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    SubjectRe: Another proposal for DAX fault locking
    On 02/09/2016 07:24 PM, 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?
    >

    You could also use one of the radix-tree's special-bits as a bit lock.
    So no need for any extra allocations.

    [latest page-lock is a bit-lock so performance is the same]

    Thanks
    Boaz


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