaction
> > and journal_dirty_data. After journal_commit_transaction adds buffer_head pointers
> > to wbuf, it might unlock journal->j_list_lock. Although all buffer head in wbuf are locked,
> > does that prevent journal_dirty_data from unlinking the buffer head from the transaction
> > and fsx-linux from truncating it?
>   Yes, it does. Because the buffers are locked *and dirty*. Nothing can
> clear the dirty bit while we are holding the lock and
> journal_dirty_data() also waits until it can safely write out the buffer
> - which is after we release the buffer lock.
With your patch, it's not true because journal_submit_data_buffers clear the dirty
flag, so later journal_dirty_data won't try to lock/flush the buffer. journal_dirty_data
would just move the jh to the t_sync_datalist of a new transaction.

> 
> > I'm not a journal expert. But I want to discuss it.
> > 
> > My investigation is below (Scenario):
> > 
> > fsx-linux starts journal_dirty_data and journal_dirty_data links a jh to
> > journal->j_running_transaction's t_sync_datalist, kjournald might not
> > write the buffer to disk quickly, but saves it to array wbuf.
> > Then, fsx-linux starts the second journal_dirty_data of a new transaction
> > might submit the same buffer head and move the jh to the new transaction's
> > t_sync_datalist.
>   Yes, but this happens only after the buffer is removed from wbuf[] as
> I explain above.
> 
> > Then, fsx-linux truncates the last a couple of buffers of a page.
> > Then, block_write_full_page calls invalidatepage to invalidate the last a couple
> > of buffers of the page, so the journal_heads of the buffer_head are unlinked and
> > are marked as unmapped.
> > Then, fsx-linux extend the file and does a msync after changing the page content
> > by mmaping the page, so the page (inclduing the last buffer head) is marked dirty
> > again.
> > Then, kjournald's journal_commit_transaction goes through wbuf to submit_bh all
> > dirty buffers, but one buffer head is already marked as unmapped. A bug check is
> > triggerred.
I think the reason that your patch fixes it is that journal_invalidatepage
will lock the buffer before calling journal_unmap_buffer. So the last step to trigger
the bug will be synced with journal_commit_transaction.

> I think the right way is to let journal_dirty_data to wait till wbuf is flushed.
>   This actually happens in my fix too. And my fix has also a bonus of
> fixing a few other flaws... Otherwise your patch seems to be right.
Other flaws could be fixed by other small patches to make it clearer.
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