Jan Kara <jack@xxxxxxx> writes: > 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? Agree, only small note: Hash locks has side effect for batch locking due to collision. Some times we want to lock several pages/entries (migration/defragmentation) So we will endup with deadlock due to hash collision. > > Honza > > [1] http://oss.sgi.com/archives/xfs/2016-01/msg00575.html > [2] https://lists.01.org/pipermail/linux-nvdimm/2016-January/004057.html > > -- > Jan Kara <jack@xxxxxxxx> > SUSE Labs, CR > _______________________________________________ > Linux-nvdimm mailing list > Linux-nvdimm@xxxxxxxxxxxx > https://lists.01.org/mailman/listinfo/linux-nvdimm
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