On 08/12/2015 12:48 AM, 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. > Exactly, thanks > Cheers, > > Dave. > -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>