Messages in this thread |  | | Date | Mon, 13 Apr 2015 12:45:32 +0200 | From | Ingo Molnar <> | Subject | Re: [GIT PULL] PMEM driver for v4.1 |
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* Christoph Hellwig <hch@lst.de> 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. )
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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