[dm-devel] Persistent memory interface

[Dan Williams]

On Fri, Jun 19, 2015 at 12:14 PM, Mikulas Patocka <mpatocka at redhat.com> wrote:
>
>
> On Fri, 19 Jun 2015, Dan Williams wrote:
>
>> On Fri, Jun 19, 2015 at 9:33 AM, Mikulas Patocka <mpatocka at redhat.com> wrote:
>> > Hi
>> >
>> > I looked at the new the persistent memory block device driver
>> > (drivers/block/pmem.c and arch/x86/kernel/pmem.c) and it seems that the
>> > interface between them is incorrect.
>> >
>> > If I want to use persistent memory in another driver, for a different
>> > purpose, how can I make sure that that drivers/block/pmem.c doesn't attach
>> > to this piece of memory and export it? It seems not possible.
>> > drivers/block/pmem.c attaches to everything without regard that there may
>> > be other users of persistent memory.
>>
>> Simply partition the pmem device however you see fit and then
>> blkdev_get_by_path(<dev>, FMODE_EXCL, <holder>) the resulting
>> partition(s).
>
> But that still means accessing it through the block layer.
>
> I need to access persistent memory directly - map it into the kernel space
> and access it as mapped memory - and if I do it, it will fight with
> drivers/block/pmem.c over ownership of the memory :-(
>

See the latest definition of struct block_device_operations in these
patches.  We now have ->direct_access() in the 4.0 kernel and will
have ->rw_bytes() in the 4.2 kernel if you want to directly access the
memory published by a pmem device.