1:35 p.m.
* Boaz Harrosh <boaz(a)plexistor.com> wrote:
On 04/13/2015 01:45 PM, Ingo Molnar wrote:
>
> * Christoph Hellwig <hch(a)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