Re: [Linux-nvdimm] [GIT PULL] PMEM driver for v4.1

[Yigal Korman]

On Mon, Apr 13, 2015 at 1:45 PM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
> * Christoph Hellwig <hch@xxxxxx> wrote:
>
>> On Mon, Apr 13, 2015 at 11:33:09AM +0200, Ingo Molnar wrote:
>> > Limitations: this is a regular block device, and since the pmem areas
>> > are not struct page backed, they are invisible to the rest of the
>> > system (other than the block IO device), so direct IO to/from pmem
>> > areas, direct mmap() or XIP is not possible yet. The page cache will
>> > also shadow and double buffer pmem contents, etc.
>>
>> Unless you use the DAX support in ext2/4 and soon XFS, in which case
>> we avoid that double buffering when doing read/write and mmap
>
> Indeed, I missed that DAX support just went upstream in v4.0 - nice!
>
> DAX may have some other limitations though that comes from not having
> struct page * backing and using VM_MIXEDMAP, the following APIs might
> not work on DAX files:
>
>    - splice
>    - zero copy O_DIRECT into DAX areas.
>    - futexes
>
>    - ( AFAICS hugetlbs won't work on DAX mmap()s yet - although with
>        the current nocache mapping that's probable the least of the
>        performance issues for now. )

mlock() and MAP_POPULATE don't work with DAX files as well.

>
> Btw., what's the future design plan here? Enable struct page backing,
> or provide special codepaths for all DAX uses like the special pte
> based approach for mmap()s?
>
> Thanks,
>
>         Ingo
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