Re: Another proposal for DAX fault locking

[Cedric Blancher <cedric.blancher@gmail.com>]

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@linux.intel.com> 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.
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-- 
Cedric Blancher <cedric.blancher@gmail.com>
Institute Pasteur

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