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?
On 11 February 2016 at 00:44, Ross Zwisler <ross.zwisler(a)linux.intel.com> wrote:
On Tue, Feb 09, 2016 at 06:24:16PM +0100, Jan Kara wrote:
> I was thinking about current issues with DAX fault locking  (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 . 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 , so far I'm not aware of a
> decent solution for . After briefly discussing the issue with Mel he had
> a bright idea that we could used hashed locks to deal with  (and I think
> we can solve  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 . 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 .
>  http://oss.sgi.com/archives/xfs/2016-01/msg00575.html
>  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.
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