Re: [PATCH, RFC 2/2] dax: use range_lock instead of i_mmap_lock

From: Jan Kara
Date: Thu Aug 13 2015 - 07:30:24 EST

On Wed 12-08-15 07:48:22, Dave Chinner wrote:
> On Tue, Aug 11, 2015 at 04:51:22PM +0000, Wilcox, Matthew R wrote:
> > The race that you're not seeing is page fault vs page fault. Two
> > threads each attempt to store a byte to different locations on the
> > same page. With a read-mutex to exclude truncates, each thread
> > calls ->get_block. One of the threads gets back a buffer marked
> > as BH_New and calls memset() to clear the page. The other thread
> > gets back a buffer which isn't marked as BH_New and simply inserts
> > the mapping, returning to userspace, which stores the byte ...
> > just in time for the other thread's memset() to write a zero over
> > the top of it.
> So, this is not a truncate race that the XFS MMAPLOCK solves.
> However, that doesn't mean that the DAX code needs to add locking to
> solve it. The race here is caused by block initialisation being
> unserialised after a ->get_block call allocates the block (which the
> filesystem serialises via internal locking). Hence two simultaneous
> ->get_block calls to the same block is guaranteed to have the DAX
> block initialisation race with the second ->get_block call that says
> the block is already allocated.
> IOWs, the way to handle this is to have the ->get_block call handle
> the block zeroing for new blocks instead of doing it after the fact
> in the generic DAX code where there is no fine-grained serialisation
> object available. By calling dax_clear_blocks() in the ->get_block
> callback, the filesystem can ensure that the second racing call will
> only make progress once the block has been fully initialised by the
> first call.
> IMO the fix is - again - to move the functionality into the
> filesystem where we already have the necessary exclusion in place to
> avoid this race condition entirely.

I'm somewhat sad to add even more functionality into the already loaded
block mapping interface - we can already allocate delalloc blocks, unwritten
blocks, uninitialized blocks, and now also pre-zeroed blocks. But I agree
fs already synchronizes block allocation for a given inode so adding the
pre-zeroing there is pretty easy. Also getting rid of unwritten extent
handling from DAX code is a nice bonus so all in all I'm for this approach.

Jan Kara <jack@xxxxxxxx>
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