Messages in this thread | | | Date | Mon, 13 Apr 2015 14:35:35 +0200 | From | Ingo Molnar <> | Subject | Re: [GIT PULL] PMEM driver for v4.1 |
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* Boaz Harrosh <boaz@plexistor.com> wrote:
> On 04/13/2015 01:45 PM, Ingo Molnar wrote: > > > > * 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 > > splice works fine. Also I sent a cleanup in this area to Andrew it will > be in for 4.1
How does splice work with DAX files? AFAICS vmsplice() won't work, as it uses get_user_pages(), which needs struct page backing. Also, how will f_op->sendpage() work? That too needs page backing.
> > - zero copy O_DIRECT into DAX areas. > > DAX is always O_DIRECT. > > What does not work is mmap of DAX file and use that pointer in an > O_DIRECT operation of another device. (unless it is a DAX device)
That is what I meant: O_DIRECT into (or from) DAX mmap()-ed areas, from a different device.
I'm not promoting the use of these APIs, some of them are quirky, just wanted to list the known limitations. The pmem driver is already useful as-is.
Thanks,
Ingo
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