Re: [PATCH 1/5] mm/persistent-memory: match IORES_DESC name and enum memory_type one

[Dan Williams]

On Wed, Jul 5, 2017 at 11:49 AM, Jerome Glisse <jglisse@xxxxxxxxxx> wrote:
> On Wed, Jul 05, 2017 at 09:15:35AM -0700, Dan Williams wrote:
>> On Wed, Jul 5, 2017 at 7:25 AM, Jerome Glisse <jglisse@xxxxxxxxxx> wrote:
>> > On Mon, Jul 03, 2017 at 04:49:18PM -0700, Dan Williams wrote:
>> >> On Mon, Jul 3, 2017 at 2:14 PM, JÃrÃme Glisse <jglisse@xxxxxxxxxx> wrote:
>> >> > Use consistent name between IORES_DESC and enum memory_type, rename
>> >> > MEMORY_DEVICE_PUBLIC to MEMORY_DEVICE_PERSISTENT. This is to free up
>> >> > the public name for CDM (cache coherent device memory) for which the
>> >> > term public is a better match.
>> >> >
>> >> > Signed-off-by: JÃrÃme Glisse <jglisse@xxxxxxxxxx>
>> >> > Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
>> >> > Cc: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
>> >> > ---
>> >> >  include/linux/memremap.h | 4 ++--
>> >> >  kernel/memremap.c        | 2 +-
>> >> >  2 files changed, 3 insertions(+), 3 deletions(-)
>> >> >
>> >> > diff --git a/include/linux/memremap.h b/include/linux/memremap.h
>> >> > index 57546a07a558..2299cc2d387d 100644
>> >> > --- a/include/linux/memremap.h
>> >> > +++ b/include/linux/memremap.h
>> >> > @@ -41,7 +41,7 @@ static inline struct vmem_altmap *to_vmem_altmap(unsigned long memmap_start)
>> >> >   * Specialize ZONE_DEVICE memory into multiple types each having differents
>> >> >   * usage.
>> >> >   *
>> >> > - * MEMORY_DEVICE_PUBLIC:
>> >> > + * MEMORY_DEVICE_PERSISTENT:
>> >> >   * Persistent device memory (pmem): struct page might be allocated in different
>> >> >   * memory and architecture might want to perform special actions. It is similar
>> >> >   * to regular memory, in that the CPU can access it transparently. However,
>> >> > @@ -59,7 +59,7 @@ static inline struct vmem_altmap *to_vmem_altmap(unsigned long memmap_start)
>> >> >   * include/linux/hmm.h and Documentation/vm/hmm.txt.
>> >> >   */
>> >> >  enum memory_type {
>> >> > -       MEMORY_DEVICE_PUBLIC = 0,
>> >> > +       MEMORY_DEVICE_PERSISTENT = 0,
>> >> >         MEMORY_DEVICE_PRIVATE,
>> >> >  };
>> >> >
>> >> > diff --git a/kernel/memremap.c b/kernel/memremap.c
>> >> > index b9baa6c07918..e82456c39a6a 100644
>> >> > --- a/kernel/memremap.c
>> >> > +++ b/kernel/memremap.c
>> >> > @@ -350,7 +350,7 @@ void *devm_memremap_pages(struct device *dev, struct resource *res,
>> >> >         }
>> >> >         pgmap->ref = ref;
>> >> >         pgmap->res = &page_map->res;
>> >> > -       pgmap->type = MEMORY_DEVICE_PUBLIC;
>> >> > +       pgmap->type = MEMORY_DEVICE_PERSISTENT;
>> >> >         pgmap->page_fault = NULL;
>> >> >         pgmap->page_free = NULL;
>> >> >         pgmap->data = NULL;
>> >>
>> >> I think we need a different name. There's nothing "persistent" about
>> >> the devm_memremap_pages() path. Why can't they share name, is the only
>> >> difference coherence? I'm thinking something like:
>> >>
>> >> MEMORY_DEVICE_PRIVATE
>> >> MEMORY_DEVICE_COHERENT /* persistent memory and coherent devices */
>> >> MEMORY_DEVICE_IO /* "public", but not coherent */
>> >
>> > No that would not work. Device public (in the context of this patchset)
>> > is like device private ie device public page can be anywhere inside a
>> > process address space either as anonymous memory page or as file back
>> > page of regular filesystem (ie vma->ops is not pointing to anything
>> > specific to the device memory).
>> >
>> > As such device public is different from how persistent memory is use
>> > and those the cache coherency being the same between the two kind of
>> > memory is not a discerning factor. So i need to distinguish between
>> > persistent memory and device public memory.
>> >
>> > I believe keeping enum memory_type close to IORES_DESC naming is the
>> > cleanest way to do that but i am open to other name suggestion.
>> >
>>
>> The IORES_DESC has nothing to do with how the memory range is handled
>> by the core mm. It sounds like the distinction this is trying to make
>> is between MEMORY_DEVICE_{PUBLIC,PRIVATE} and MEMORY_DEVICE_HOST.
>> Where a "host" memory range is one that does not need coordination
>> with a specific device.
>
> I want to distinguish between:
>   - device memory that is not accessible by the CPU
>   - device memory that is accessible by the CPU just like regular
>     memory
>   - existing user of devm_memremap_pages() which is persistent memory
>     (only pmem seems to call devm_memremap_pages()) that is use like a
>     filesystem or block device and thus isn't use like generic page in
>     a process address space
>
> So if existing user of devm_memremap_pages() are only persistent memory
> then it made sense to match the IORES_DESC we are expecting to see on
> see such memory.
>
> For public device memory (in the sense introduced by this patchset) i
> do not know how it will be described by IORES_DESC. i think first folks
> with it are IBM with CAPI and i am not sure they defined something for
> that already.
>
> I am open to any name beside public (well any reasonable name :)) but
> i do need to be able to distinguish persistent memory as use today from
> this device memory.

Right, so that's why I suggested MEMORY_DEVICE_HOST for memory that is
just normal host memory and does not have any device-entanglements
outside of the base ZONE_DEVICE registration.