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

From: Ingo Molnar
Date: Mon Apr 13 2015 - 08:35:51 EST

* Boaz Harrosh <boaz@xxxxxxxxxxxxx> wrote:

> On 04/13/2015 01:45 PM, Ingo Molnar 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
> 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.


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