There is another "twist" in this game: If there is a huge page with 1GB with a small 4k page as "overlay" (e.g. mmap() MAP_FIXED somewhere in the middle of a 1GB huge page), hows that handled? Ced On 11 February 2016 at 00:44, Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx> wrote: > 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 from this list: send the line "unsubscribe linux-fsdevel" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- Cedric Blancher <cedric.blancher@xxxxxxxxx> Institute Pasteur -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
