Re: DAX data corruption for mmaped and written files

[Ross Zwisler <ross.zwisler@linux.intel.com>]

On Thu, Mar 24, 2016 at 02:12:23PM +0100, Jan Kara wrote:
> Hello,
> 
> yesterday I have been stress-testing mmap code with my new fault locking
> patches and I have found a data corruption issue when file is written both
> via mmap and standard write(2). The problem is following:
> 
> CPU1					CPU2
> dax_io()				dax_fault()
>   get_block() - allocates block
>   ...					  get_block() - finds allocated block
> 					    - zeroes it inside fs
> 					  fault completese
> 
>   if (buffer_unwritten(bh) || buffer_new(bh)) -> new buffer
>     dax_new_buf() -> zeroes buffer which may
> 		     overwrite user data
> 
> In some cases the race can also go the other way around and we lose data
> written by write.
> 
> So either we need to do the zeroing inside fs also for write(2) path (but
> that would essentially mean we would write the block twice for each
> allocating write) or we would need dax_io() to also use radix tree locking
> to serialize against page faults (in the same way page cache does this with
> page lock). Any opinion on what would be better?

If the radix tree locking is essentially trying to be analogous to the page
lock, and the page lock is already used to serialize I/O vs page faults for
the page cache case, I guess doing that same serialization using our new DAX
locking seems like a natural fit.