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

[Janet Morgan <janetmor@us.ibm.com>]

Andrew Morton wrote:

>Daniel McNeil <daniel@osdl.org> 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.
>
>  
>