Re: [PATCH 2.6.2-rc3-mm1] DIO read race fix

[Janet Morgan]

Andrew Morton wrote:

> Daniel McNeil <daniel@xxxxxxxx> wrote:
>  
>
> > I have found (finally) the problem causing DIO reads racing with
> > buffered writes to see uninitialized data on ext3 file systems 
> > (which is what I have been testing on).
> >    
>
> What kernel?  If -mm, is this the only remaining buffered-vs-direct
> problem?
>
>  
I think there was consensus on two other patches along the way:

   http://marc.theaimsgroup.com/?l=linux-kernel&m=107286971311559&w=2
   http://marc.theaimsgroup.com/?l=linux-aio&m=107291089712224&w=2

-Janet

> > The problem is caused by the changes to __block_write_page_full()
> > and a race with journaling:
> >
> > journal_commit_transaction() -> ll_rw_block() -> submit_bh()
> > 	
> > ll_rw_block() locks the buffer, clears buffer dirty and calls
> > submit_bh()
> >
> > A racing __block_write_full_page() (from ext3_ordered_writepage())
> >
> > 	would see that buffer_dirty() is not set because the i/o
> >        is still in flight, so it would not do a bh_submit()
> >
> > 	It would SetPageWriteback() and unlock_page() and then
> > 	see that no i/o was submitted and call end_page_writeback()
> > 	(with the i/o still in flight).
> >
> > This would allow the DIO code to issue the DIO read while buffer writes
> > are still in flight.  The i/o can be reordered by i/o scheduling and
> > the DIO can complete BEFORE the writebacks complete.  Thus the DIO
> > sees the old uninitialized data.
> >    
>
> ow.  How'd you work this out?
>
>  
>
> > Here is a quick hack that fixes it, but I am not sure if this the
> > proper long term fix.
> >    
>
> The problem is that ext3 and the VFS are using different paradigms.  VFS is
> all page-based, but ext3 is all block-based.  One or the other needs to do
> something nasty.
>
> One approach would be to change the JBD write_out_data_locked loop to use
> block_write_full_page(bh->b_page) instead of buffer_head operations.  But
> that could get hairy with blocksize < PAGE_SIZE.
>
> Thanks for working this out.  Let me ponder it for a bit.
>
>  


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