On Tue, Feb 09, 2016 at 06:24:16PM +0100, Jan Kara wrote: > Hello, > > I was thinking about current issues with DAX fault locking [1] (data > corruption due to racing faults allocating blocks) and also races which > currently don't allow us to clear dirty tags in the radix tree due to races > between faults and cache flushing [2]. Both of these exist because we don't > have an equivalent of page lock available for DAX. While we have a > reasonable solution available for problem [1], so far I'm not aware of a > decent solution for [2]. After briefly discussing the issue with Mel he had > a bright idea that we could used hashed locks to deal with [2] (and I think > we can solve [1] with them as well). So my proposal looks as follows: > > DAX will have an array of mutexes (the array can be made per device but > initially a global one should be OK). We will use mutexes in the array as a > replacement for page lock - we will use hashfn(mapping, index) to get > particular mutex protecting our offset in the mapping. On fault / page > mkwrite, we'll grab the mutex similarly to page lock and release it once we > are done updating page tables. This deals with races in [1]. When flushing > caches we grab the mutex before clearing writeable bit in page tables > and clearing dirty bit in the radix tree and drop it after we have flushed > caches for the pfn. This deals with races in [2]. > > Thoughts? > > Honza > > [1] http://oss.sgi.com/archives/xfs/2016-01/msg00575.html > [2] https://lists.01.org/pipermail/linux-nvdimm/2016-January/004057.html Overall I think this sounds promising. I think a potential tie-in with the radix tree would maybe take us in a good direction. I had another idea of how to solve race #2 that involved sticking a seqlock around the DAX radix tree + pte_mkwrite() sequence, and on the flushing side if you noticed that you've raced against a page fault, just leaving the dirty page tree entry intact. I *think* this could work - I'd want to bang on it more - but if we have a general way of handling DAX locking that we can use instead of solving these issues one-by-one as they come up, that seems like a much better route. -- 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>